PRELIMINARY
SITE -SPECIFIC EARTHQUAKE HAZARD
DETERMINATIONS FOR APIA, UPOLU, SAMOA
Geraldine Teakle and Monika Swamy
SOPAC Secretariat
January 2001
SOPAC Preliminary Report 126
Funding Support for this project was secured from
The New Zealand Overseas Development Assistance
(NZODA)
Government of New Zealand
DISCLAIMER
Opinions expressed in this report are not necessarily those of the SOPAC Secretariat, neither
does the mention of any agency, firm or licensed process imply endorsement by SOPAC. The
opinions expressed in this report are wholly those of the authors whom are not liable for the
misinterpretation, use or misuse of the information presented herein.
[2]
CONTENTS
ABSTRACT
ACKNOWLEDGEMENTS
3
GOAL & METHODOLOGY
ACTIVITIES
4
1.
2.
3
4
INTRODUCTION
1.1
Background
1.2 Global Advances in Site -Specific Hazard Determination Techniques
1.3 Theory of Microtremors
1.4 Nakamura Site Response Determination Method
1.5 SOPAC Resource Person and Counterparts
METHODOLOGY
Summary of Methods
2.2 Equipment and Acquisition Software
2.3 Site Response Determination (SRD) Procedure
2.4 Grouping of Nakamura Spectral Responses
2.5 Geotechnical Input
2.6 Data Uses and Limitations
2.7 GIS Database
2.1
5
7
7
8
8
8
9
9
13
13
14
14
15
SUMMARY OF KNOWN SEISMO- TECTONIC AND GEOLOGICAL INFORMATION
Regional Seismo- Tectonic Setting
3.2 Geological Setting
16
4.
MICROTREMOR DATA ACQUISITION AND ANALYSIS
20
5.
SUMMARY, DISCUSSION AND FURTHER WORK
5.1 Summary and Discussion
5.2 Further Work
36
37
3.
3.1
REFERENCES
16
38
APPENDICES
A: Data Sources - Borehole Information
B: Data Sources - GIS Information
C: Data Sources - Projection and Coordinate System
2
Equipment
3
Catalogue of Large Shallow Earthquakes around Apia, 1970 -2000
4
Apia Site Microtremor Field Notes and Raw Data File Names
1
[SOPAC Preliminary Report 126 - Teakle & Swamy]
40
41
42
43
45
50
[3]
ABSTRACT
Equipment originally supplied by the Geophysical Institute of Israel was upgraded and used to acquire microtremor data
by the Nakamura (1989) method in the city of Apia, Upolu, Samoa. Specific site response spectra of soft soils in Apia
were determined using the Nakamura technique incorporated in the GII -SRD software. Resonance, possible resonance
and non -resonant sites are identified and the site -specific Nakamura ratios are analysed. Preliminary subsurface
geological models and the Nakamura site response ratios are used to calculate the analytical response functions
(models) by using the computer code of Joyner (1977). Each site spectral ratio is grouped based on similarity of
response; that is the resonant frequency and the preliminary model. A preliminary seismic microzonation map of Apia is
produced and recommendations for further work are outlined.
Key words: Nakamura site response determination method, site response spectra, seismic microzonation, Pacific Cities
concept.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the important contributions of Nilesh Kumar, Sakaraia Vunisa, Arvin Singh and
Lasarusa Vuetibau of the Mineral Resources Department of Fiji for their assistance with the seismic instrumentation
upgrade and for use of their seismic acquisition system. Christopher loan and Morris Stephen of the Geology, Mines
and Water Resources Department, Vanuatu, promptly mailed us their seismic acquisition system and its use for an
extended period. We also acknowledge Lameko Talia, SOPAC's Samoan country counterpart, from the Department of
Meteorology, Geophysics Division, Apia, Samoa, and Leatuaoa Sakaria and Johnny Ah Kau for their continued
assistance in data gathering and field work. The Government of Samoa gave permission to publish this data and
information. We are grateful for the invaluable input of staff of the Geophysical Institute of Israel and the SOPAC
Secretariat. New Zealand Overseas Development Assistance (NZODA) is gratefully acknowledged as the major source
of funding for the project.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[4]
GOAL & METHODOLOGY
Earthquake hazard of Apia quantified in terms of site -specific, uniform -hazard, acceleration- response functions and
made available for the purpose of seismic hazard assessment, planning, management and mitigation activities in the
city of Apia, Upolu, Samoa.
Specific site effects of amplification of earthquake shaking in soft soils in Apia assessed using the Nakamura (1989)
method and available geological and seismo- tectonic information.
1.
2.
3.
Upgraded and improved seismic microzonation equipment.
Preliminary seismic microzonation of Apia.
Preliminary seismic microzonation map of Apia.
ACTIVITIES
1.
2.
3.
4.
5.
6.
7.
Analyse surface and subsurface geology and seismo- tectonic setting of Apia.
Upgrade seismic microzonation equipment.
Carry out seismic micro -tremor survey using the Nakamura Method in Apia.
Process and analyse Apia microtremor recordings using the Nakamura Method.
Determine the specific site effects of amplification of earthquake shaking in soft soils in Apia by interpreting the
microtremor analysis in the context of the geological and seismo- tectonic analysis.
Report on the results including a preliminary seismic microzonation map of Apia.
Make recommendations for further work.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[5]
1. INTRODUCTION
The islands of Samoa (formerly Western Samoa), lie in a region that is vulnerable to many natural and human -induced
hazards including earthquakes, tsunamis, cyclones, extreme storms, flooding, pollution and El Niño events. In 1991,
Samoa had a total population of 161 298 and the capital, Apia, a population of 34126; 21 % of Samoa's total
population (Department of Statistics, 1991). There is a danger in that if city growth and ever -increasing development
activities are not adequately managed, then in times of disaster, community resilience could be greatly compromised. In
addition, migration of rural people to the urban centre and greater variations in the usage of city land area, such as
reclamation in areas previously occupied by mangrove swamps, compound the problem. For these reasons alone, the
hazards that pose a risk to the people and their socio- economic well being, must be clearly identified, understood and
subsequently mitigated.
Earthquake risk in Apia is mainly dictated by its location relative to an active seismic belt (the Tonga- Kermadec Trench)
that spreads from the south of Tonga to approximately 200 kilometres south of Apia (see Figure 1). Building damage
can be mitigated if the construction of new buildings and structures, and extensions and alterations to old buildings
adhere to a standard that takes into account amplified site -effects that can occur during an earthquake. Incorporating
quantified site -specific earthquake hazard into building codes can mitigate building damage. However, earthquake
damage will only be reduced ultimately if the code is enforced through strict policy and legislation.
Interpretation of microtremor analysis in the context of the geological and seismo- tectonic setting of Apia enabled
specific site effects of amplification of earthquake shaking in soft soils to be estimated. A microtremor survey and
analysis was undertaken using the internationally accepted Nakamura technique (1989). Through this technique
spectral ratios were calculated and grouped. The groups are based on the criteria of similarity of resonance and
amplification characteristics. Preliminary seismic microzones, or areas of differing levels of earthquake risk (ground
response and ground motion), were identified and delineated.
More detailed information on geological mapping and boring and seismic cone penetration tests must be acquired if a
more robust, detailed and accurate interpretation of the seismic microzonation for Apia to be produced. In addition, the
earthquake hazard should also be quantified by calculating site -specific uniform hazard, acceleration response
functions. In other countries this has been undertaken by Dr Avi Shapira of the Geophysical Institute of Israel (SOPAC's
former project counterpart), by application of the semi -empirical SvE method (Shapira and van Eck 1993). This
constitutes the major goal of this project but will not be discussed in this report. A follow -up study quantifying Apia's
seismic hazard using the SvE method is planned, and will be presented in a later technical report.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[7]
1.1 Background
Damage during an earthquake is controlled by many factors other than the earthquake source characteristics and focal
distance. Shallow geological structures and soil conditions are also key parameters that can account for observed
damage and site response effects. A fast and reasonably accurate technique for assessing site response is to carry out
a microtremor survey of the target area and apply the Nakamura method to analyse the data.
The investigation of earthquake- shaking risk in Apia is essentially a new addition to a 3 -year study completed in 1998.
The study investigated the site -specific earthquake hazard determinations in four capital cities of the South Pacific,
namely, Suva, Port Vila, Nuku'alofa and Honiara (Shorten et al. 1999). This was a USAID funded project (Grant number
TA- MOU- 95 -C13 -024) with collaborating investigators from the Geophysical Institute of Israel (GII), IRD /ORSTOM and
participating country organizations. The results were incorporated in the SOPAC Pacific Cities concept which
encompasses a broad range of activities such as seismic microzonation and asset surveys of buildings in capital cities.
The concept is founded on the construction of a comprehensive GIS database of geographically located information
including aerial photography; digital terrain models; bore -hole data; geology; seismic microtremor sites and zonations;
roads and population information from each country's census bureau.
1.2 Global advances in site -specific hazard determination techniques
Over the past decade, investigators have made significant advances in site -specific hazard determination techniques
across the globe. Shapira and Hofstetter (1993) field- tested the Nakamura technique in Israel and took the analysis one
step further by defining a method for synthetic uniform -hazard site -specific response spectrum (Shapira and Van Eck,
1993), now referred to as the SvE method. Stephenson et al (1990) assessed the seismic hazard for resonant alluvium
conditions for part of the Porirua Basin in Wellington, New Zealand. Their methodology used a combination of
geomorphological studies together with penetrometer tests and seismic cone penetrometer tests. Stephenson et al
describe the phenomena whereby topographic features, such as sharp ridges or plateau edges can cause increased
ground motions. Stephenson and Baguley (1996) undertook an assessment for amplification of earthquake shaking in
soft soils in Wanganui, New Zealand using the Nakamura method. Again the concept of resonance of topographic
features (e.g. buildings, ridges and embankments) was discussed. They noted that during microtremor analysis the
resonant character of a feature beneath the site investigated should be revealed, although the identity and extent of the
feature may not be obvious. Stephenson and Baguley (1996) also noted that microtremor recording made near these
features may reveal the resonant frequency of the feature as opposed to the site itself.
Irrespective of these findings, it is generally agreed that the Nakamura site response determination method provides a
good estimate of the natural frequency of a resonant site and a rough estimation of the amplification expected at that
site (Lermo and Chavez -Garcia 1994, Stephenson et al 1990, Shapira 1999, Shorten et al 1999). Interestingly,
Stephenson and Baguley (1996) noted that some sites covered with a significant depth of soft soil have not shown the
high amplifications expected. Also, in other places anomalous effects were shown to have been caused by a gradual
change from soft surface soils to stiff basement rocks, or by the scattering of the arriving earthquake waves in the
inhomogeneous strata underlying the soft surface materials. Nevertheless, Stephenson and Baguley indicate that if a
widespread soft layer has an abrupt interface with the firmer, underlying material then the method is clearly capable of
locating highly resonant areas although further characterisation must be achieved through other methods. As far as it
was possible, these findings were considered and incorporated into the seismic microtremor data acquisition and
analysis, and preliminary microzonation for Apia.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
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[8]
1.3 Theory of Microtremors
Microtremors are the continuous microscopic vibrations of the ground. They result from ever -present surface waves
generated by such sources as human activities, wind and surf, as well as natural ground motions such as earthquakes
(Nakamura 1989, Singh et al 1 998).
The use of microtremors to characterise soft soils was developed by Nakamura to determine the resonant frequency of
ground shaking and the degree of amplification arising from the shaking of soft soils overlying hard bedrock. This
process of characterisation is referred to as site -specific earthquake hazard determination, more commonly known as
seismic microzonation. Microseismic zones delineate areas of different seismic hazard due to variable amplification at
specific sites. The seismic hazard is more acute for those buildings or structures having a natural period of resonance
matching that of the ground on which it is built.
1.4 Nakamura Site Response Determination Method
Nakamura's method attracted many researchers around the world looking for a simple, inexpensive technique to
estimate the response of a site to seismic waves. World -wide experience has shown that the site effect is one of the
major factors that controls the intensity of damage caused during earthquakes and consequently is an important
parameter in the process of estimating earthquake hazard and risk.
The method considers microtremors to be constituted mainly of seismic surface waves that are generated by the
constructive and destructive interference of the upward propagating body waves. Based on theoretical considerations
as well as on some empirical evidence, Nakamura applied the assumption that the spectral ratio between the vertical
motions on the surface and those of the horizontal motions on the interface between the rock and the soft layer is close
to unity. Consequently, the ratio between the horizontal motions and the vertical motions on the free surface closely
represents the transfer function of the soft layer to seismic shear waves (S- waves). The generation process of the
surface waves (i.e. microtremors) basically implies that seismic energy is trapped between the free surface and the
surface of the basement rock, focussing seismic energy in discrete frequencies. The fundamental frequency is known
as the resonant frequency of the soft layer (Nakamura 1989).
1.5 SOPAC Resource Person and Counterparts
Geraldine Teakle
Lameko Talia
Leatuaoa Sakaria
Johnny Ah Kau
Environmental Geophysicist, Hazards Assessment Unit, SOPAC, Suva, Fiji.
Geophysicist and country counterpart, Apia Observatory, Department of Meteorology, Geophysics
Division, Apia, Samoa.
Field Assistant, Apia Observatory, Department of Meteorology, Geophysics Division, Apia, Samoa.
Field Assistant, Apia Observatory, Department of Meteorology, Geophysics Division, Apia, Samoa.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[9]
2. METHODOLOGY
2.1 Summary of Methods
The first step in undertaking site -specific hazard determinations requires the geology and seismo- tectonic setting of
Apia and the region to be analysed (see Chapter 3).
Following this, appropriate microtremor field sites are chosen and the survey team prepares equipment for data
acquisition. The seismometers are set up at the specific sites and the microtremor data is logged on field laptop using
the Seismic Data Acquisition (SDA) software (Shapira and Avirav 1996). If it is not possible to take a GPS position of
the microtremor site then the location is plotted on an aerial photograph, cadastre or street map. Pre -processing is
performed in the field so that if resonance is detected then a site traverse can be carried out to characterise the area in
more detail.
After the field survey is completed the microtremor site locations are input into the GIS database at the SOPAC
Secretariat. Contour lines and the roads network of Apia are also inserted into the database (see Appendix lb, Data
Sources - GIS Information). It is important that the projection of the working maps (GIS layers) are known so that a
directionality analysis can be undertaken later if required. The microtremor data is then post -processed using the Site
Response Determination (SRD) software incorporating the Nakamura method. The spectral responses are grouped
based on the criteria of resonance detection and the degree of similarity in response. If sure resonance is detected the
most dominant spectral characteristic is indicated by amplifications that have a narrow peak with low ratio values at
high frequencies (Singh et al 1998). The remaining spectra are then grouped based on the criteria that they are either
possibly resonant or that no significant response was determined. The representative grouped site responses are then
modelled using the method of Joyner (1977). This enables the analytical response function in the SRD software to be
calculated. Based on grouped spectral responses preliminary seismic microzones of Apia are delineated and presented
on a GIS map.
2.2 Equipment and Acquisition Software
2.2.1 Seismic Acquisition System Upgrade
In the GII -USAID Seismic Microzonation of Pacific Cities project, the seismic acquisition systems were purchased for
use in microtremor data acquisition, and also for permanent deployment in each country for earthquake recordings.
Since the data -logging instrument was required for a dual purpose, the project supplied desktop computers for field
data acquisition, and subsequently for earthquake recordings in a permanent in -house setup.
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However, in the Apia microtremor survey, it was a priority to have equipment that increased the
the field. This required the purchase and use of a laptop computer; an analog -to- digital (ND) con
connecting cable and 37 -pin adapter, and a connector box (terminal panel). Some of the existing
adapted to accommodate the new parts. Adaptation and field testing of equipment was undertak
SOPAC's Hazard Assessment Unit staff; SOPAC's Laboratory staff, and staff of the Seismology
Resources Department of Fiji.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[10]
Shown in Figure 2 is a schematic diagram of the seismic acquisition equipment used in Apia. Note that although
schematically simple there are numerous setup connections to be made, in reality the system was reduced
considerably in weight and size. The bulk of the equipment can be found in the seismometer cables and the
seismometers themselves. To minimise downtime in case of equipment failure, redundant parts were also taken for
data acquisition as spares, that is, 2 sets of seismometers, cables and amplifier -filter boxes. The seismometers, cables,
filter -amplifiers (blue boxes) were made available by for use in the Apia survey by both the Mineral Resources
Department (Fiji Islands) and Geology and Mines (Vanuatu).
The filters were set at 0.2 Hz/12.5 Hz band -pass filter. The amplifiers were tested in the field and set at a CONSTANT
GAIN THROUGHOUT THE SURVEY of 40 dB. The constant gain is required so that if estimation of the level of
amplification is to be made then each site must have the same base -reference amplification.
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In Appendix 2 is a list of the equipment source and serial number, where applicable.
The recording system was located in a vehicle and the seismometers are led out from the vehic
The seismometers were set up distant enough from the recording and processing area not to b
caused by walking near the recording system or running the car's engine. However, in practice
by the location of suitable parking for the car and the site to be monitored. In some cases this w
which was acceptable provided the car was not running.
The power supply to the Compaq Notebook computer can be either an inverter from 12V DC to
power booster for 12 V DC. The choice depends on whether the laptop has an internal or exter
laptop used for seismic data acquisition in Apia had an internal power supply. Thus, an inverter
from the SOPAC Laboratory to invert 12V DC (the car battery) to 220V AC for powering the lap
The system was originally designed by Gil to be used with a GPS for timing. This is shown in da
as it was not required for the microtremor survey in Apia.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
2.2.2 Seismic Acquisition System Site Set -Up
To ensure the seismometers are not damaged in transit, they are connected to the cables one -by -one at each site. For
long distance shipping and handling the seismometer pins need to be short- circuited. This should disable the coil inside
the seismometer and minimise risk of damage. This precaution is not necessary for movement around the survey area
provided they are kept safely in their locked positions (pin- connector end down) in their carry boxes within the transport
vehicle.
At each site the two horizontal components are aligned at right angles, one north and one east, and the vertical
component is placed upright. Alignment is achieved with a compass and a set -square. Care is taken to ensure that the
compass needle is not deflected by the magnetic components inside the seismometers. The directional set -up of the
seismometer components was assumed so that, if required at a later date, directionality of wave propagation can be
incorporated into analyses. The seismometers can be placed on firm ground or a concrete or tar -sealed surface
provided it's a surface that enables good coupling between the seismometer and the ground. If the surface area is soft
soil or grassy, it should be cleared and flattened. If this is not possible a moveable concrete block can be used to
increase the frequency of any foundation resonance that may occur between the seismometer and the ground, to a
value out of the range of interest (Singh, et al 1998).
As depicted in Figure 2, the three seismometer cables are wired to a single connector that is connected to the amplifier filter box. The gain and filter settings in the amplifier -filter box are checked. The amplifier -filter box and the laptop are
powered via jumper leads by the 12 -Volt car battery, although the laptop is connected to the car battery via the DC /AC
inverter. For ease of transportation, the terminal panel, also known as the connector box, is mounted inside the
amplifier -filter box.
The four output wires from the amplifier -filter cards (in the amplifier -filter box) are connected to the terminal panel. One
output wire (generally black) is earthed to the common ground (channel 8 and channel 17) on the terminal panel. The
other 3 wires carrying the filtered and amplified analogue vertical (Z), north and east component signals are connected
on the terminal panel to channels 18, 19, and 20 respectively.
The analogue signals are then passed, via a connector cable, to the A/D converter card in the laptop. The signal is
recorded digitally on the computer using the SDA (seismic data acquisition) software supplied by GII.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[12]
12V Car Battery
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Seismometers
z
N
Seismometer Cables
Length 50 m or more
E
Figure 2. Schematic Diagram of Seismic Acquisition Equipment
2.2.3 Seismic Data Acquisition Software
Firstly, the A/D PCMCIA -card software (InstaCal Version 5.02, Computerboards 1994) must already be loaded onto the
laptop and tested. Next, seismic data acquisition is achieved by initiating the PC -SDA software (Shapira and Avirav
1996). The SDA software enables simultaneous digital registration of up to 16 seismic channels. Note, only 3 channels
plus ground were used for seismic data acquisition in Apia. The analogue signal input is digitised with a 16- bit/word
digitizer and a sampling rate set to 100 samples per second per channel.
The PC -SDA was developed during the initial stages of the GII -USAID Seismic Microzoning project. The first version
was prepared in 1995 and the PC -SDA manual (see Shapira and Avirav 1996) provides a detailed description of the
system. The PC -SDA was originally designed to be used by a PC 486 using DOS, however, Shapira (1999) upgraded
the system, so that it can now be effectively used on WINDOWS -95 and has bug fixes for WINDOWS -NT.
Recording can be made in both trigger mode and scheduled mode. Trigger mode records only when the input signal
fulfils certain pre -defined conditions. In scheduled mode, start and end recording is achieved according to a pre -defined
schedule. Triggering is based on the STA/LTA algorithm (for a single channel) and coincidence criterion (number of
channels triggered within a prescribed time window). Scheduled recording can be defined in terms of starting time and
length of recording or in a periodic manner, that is, t seconds are recorded every T minutes. Manual trigger mode was
used for microtremor acquisition in Apia.
Note that the PC -SDA can produce an output signal that is used to trigger a digital strong motion accelerograph. This is
optional, and was not required for site -acquisition in Apia.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[13]
2.3 Site Response Determination (SRD) Procedure
The site response determination (SRD) software was written for the interactive analysis of site response measurements
and was developed by the Geophysical Institute of Israel from their seismic data processing (SDP) software in the initial
stages of the GII -USAID project (Shapira 1999). While SDP supports the need for phase picking, location and
magnitude determinations, the SRD algorithm enables the analysis of the recorded waveform in the spectral domain to
determine site -response.
The SRD software was used to:
1.
Calculate the spectral -ratio functions of individual sites within Apia, and
Fit a subsurface model that will yield a theoretical response function, that is similar to the observed pattern.
2.
To minimise the uncertainty in estimating known resonance by using the Nakamura method, the empirical site response
function (i.e. the horizontal to vertical spectral ratio) was matched to an analytical function using geotechnical
parameters. The computation of the analytical function required the input of subsurface information using the computer
code of Joyner (1977).
In theset
first
the
assign
module,
appropriate
choose
the
assign
the
window
the
define
SRD
number
the
number
program
length;
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ratio
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shape;
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and
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seismic
to:
the components;
data acquisition period;
In the second module the SRD program also allows the computation of the analytical response function for a user defined subsurface model, by integrating Joyner's (1977) non -linear site response determination program. The Joyner
(1977) method calculates non -linear seismic response of a system of horizontal soil layers underlain by a semi -infinite
elastic medium representing bedrock.
In quantifying the seismic response at a site with soft soil, the most significant data required are the shear -wave
velocities; the thickness of the soil layers; and depth to bedrock. Shear -wave velocities measured using a seismic core
penetrometer to a depth of several tens of metres are generally best for this purpose. In the absence of direct
measurements, velocity profiles may be estimated from geotechnical data, attained from drillhole logs and p -wave
velocity profiles or cone penetration tests can be used. Estimation was difficult in Apia although several drillholes had
recorded N- values. N- values are estimated by undertaking standard penetration tests (SPT) and are used to assess in
situ relative density of sand deposits (Craig 1987) or consistency of cohesive soils for preliminary site studies (Berkma
and Ryall 1976). The N- values given for some of the Apia boreholes are in some cases incorrectly documented and ar
only accompanied by basic non -technical soil descriptions. In the cases where the N -value could be interpreted
together with a soil description, shear wave velocities were estimated, somewhat crudely, for that site.
2.4 Grouping of Nakamura Spectral Responses
The individual site responses were determined using the Nakamura technique. This required the computation of the
spectral ratio of the average of each of two orthogonal horizontal components; arbitrarily the north -south (x) and east west (y) components relative to the vertical component (z) of the ground motion, ie. x/z and y /z. The calculations were
performed within the SRD software. The spectral response curve determined for each site was plotted as spectral rati
versus frequency. Plots of the site -specific spectral responses can be found in Section 4.
Model controls and boundary conditions can be defined if sufficient subsurface geology, geotechnical parameters and
shear -wave velocities are provided. In turn, the geographic distribution of similar grouped site responses can then
define the seismic microzones. Grouped, or stacked, Nakamura spectral ratios are based on correlation of resonant
frequency response curves and amplification characteristics of all recordings obtained across the surveyed area.
As mentioned above, the Nakamura technique results in a good estimate of the resonant frequency of the soil layer, b
does not necessarily give a reliable estimate of amplification. Where sufficient and reliable baseline data, is available,
the next step in seismic microzonation is to fit a unique analytical function to each Nakamura stack. In -situ site
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[14]
geotechnical parameters and known layer thicknesses for representative site -responses in that zone are used. The
analytical response function is then incorporated into subsequent SvE analysis (Shapira and Van Eck 1993) to
determine the acceleration responses of each zone. This information is used in a comprehensive earthquake shaking
risk analysis of the city area. Such a risk analysis is planned in a follow -up study (see Chapter 5).
Grouping or stacking of site responses in this study will refer to those cases where the spectral ratio indicates
resonance and will be plotted together with the best -fit analytical response function model, that is:
1.
Resonance and model,
Possible resonance and model,
2.
No resonance and model.
3.
2.5 Geotechnical Input
A detailed analysis of the geology and geotechnical information can be found in Chapter 3. The geotechnical
parameters required for modelling site -response in Apia, and subsequently delineating microzone boundaries are listed
in Box 1 below. Although Apia lacks sufficient subsurface geology and geotechnical information, representative sites in
each group have been modelled and some estimates of layer depth and s -wave velocity have been made (see Chapter
4).
Box 1.
Input Geotechnical Parameters
For surface soil layer:
thickness (or depth)
natural density (bulk density)
pre -consolidation vertical effective stress
depth to water table
degree of saturation of phreatic zone
dynamic shear strength
shear modulus (low- strain)
shear velocity (low- strain)
For the elastic substratum:
Density
shear velocity
2.6 Data Uses and Limitations
As identified in past investigations (Hull et al., 1997, Shorten et al., 1999) there are some limitations associated with
microtremor data collection and analysis. A major and unavoidable limitation is that of sampling at specific sites (Singh
et al 1998). The spectral ratio is determined at discrete locations only, so that some localities, even directly adjacent,
potentially having high amplifications may not be sampled. As discussed in Section 4.2, a dominant topographic feature
or a resonant object located at the investigated site (buildings, ridges, embankments, etc), after microtremor analysis,
may reveal the resonant character of the object although the extent of the object or texture would not be obvious. As a
result of their findings, Singh et al recommend that "when assessing sites by the Nakamura method it is important to
avoid over -interpreting spectral -ratio plots. In particular the heights of resonant peaks should not be taken as indicating
site amplifications, and a narrow peak with low ratio values at high frequencies should be the main criterion for
assessing amplification ".
Other limitations identified by Singh et al (1998) include the small- strain nature of microtremors recorded where the
onset of non -linear characteristic of soils (where plasticity is not documented) is unknown. Also it is postulated by Singh
et al that the soil -to -rock spectral ratios may respond differently during large and small earthquakes. They claim that
any method that estimates amplifications, including the Nakamura method, cannot predict the amplification effects of all
earthquakes. Depth, magnitude, fault -plane orientation, and rupture mechanism are not considered in the method
although all play part in amplitude and are variable for different earthquakes.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[15]
It is understood, however, that the frequency characteristics of the Nakamura spectral -ratio plots will allow
discrimination between the possibility of resonance or no resonance at a site. Using those site -response ratios showing
a narrow peak with low ratio values at high frequencies, the area of interest (Apia city) was zoned accordingly.
Amplification and acceleration response of resonant areas can be estimated by other methods, such as the SvE
method (Shapira and Van Eck 1993).
Other limitations associated with microtremor analysis are inconsistencies in data collection and processing. Rigorously
controlled acquisition and processing of microtremor data was one of the main goals of the Apia microtremor survey. In
the cases where it was not possible to retain consistency of set -up, acquisition, processing and analysis, the
inconsistency has been noted and possible effects on the results have been assessed.
Accurate Apia borehole positions are not available so estimates of their positions have been plotted on the GIS for
illustrative purposes.
Microtremor site positions were identified on the aerial photograph and on the roads, cadastral and contour maps of
Apia. They were subsequently transferred to the Apia GIS database. Where there was no data (missing roads or land
fill where there was previously mangroves) the site position plotted on the map (refer Figure 6) may be in error up to
100 m. However, this is generally not the case and so the micro -tremor site position error generally ranges from 0 - 20
m. The error of positioning of microtremor sites for this study is acceptable since it is not crucial to know the exact site
location. The resolution of the contours and roads are not greater than about 20 m (possibly due to digitising off of a
large -scale map of 1:20,000). In the cases where a microtremor site was located where there was aerial photograph or
cadsatral map coverage, the error in position is less than 5 m. An accuracy of microtremor site position of about 20 m is
acceptable for the purpose of delineating seismic microzones. The error in site positioning becomes much more
significant when multiple microtremor recordings are undertaken in a small area to determine the variability of soil
resonance and amplification.
The most noteworthy aspect of the microtremor analysis and preliminary seismic microzonation of Apia is the lack of
control data from borehole logs, known geotechnical parameters and shear -wave velocities. Several borehole logs
reach bedrock (e.g. boreholes AB013 and AB014ACB at the ACB building site, see Figure 3 and Appendix la). In some
cases there is a lack of consistent information regarding geological formation, natural density, shear strength etc. Most
of the boreholes in Apia are located in a concentrated area in the downtown district and not spread across the whole
survey area. In general the geotechnical parameters used for the Apia microtremor study were estimated based on the
information from boreholes that reach bedrock and on other geotechnical studies.
Lack of geotechnical and geological data hinders a thorough data analysis and interpretation of site responses and site
spectral ratios. Nevertheless, the Nakamura method itself is sufficient to identify resonant sites and, together with the
data available, a preliminary interpretation and microzonation of Apia was able to be made.
Apia aerial photography was scanned from photographs flown in 1990 at a scale of 1:6000 (NZAM 1990). The aerial
photography was limited in extent and did not include extra roads and buildings, constructed, nor mangrove areas that
had been reclaimed and developed in the period form 1990 to 1999. Nevertheless, it proved a valuable asset for
locating microtremor sites downtown and for undertaking the SOPAC Hazard Assessment Unit assets survey.
Concurrently with the Apia microtremor survey, new aerial photographs were being flown by Airesearch Mapping Pty.
Ltd and are now available for acquisition.
2.7 GIS Database
All the available secondary data, such as roads, contours, rivers, etc, were used as a base for developing a preliminary
Apia seismic microzonation map on a GIS database. Each delineated microzone represents a layer on the database.
Borehole locations and microtremor site locations are compiled in one layer on the GIS database. Primary and
secondary data are housed in the SOPAC Regional Data Centre and are listed in Appendix 1 a, 1 b and 1 c including the
data source and other information such as the projections and coordinate system. For more information regarding the
available or acquired data from the Apia microzonation study, contact the SOPAC Secretariat in Suva. SOPAC's
website address is http: / /www.sopac.org.fj.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[16]
3. SUMMARY OF KNOWN SEISMO- TECTONIC AND
GEOLOGICAL INFORMATION
3.1 Regional Seismo -Tectonic Setting
The broad distribution of shallow, large earthquakes throughout the North and South Fiji and Lau Basins can be seen in
Figure 1. The areas of medium -high intensity, shallow (crustal) seismic activity represent a major concern for the cities
in this study. Those include clusters of earthquake events located in the Fiji Fracture Zone; the Hunter Fracture Zone;
the Hazel Holme Ridge; the New Hebrides Arc and San Cristobal Trench; the Tonga Trench; and the proposed
spreading centre immediately west of Fiji (Hamburger and (sacks 1993). The northern and western North Fiji Basin and
the western Lau Basin are typically aseismic and the area as a whole is dominated by strike -slip deformation
(Hamburger and (sacks). The boundary between the Pacific and Indo- Australian Plates is convergent. In the North Fiji
Basin complex processes and active crustal deformation of back -arc extension are taking place (Hamburger and Isacks
1993, Cooper and Kroenke 1993). The North Fiji Basin is sandwiched between the Tonga and New Hebrides Trenches
where there are notably two active subduction zones of opposite polarity (Hamburger and (sacks 1993). The Vitiaz
Trench lies north of Fiji where there is an inactive subduction zone.
A catalogue of large, shallow earthquakes that occurred in a radius of 250 kilometres around Apia from 1900 to 2000 is
listed in Appendix 3. The data selection includes historical and preliminary data and significant earthquakes occurring
within that radius. For more information on the data selection, see the National Earthquake Information Center web -site
(NEIC 2000).
Listed in Table 1 are the five largest magnitude earthquakes that were recorded during the period 1900 -2000
(extracted from the catalogue in Appendix 3). The shallowest earthquake in the selection had a depth of 21 km, which
was recorded on the 7,h of April 1995 and had a surface wave magnitude, Ms = 8.1. This record is an illustration of the
type of high magnitude earthquake event that can occur around Apia. The highest magnitude event ever known globally
was about 9.5 (NEIC, 2000). On this logarithmic scale a magnitude 8.0 shallow -focus earthquake represents elastic
wave energy about 30 times larger than that generated by a magnitude 7.0 earthquake; 900 times (30 x 30) larger than
that of a magnitude 6.0 earthquake (NEIC, 2000). Depending on local ground conditions (soil strength, building height,
etc) the resultant damage could be catastrophic. Magnitude solutions are described in Appendix 3.
Table 1.
Five Largest Magnitude Earthquakes recorded in a radius of 250Kms during the period 1900 - 2000.
Date
Origin Time
Latitude
Longitude
Longitude (E)
Depth
(km)
Magnitude
Solution
Recorder
Epicentral
Distance from
Apia (km)
(UTC)
26- Jun -17
054900.00
-15.50
-173.00
187.00
25
8.7
Ms
NOAA
227
14- Apr -57
191800.00
-15.50
-173.00
187.00
60
7.6
Ms
NOAA
227
1- Sep -81
092931.54
-14.96
-173.09
186.91
25
7.9
Ms
BRK
188
3- Jun -85
120621.13
-15.29
-173.52
186.48
33
7.0
Ms
BRK
247
7- Apr -95
220656.89
-15.20
-173.53
186.47
21
8.1
Ms
BRK
242
(Source: NEIC, 2000)
3.2 Geological Setting
The geology of Western Samoa (now Samoa) was described by Hedge et al (1972) to consist mainly of olivine -rich
basaltic lava flows that are associated with marine tuffs, volcanic breccia and alluvial deposits.
The majority of these younger basaltic lava flows had erupted from the large number of volcanic cones distributed along
the crest of the island (Fepuleai 1997). Kear and Wood (1959) identified six geological units. The identification was
based upon the geomorphology and degree of weathering, although the units were named geographically. See Table 2
below.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[1 7]
Table 2.
Six volcano -stratigraphic units of the Western Samoa sequence and their distinguishing features
Units
Features
Name Origin
Aopo Volcanics
Fresh or slightly weathered with very thin soil or no soil; pahoehoe and aa
flows common only around cones which fill older valleys and spill out over
coasts to fill lagoons and cover the barrier reef.
Slightly weathered with thin soil; lava flows offshore and also form rocky (or
ironbound) coasts; ubiquitous aa and pahoehoe structure form broad
domes.
Intermediate, weathered, thin soil; of these outcrops only narrow fringing
reefs are present offshore.
Intermediate, weathered, soil; existence of wide barrier reefs offshore.
Village on the northwest Savaii, about
8 km inland.
Puapua Volcanics
Lefaga Volcanics
Mulifanua Volcanics
Village on northeast Savai'i.
District in southeast Upolu.
Village on the southwestern part of
Upolu.
Salani Volcanics
Fagaloa Volcanics
Thick soil and deeply weathered; reef is quite far offshore and gorges cut in District on the southeast of Upolu.
flanks.
Very thick soil and deeply weathered; reef is close to the inshore or none
District on the northeast Upolu.
occur in some parts; deeply eroded and dissected volcanic terrain and lava
associated with series of dykes.
(after Kear and Wood, 1959, and adapted from Fepuleai, 1997)
Fepuleai (1997) states that the Fagaloa Volcanics is the oldest unit (see Table 3) of the stratigraphic sequence on the
northeast and southwest sides of Upolu. He also states that it is unconformably overlain by the younger rocks of the
Salani, Mulifanua, Lefaga, Puapua and Aopo Volcanics. It is interesting to note that the topography developed on the
Fagaloa Volcanics is very steep (sometimes greater than 50 °). This is demonstrated in Apia where Mount Vaea rises to
approximately 450 metres above the city's coastal plain in less than 2 Kms and has slopes ranging from 10° to 52 °.
Mount Vaea is made up of weathered Fagaloa Volcanics and dominates the centre of the project area, shown in Figure
3. The majority of the exposed terraces of the older lavas are covered with Holocene alluvium, including very thick
deposits along the eastern and northeastern sides of Upolu (Fepuleai 1997). Holocene sediments form the coastal plain
of Apia. Although borehole information for Apia is very scant, some of the logs indicate that the depth to the bedrock
surface in the alluvial plain can reach up to 30 metres (see Appendix la for summary). The remainder of the Apia plain
is characterised by Salani and Mulifanua Volcanics, with occasional outcrops of Fagaloa Volcanics. The surface
geology is outlined in Figure 3.
Table 3.
Western Samoa volcanic formations and their ages
Units
Age
Aopo Volcanics
1905 -1911
Puapua Volcanics
Middle to Late Holocene
Lefaga Volcanics
Early Holocene
Mulifanua Volcanics
Last Glaciation
Salani Volcanics
Penultimate Glacial to Last Inter Glacial
Fagaloa Volcanics
Plocene to Middle Pleistocene
(after Kear and wood, 1959 and adapted from Fepuleai, 1997).
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[18]
3.2.1 Apia Borehole Information
Apia borehole (or drillhole) information was obtained and summarised mainly from the records of Geotechnical
Investigation Drilling of the Meteorology Division, Ministry of Agriculture Forests Fisheries and Metorology. A simplified
borehole summary table is given in Appendix la. The authors obtained the majority of the records from the Apia
Observatory. Most of the borehole logs did not contain detailed geological information, and information provided was
inconsistent (for example penetration resistance was often calculated incorrectly). It was not possible to undertake a
detailed geotechnical analysis and interpretation using the borehole information that was made available. Due to scant
information, development of reliable geological cross -sections within the project area was not possible. Lack of
substantial geological mapping and geotechnical borehole information for Apia remains the largest obstacle in
developing a comprehensive, reliable seismic microzonation map and interpretation of Apia. Approximate borehole
locations are shown in Figure 3.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[19]
Figure 3.
Project Area, Apia, Upolu, Samoa
[SOPAC Preliminary Report 126 - Teakle & Swamy]
Mulinuu Point'
017
Apia Harbour
Inset
LEGEND
Scale: 1:30,000
Au5Or: HAU
300 0
Date: 22/12/2000
(c) SOPAC Regional Data Centre 1998 (FM)
Upolu
Samoa
APIA
Figure 3: Project Area
www.sopac.org.lj
South Pacific
Applied Geoscience
Commission
SORE
wr.a, Western Samoa iw<,., r.
Projection:
Borehole locations estimated from Geotechnical Investigation Drilling
records of the Ministry of Agriculture Forests Fisheries and Metorology,
Apia, Samoa.
WO 0
Geology adapted from B. Richmond, SOPAC Technical Report 90 (1991)
Apia Coastline, Rivers and Contours from SOPAC Regional Data Centre
Data Sources
Fagaloa Volcanics
(Age: Pliocene to Mid -Pleistocene)
Salani Volcanics
(Age: Penultimate Glaciation to
Last Interglacial)
Mulifanua Volcanics
(Age: Last Glaciation)
Recent sediment, colluvium and fill
(Age: Holocene)
Contours (metres above sea level)
Rivers
Coastline and Roads
Borehole Estimated Position
(ID number assigned by SOPAC)
[20]
4. MICROTREMOR DATA ACQUISITION AND ANALYSIS
Microtremor recordings were made at 61 sites in Apia over a period of three weeks during August and September,
1999.
At each site, the ambient noise was recorded continuously for 20 minutes. The ambient noise is a term used to describe
all surface waves generated from any source (eg, human activities, wind and surf, earthquakes, etc) that impact the site
and are detected by the seismometers during the recording period.
During post -processing procedures, site -response spectra are grouped in terms of "resonance ", "possible resonance"
and "no resonance ". To ensure meaningful spectral groupings, consistency of data collection (and subsequent
processing) across Apia was maintained at all times where possible. At each site, one recording of 3 or 4 minutes was
acquired every 10 minutes for a duration of 30 or 40 minutes. The window chosen by the operator was box shaped.
Analysis of microtremor site recordings requires that the spectra be sampled for a prescribed number of windows of
equal size across each component. Four discrete time windows, of 20.48 sec (1024 samples), were sampled across
each component of each microtremor spectra and then averaged. However, this was not done for those sites where the
microtremor recording (seismogram) contained excessive cultural noise, such as continuous unexpected traffic noise,
making it difficult for the operator to choose the prescribed time windows (length and number). In these cases, three
time windows of length 10.24 sec (512 samples) were sampled or 5.12 sec (256 samples) for extreme cases. There
were cases where the frequency of the natural ground resonance was swamped by a dominant frequency such as
those surface waves generated by a nearby factory, power station, steady wood -chopping and in one case there was
an explosion in the vicinity of Mount Vaea. This was recognised on -site during data acquisition and, although the
microtremors were recorded for sake of correlation, they were not used for the purposes of analysis or microzonation.
Averages of the east (x), north (y) and vertical (z) components were used to calculate the value of the Nakamura
spectral ratio. That is, x and y are averaged and then divided by z (as opposed to the average of the ratios).
The microtremor site locations shown in Table 4 are in WSIG1 coordinates; site name and number; UTC time and date
of acquisition. The file names of the raw data, location description and field comments are listed in Appendix 4.
Grouped site response spectra are shown Figure 4a - Figure 4m and are plotted together with the preliminary site response models. Listed in Table 5 are the parameters used for modelling; note that, Models Al - A5 have the same
properties with the only difference being the sediment thickness. This is also the case for the B models. For the purpose
of comparison of the site response models, the A models, B models and C model, are plotted together in Figure 5a, 5b
and 5c respectively. The different sediment thicknesses used in the A models result in different peak frequencies,
however, similar characteristic spectra are retained (see Figure 5a). This is also the case for the B models (see Figure
5b).
WSIG. Western Samoa Integrated Grid, is the local coordinate system, see Appendix lc.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[21]
Table 4.
Sie.
I
Microtremor Site Response Recordings in Apia
UC7
'
lT 11fl
g.:. o
iiy
_..
1
8/23/99
22:56
724360.32
5467252.42
Mt Vaea Radio Phone
2
8/24/99
3:17
724533.69
5467612.28
3
8/24/99
4:16
724856.87
5468653.8
4
8/28/99
3:24
725003.53
5469119.68
Mt Vaea Our Landing
Mt Vaea Mango Peak
Leadin. Licht MtVaea
5
8/26/99
8:16
725317.46
5469873.08
Marist Sports Ground
8
8/26/99
21:42
726571.84
5470155.36
Vaipuna CCCS
10
8/27/99
23:25
724343.71
5470414.06
Savalalo FM Station
11
9/2/99
3:17
724223.82
5471063.23
Mobil Oil
12
8/26/99
5:14
723712.09
5472113.01
Apia Observatory
16
9/3/99
0:37
727485.66
5468434.6
Adjacent Samoa College
17
8/27/99
21:03
725182.56
5469524.58
Mt Vaea Club
18
8/30/99
22:21
724423.88
5469807.18
Fugalei Home
19
8/27/99
22:19
725460.75
5469729.8
20
8/29/99
6:04
726076.7
5469702.36
Tufuiopa Cemetary
AOG Leone
21
8/26/99
11:29
725747.48
5470015.77
John Williams Building
22
8/31/99
0:13
726860.11
5467892.42
Magiagi Primary School
23
8/27/99
0:49
727667.45
5469926.22
Moata'a CCCS
24
8/24/99
23:30
726961.63
5469520.63
Apia Park
25
8/28/99
2:22
728463
5468370.5
26
8/27/99
22:44
725376.13
5468472.42
Royal Samoa Golf Course
Mootootua National Hospital
27
9/2/99
21:08
725681.64
5467745.52
28
9/1/99
23:17
725295.03
5466558.1
Papauta Girls School
Robert Luis Museum
29
9/1/99
22:09
726051.13
5465939.26
Avele College
30
9/2/99
0:40
723676.1
5469261.87
31
8/25/99
4:40
723336.01
5470525.98
St Marys College
Fish Pond
32
8/25/99
3:12
722805.49
5468730.38
Old St Josephs College
33
8/25/99
2:02
722142.34
5467062.31
USP Alafua
34
8/25/99
21:58
721591.43
5469107.86
35
8/26/99
3:36
722249.47
36
8/31/99
4:19
721907.7
5470128.09
5471189.13
Chinese Cemetary
Vailoa CCCS
Vaigaga Primary School
37
9/1/99
22:07
726403.28
5469149.28
Faatoia
38
8/26/99
3:36
721139.49
5466168.17
39
8/26/99
2:12
721144.88
5467685.41
Papaseea Sliding Rock
Tuaefu Tupuas House
40
9/1/99
3:46
722774.89
5468102.95
Lepeka Hotel
41
8/31/99
0:56
727710.66
5467511.05
42
8/25/99
0:35
728893.29
5467778.59
Vaivaseuta
Vailele Rd
43
8/28/99
1:15
728681.26
5469059.2
Fangalii CCCS
45
8/24/99
2:54
726037.19
5470294.95
Pasefika Hotel
46
8/26/99
7:02
724883.25
5470324.39
AC Building
47
8/26/99
20:42
5470990.25
Wharf
48
8/29/99
5:07
725781.73
725923.94
5469973.63
ORUM Catering
49
8/30/99
23:08
723768.1
5468118.73
Sinamoga
50
8/29/99
7:22
724683.47
5469689.19
Taufusi
51
9/1/99
1:49
5469464.94
Vaiusuuta
52
9/2/99
1:43
721326.17
724391.94
5469042.82
Alamaoto CCCS
53
8/27/99
23:50
725750.12
5469136.11
Leififi School
54
8/31/99
2:53
726893.15
5468563.28
Magiagi Cemetary
55
9/9/99
21:52
723366.33
5466582.06
56
9/9/99
22:46
722942.02
5470398.36
Abandoned Quarry
Vailoa Tai
57
9/9/99
23:48
722887.36
5469667.9
Lepea
58
9/10/99
2:04
723483.65
5469677.85
Vaimoso MM
59
9/10/99
2:51
724248.88
5469906.42
Vaimoso House
60
8/30/99
21:08
723170.6
5470115.12
13
9/1/99
23:04
723886.28
5471622.39
Vaitoloa Primary School
Fale Fono
7
8/24/99
22:23
726176.21
5470817.11
Mataututai
6
9/3/99
1:23
725156
5470492.92
15
9/2/99
23:50
726909.6
5466185.59
Government Building Reclaimed
Magaigi Inland
14
9/10/99
3:49
725829.13
5469408.69
Teuila Hotel
-
-
724233.97
5470766.06
Adjacent Tusitala Hotel
61
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[22]
5-
Dl
ege
+11161.-.
Apia Al Model
s
Figure 4a.
Frequency , Hz
s
Apia Al Grouped Site Response Spectra, Resonance Identified
Alamagato CCCS
Vailoa CCCS
Taufusi
Adfaecent
Samoa College
3
Apia A2 Model
5
Figure 4b.
1
Frequency , Hz
S
Apia A2 Grouped Site Response Spectra, Resonance Identified
[SOPAC Preliminary Report 126 - Teakle & Swamy]
igure 4d.
[23]
Figure 4c.
Apia A2 Grouped Site Response Spectra, Possible Resonance Identified
5
/Ai/
2
1
\
P-0r,at3a
C
ORUM Catering
1
1
John William's
1
Building
Abandoned Quarry
ì
3
Apia A3 Model
2'
5
1
Frequency Hz
5
Apia A3 Grouped Site Response Spectra, Resonance Identified
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[24]
.
Varvaseuta
-f
-f-.
S
Figure 4e.
LFfifi School
Mobil Oil
5
1
Frequency , Hz
Apia A4 Grouped Site Response Spectra, Resonance Identified
s
;ra'.e Fern_
2
Government Building
- Reclaimed
Turf opa Cemetery
3-
2
Apia A4 Model
i
Frequency , Hz
Figure 4f.
5
Apia A4 Grouped Site Response Spectra, Possible Resonance Identified
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[25]
'v'ailoa Tai
AOG Leone
Vaimoso MM
Apia A5 Model
aimoso House
Manst Sports Ground
5
Figure 4g.
Frequency , Hz
Apia A5 Grouped Site Response Spectra, Resonance Identified
' 1'
T.
.
F
AC Building
V
Apia A5 Model
o.
5
Figure 4h.
Frequency , Hz
5
Apia A5 Grouped Site Response Spectra, Possible Resonance Identified
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[26]
.
51t
V
.-9:;.
Mt Vaea Mango
v
P.
Apia B1 Model
5
Figure 4i.
1
Frequency , Hz
Apia B1 Grouped Site Response Spectra, Resonance Identified
1 Tuaefu Tupua's House
Apia B2 Model
5
Figure 4j.
i
Frequency , Hz
5
Apia B2 Grouped Site Response Spectra, Resonance Identified
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[27]
:1
Maigagi Cemetery
S-"' `j `_
2
I
I
^
12i..,
t
-
.5
I
i
/
Fatnala
Magagi Inland
Vailele
,
I_eP,,,:.i Hotel
I
l
.
-I_
Royal Samoa Golf
!
Old 5c Joseph's
College
Apia C Model
l
5
Figure 4k.
t
Frequency , Hz
5
Apia C Grouped Site Response Spectra, No Resonance Identified
Vaigaga Primary
School
a
rn
Apia C Model
t
Frequency , Hz
Figure 41.
4''"-'...':'''I:\.\\\\
5
Apia C Grouped Site Response Spectra, No Resonance Identified
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[28]
Mt vaea Club
Al3fua Campus
Papaseea Sliding
Rock
r,ip9_t3 Grlì r;_l_.:
Robert Louis
Museum
Apia C Model
Frequency , Hz
Figure 4m.
5
Apia Grouped Site Response Spectra, Possible Resonance Identified, No Definite Signature
Figure 5.
SITE RESPONSE MODELS, APIA
Model A5
Model A3
Model AI
Model A2
5
Figure 5a.
1
Frequency , Hz
Apia Site Response "A" Models (Analytical Response Functions).
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[29]
Figure 5b.
Apia Site Response "B" Models (Analytical Response Functions)
Figure Sc.
Apia Site Response "C" Model (Analytical Response Functions)
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[30]
Definition of Site Response Models in Apia
Table 5.
Model
Layer
Apia Al
Apia A2
Apia A3
Apia A4
Apia A5
Apia B1
Apia B2
Apia C
Thickness
Shear* Velocity
(m)
Density
(9 cmr')
Geotechnical Description
15
(m s -i)
300
1,9
Holocene Alluvium, sands. gravel. some soils
2
Half Space
900
3.0
Weathered Basalt
1
30
300
1.9
Holocene Alluvium, sands gravel, some soils
2
Half Space
900
3.0
Weathered Basalt
1
35
300
1.9
Holocene Alluvium, sands, gravel, some soils
2
Half Space
900
3.0
Weathered Basalt
1
40
300
1.9
Holocene Alluvium, sands, gravel. some soils
2
Half Space
900
3.0
Weathered Basalt
1
25
300
1.9
Holocene Alluvium, sands, gravel, corals some soils
2
Half Space
900
3.0
Weathered Basalt
1
45
500
2.2
Weathered Volcanics - Soil
2
Half Space
900
3.0
Slightly Weathered Basalt Rock
1
45
500
2.2
Highly Weathered Basalt - Soil
2
30
900
3.0
Slightly weathered Basalt
3
Half Space
2000
3.1
Basalt Bedrock
1
10
500
2.2
Highly Weathered Basalt
2
Half Space
900
3
Slightly Weathered Basalt
1
*Note, empirical measurements of shear wave velocities of rocks in Apia have not been carried out, hence shear wave velocities have been estimated from like rock
types classified by Borcherdt (1994) and Crouse and McGuire (1996).
Location of microtremor sites in Apia were chosen strategically so as to characterise the downtown area (highest socioeconomic risk area), and to distinguish the areas between soft sediments (alluvial plains, marine sediments, reclaimed
and fill areas) and rock.
In Apia, site response spectra that display a characteristic resonant peak, generally have amplitudes that fall off at low
frequencies above the peak. In the cases where the signature of the spectral amplitude was not consistent with the
latter, they have generally been referred to as having no response or "not resonant ". However, in ambiguous cases
where the response spectra exhibits high amplitude but does not fall to low values at high frequencies, the sites may be
interpreted to be "resonant" or "possibly resonant ". In the case of the latter, ambiguities of classification may arise and
so some reservations must be made during interpretation. In addition, those sites that have spectra indicating a low
response are also referred to as "possibly resonant ". It is difficult to comment on the "possibly resonant" sites without
further investigation, except to emphasise that the method strictly applies only to sites where Rayleigh waves or
mixtures of s -waves and p -waves propagate in a soft layer overlying a stiff basement (Singh 1998). Further
investigations of such sites could include a detailed microtremor survey and analysis of that site, together with a
standard penetration test or SPT2. At some sites in Apia, the peaks are broad, and their frequencies are consistent with
possible resonance in quite shallow soil layers; these cases are likely to be of little importance in determining damage
to structures. No significance should be attached to the sizes of the spectral ratios obtained from the sites classified as
non -resonant.
z Whitlow (1995) defined the SPT, standard penetration test, as a dynamic test carried out in boreholes during site investigations. He stated that "a split -barrel
sampler is driven into the soil at the bottom of a hole using a standard hammer. After an initial drive of 150mm, the number of blows required to drive the sampler a
further 300mm is recorded. This number of blows is referred to as the standard penetration resistance or N- value" (Whitlow, 1995, p474). The resistance to
penetration of a "conical point offered by the soils at any particular depth...creates a complex shear failure and thus provides an indirect measure of the in -situ shear
strength of the soil" (Bell, 1993, p215).
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[31]
The site -response models were chosen using limited information. However, to aid as model controls or boundary
conditions, the information from several boreholes proved useful. For example, borehole number AB013 (see borehole
information in Appendix 1 a and Figure 3 for locality) located at the ACB site had some basic geological descriptions,
depth information and the N- values from SPT tests. The material in the first 4 m is described as being loose, silty,
coarse terrigenous sands containing large pieces of coral. The sand are loose with low N- values of 6 -8. From 4 m to
23 m, the N- values range from 30 to over 50 indicating dense - very dense gravels consisting of broken corals with
some coarse terrigenous sands, pebbles and shell fragments. This is consistent with a back -reef lagoon environment
with interfingering coral heads. From 23 m to 30 m the drilling is reported to be very hard and samples contain
indurated basalt, with N- values of 50 or more3. The model fitted for this site is Apia A5, and although there are no in -situ
geotechnical or s -wave velocity measurements, velocities have been estimated from known values in like materials
(i.e., from classifications defined in Borcherdt, 1994). The site -response recording at the AC Building (Figure 4h) has
relatively low amplitude, but the spectral response drops off at high frequencies, consistent with possible resonance.
The model adopts a much higher peak in amplitude to match other sites like Vaimoso MM, Vailoa Tai and Fungalei
Home (Figure 4g). However, with respect to the ACB site, this could indicate that there is not such a high contrast
between the s -wave velocity of the top layer(s) and the basement (the basalt), producing a lower amplitude response.
The top 2.8 m of the core from borehole AB016, located on Mount Vaea (see Figure 3) contained loose topsoil, while
the following 3 m contained solid basalt'. There was weathered basalt from 6 m -12 m, and the remainder of the core
contained solid basalt, considered to be basement. The model fitted the spectral response of the nearby microtremor
Site 4 (Leading Light), was Apia B1 (Figure 4i). The spectral response resembles sites with high resonance (see model
Apia Al and Site 30, St Mary's College in Figure 4a) since the spectral ratio plot (Figure 4i) contains a classic signature
of the response falling to low values at high frequencies.
Normally it is not expected that a site with such a thin layer of topsoil followed by rock, would have such a resonant
signature as detected at Site 4 (near borehole AB016). There may be cases when a borehole may not have penetrated
through to true basement, especially when solid basalt is detected at very shallow depths followed by weathered basalt.
Stephenson and Baguley (1996) had noticed in New Zealand, that the resonance detected in such cases could be real
and perhaps result from the presence of underlying topographic features (e.g., the ridge -like Mt Vaea). Further
investigations (geological and geotechnical) must be undertaken in Apia to clarify such cases.
True resonance characteristics (narrow peak with low ratio values at high frequencies) are represented in
approximately 50 % of the spectral ratios plotted in Figure 4 (i.e., 50 % of the sites surveyed). They exhibit site
responses with considerable amplification and resonance at frequencies of 1 to 5 Hz.
Summarised in Table 6 are:
1.
characteristic resonant frequencies of the spectral ratio plots that can be grouped together due to like- response
(see figures 4a - 4m);
2.
applicable models, meaning those models that fit a characteristic group of responses with only slight variations
(see figures 5a - 5c); and
3.
assigned map (or hazard) zones that encompass each one of those site groupings.
3 This information was obtained from the records of Geotechnical Investigation Drilling of the Meteorology Division, Ministry of Agriculture Forests Fisheries and
Metorology in Apia, see Appendix la for further information.
° See footnote 3.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
511141C
Correnkdon
Soul, Faulk
Applied Geacince
lowaopicorg4
2hdareeopmerp$
P.ojeciion:
Da1e:11212001
ScAlc1:30,000
Athori-JAL
Figure 6: Preliminary Seismic
Microzonation Map
APIA
Upo lu
Samoa
(E)SOPAC Rozkznoil Dap Centre 1098 (FM)
5.170030mN
5A03,001141
720edernE
watoTAirsHouse
VAlgpAPIAAAry$chtza
T22.000mE
_VersIntbert
ss
ffinamAq.
AtaAAAAAAA2a,
72.4.21X1rnE
2
PAN
rL.M.rg
IIdInØ
ne.odome
-
AwleGollopor
22
Primsrea. had
72G.000mE.
Legend
MiCrOtrernor Recording Sde
Poilimrnary Seismic Microzone A
Preiimmary Seismoo Microzone
Prelirrinary Seismic Mrcronne C
- Coasiline and roads
Mai)
E.O.M. Imo MAC N..ISD ia Lorin.
Contours [MODES ElbUY/1
- !beers
Itstett
1.11. &maw.. M.N.
Va11.411A
AZ
.Moren. soma Ificoma.D.r.m. wadmc
J.
731:1.000mE
[32]
Table 6.
Frequency Characteristics of Seismic Microzones and Applicable Models in Apia
Map zonas .. Apia
irrequoct )
ar *atilt #iC
Applkable l4fodels
A
2- 5 Hz
A1, A2, A3, A4, A5
B
3-4 Hz
B1, B2
C
No response
C
The estimated amplification; characteristic peak frequency of each site spectral ratio; resonant (r), possible resonant
(p), non resonant (n); the preliminary assigned site response model name and the preliminary microzone name are
listed in Table 7. Note the amplification factor given is approximate and as mentioned earlier should be determined
using methods other than the Nakamura method.
[SOPAC Preliminary Report 126- Teakle & Swamy]
[33]
Table 7.
Microtremor Site Response Recordings in Apia
OBSERVED
ESTIMA7Etk BYMO BUNG
1
2.9
5
B2
B
Mt Vaea Radio Phone
2
3
2.4
B1
B
Mt Vaea Our Landing
3
3
3.8
B1
B
4
4.2
4.2
B1
B
Mt Vaea Mango Peak
Leading Light MtVaea
3.2
5
A5
A
Marist Sports Ground
3.8
3.5
p
A5
A
Vaipuna CCCS
10
3
1.2
p
A2
A
Savalalo FM Station
11
2.3
1.2
A4
A
Mobil Oil
12
2.5
3.8
A2
A
Apia Observatory
16
3
2
A2
A
17
0
1
18
3.7
5
Adjacent Samoa College
Mt Vaea Club
Fugalei Home
19
1.7
1.7
20
3
3.5
21
2.6
1.3
22
0
23
2.7
1.5
24
3.2
1.5
25
0
1
n
26
0
1
n
27
0
1
p
28
0
1
p
29
1.2
30
5.3
31
4.7
2.8
32
0
1
n
33
0
1
p
p
8
1
p
C
p
p
n
p
A5
A
A4
A
AS
A
A3
A
C
C
Tufuiopa Cemetary
AOG Leone
John Williams Building
Ma iagi Primary School
A3
A
Moata'a CCCS
AS
A
Apia Park
Royal Samoa Golf Course
C
Mootootua National Hospital
C
C
C
C
Papauta Girls School
Robert Luis Museum
2.5
A4
A
4.5
Al
Al
A
Avele College
St Marys College
A
Fish Pond
C
C
34
0
1
35
3.4
5
36
0
1
37
n
C
C
A2
A
Old St Josephs College
USP Alafua
Chinese Cemetary
Vailoa CCCS
C
C
Vaigaga Primary School
C
Faatoia
C
C
Papaseea Sliding Rock
B2
B
C
C
Tuaefu Tupuas House
Lepeka Hotel
A4
A
Vaivaseuta
C
Vailele Rd
Fangalii CCCS
Pasefika Hotel
C
n
38
0
1
39
3.8
4.1
40
0
1
41
2.2
3.2
42
0
1
43
2.5
2.5
A2
A
45
2
3
A4
A
A5
A
A4
A
ACB Building
Wharf
A3
A
ORUM Catering
C
C
Sinamoga
A2
A
Tauf usi
C
C
Vaiusuuta
P
n
n
46
3
1.7
47
2.4
2.5
48
2.5
3.5
49
0
1
50
2.6
2.5
51
0
1
52
4.6
4
A2
A
Alamagoto CCCS
53
1.7
2.2
A4
C
Leififi School
54
0
1
C
C
Magiagi Cemetary
55
2.7
2.2
A3
A
56
3
2
A5
A
Abandoned Quarry
Vailoa Tai
57
4
2.4
Al
A
Lepea
58
3.2
6
A5
A
Vaimoso MM
59
3
6
AS
A
Vaimoso House
60
0
1
n
C
C
13
2.3
1.3
p
A4
A
Vaitoloa Primary School
Fale Fono
7
3
1.8
p
A5
A
Mataututai
6
2.2
1.5
p
A4
A
Government Building Reclaimed
15
0
1
n
C
C
Magaigi Inland
14
3.2
2.2
A2
A
Teuila Hotel
61
2.3
1.2
A4
A
Adjacent Tusitala Hotel
p
p
n
n
p
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[34]
The microtremor site locations are plotted on Figure 6. The majority of the sites that display resonance characteristics
are located in the coastal downtown area and also where alluvial /marine deposits have been mapped. The sites that
contain some of the most prominent resonance characteristics include: Vailoa CCCS (Figure 4b); Mobil Oil (Figure 4e);
Vaimoso MM (Figure 4g); Fugalei Home (Figure 4g); Vaimoso House (Figure 4g); Marist Sports Ground (Figure 4g);
AOG Leone (Figure 4g); Vailoa Tai (Figure 4g); Fagalii CCCS (Figure 4b); Taufusi (Figure 4b); Alamagato CCCS
(Figure 4b); St mary's College (Figure 4a); Mt Vaea Mango Peak (Figure 4i); Mt Vaea Our Landing (Figure 4i); Leading
Light Mt Vaea (Figure 4i); Mt Vaea Radio Phone (Figure 4j) and Tuaefu Tupua's House (Figure 4j).
Other sites displaying more moderate resonance characteristics include: ORUM catering (Figure 4d); John Williams
Building (Figure 4d); Vaivaseuta (Figure 4e); Pasefika Hotel (Figure 4e); Lepea (Figure 4a); Fish Pond (Figure 4a);
Adjacent Samoa College (Figure 4b); Teuila Hotel (Figure 4b); Moataa CCCS (Figure 4d); Abandoned Quarry (Figure
4d); Lefifi School (Figure 4e) and the Wharf (Figure 4e).
The spectra analysed from the sites on Mt Vaea display characteristics of strong resonance. Initially it was expected
that those sites would represent bedrock. However, it is possible that Mt Vaea is covered with some highly weathered
material for the first 30 to 45 metres (Aleni Fepuleai, pers. comm.). Further investigations of the Mt Vaea area are highly
recommended especially where development is anticipated. The microtremor recordings from downtown and along the
coastal area, however, produced spectra very much as expected. The depths used in the models ranged from 5 to 45
metres, in loose, alluvial sediments and reworked alluvium and carbonate deposits in the marine environments.
The delineation of the preliminary seismic microzones in Apia (Figure 6) are based on a combination of:
1.
regions where microtremor site analyses produced similar, prominent spectral ratio responses,
2.
models that match known or expected surface and subsurface geology,
3.
topography.
The general geology of the preliminary seismic microzones developed is:
Zone A
Holocene terrigenous sands and gravels, marine carbonates, and colluvium fill 15 m to 40 m thick,
overlying weathered and slightly weathered Fagaloa and Salani basalt.
Zone B
Thick soil derived from highly weathered volcanics, 30 m to 45 m thick, overlying a prominent bedrock
ridge of slightly weathered Fagaloa basalt.
Zone C
Flows of weathered ( <10 m) or exposed Fagaloa, Salani and Mulifanua basalt bedrock.
Resonance has been observed both in Zone A due to a variable thickness of weak sediments overlying bedrock, and in
zone B due to a thick mantle of weathered soils overlying a prominent basement -ridge feature in Mt. Vaea.
Non -resonant conditions exist where only their sediment cover or weathered mantle overlies basement basalts with
subdued topography.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[36]
5. SUMMARY, DISCUSSION AND FURTHER WORK
5.1 Summary and Discussion
In Apia seismic microtremor recordings were undertaken in a repeatable systematic fashion so as to minimise operator
bias and maximise reproducibility. Microtremor recordings, processing and analysis were achieved by application of the
Nakamura Site Response Determination (SRD) method using the GII -SRD software. The analytical response functions
(models) were determined by using the Joyner code that was incorporated within the GII -SRD software.
Greater than 50 % of the sites within Apia had spectral ratio plots with narrow peaks together with low -ratio values at
higher frequencies above the peak. These sites have been classified as showing resonance, with resonant frequencies
ranging from 1 Hz to 5 Hz. The environs of Apia include weathered basalt; lava domes (on the south -east side of Apia);
basalt flows of different ages; and carbonate reef material along the coast. Depending on their age and history, some
basalt flows are highly weathered, some are only partially weathered and some remain as fresh hard rock. It is also
evident that there has been faulting and uplift as well as massive landslides. It appears that for these reasons the
resonant responses are more variable across Apia than was initially anticipated.
Apia downtown area is characterised by alluvial sediment and reef deposits, and contains some of the highest resonant
responses assessed in the survey. The city sites of particular concern are those from the Vailoa; Alamagato; Vaimoso
(Fugulei); Fagali'I; Vaisigano river flats (Vaipuna, Fa'atoia etc); and Mulinu'u Point, especially the Mobil Oil site. Also, as
expected, sites located near riverbanks or old river channels also had resonant responses, especially ORUM catering
(Vaisigano river flats) and Tuaefu Tupua's house (Fulu'asou river flats). Sites located on Mt Vaea also had resonant
responses, which may have resulted from the presence of deeply weathered basalt and /or resonance effects resulting
from the ridge -like structure of Mt Vaea itself. Mt Vaea needs to be thoroughly investigated so as to determine the origin
of resonance.
Fill sites across the city (Government Buildings, Wharf etc) showed varied responses. The wharf had a particularly
amplified peak, but did not represent a classic resonant response (ie, a narrow peak with low values at high frequencies
above the peak). No resonance was detected at the Government Buildings site. Both of these sites and any other
infilled sites should be investigated further, since fill often shows anomalous spectral characteristics.
Sites located inland and not on the alluvial plain, or river flats, were generally not resonant. These included Saina and
Vaigaga (where fresh basalt rock was seen outcropping); Vailele; and Magiagi. Sites classified as not resonant, but that
were possibly resonant, lie at different localities across the city including Lalovaea, Papase'ea, Papauta, and Vailima
areas. If development is to take place then these sites should be thoroughly investigated.
The response spectra of microtremor sites in Apia were grouped according to their resonance characteristics: that is,
resonant, possible resonant and non resonant; and their preliminary response model (analytical response function). A
total of three main groups were identified, and then based on these groups, a preliminary microzonation map of Apia
was produced (Figure 6). Zone A mostly encompasses the downtown area and alluvial plain; Zone B corresponds to
the Mt Vaea weathered basalt -ridge area; and Zone C corresponds to the subdued topography developed on thinly weathered basalt flows lying east and west of the city. In general, sites in Zone A show strong resonance, although
some are only possibly resonant. Zone B sites are resonant, but are characterised by a different geology from Zone A.
In Zone C most sites are not resonant or, where ambiguous, have been classified as possibly resonant.
The relative level of amplification due to soft soils during earthquake shaking can be broadly interpreted from the
preliminary microzonation map of Apia. Zone A can be seen as having the highest amplification potential; Zone B,
medium; and Zone C, the lowest. Nevertheless, seismic microzonation of Apia cannot be finalised until further
geological, geotechnical, and s -wave velocity investigations have been undertaken.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[37]
5.2 Further Work
To complete the objectives of producing a reliable seismic microzonation map of Apia the following work is
recommended:
1.
Undertake detailed surface and subsurface geological and soils mapping of Apia city by using remote sensing
methods and /or 1999 aerial photography, and confirm with geological ground truthing.
2.
Acquire Apia city aerial photographs from Airesearch Mapping Pty. Ltd (flown in 1999) and Government of Samoa.
3.
Undertake detailed geological borehole investigations at representative sites across Apia using boreholes that are
deep enough to reach bedrock.
4.
Re- calculate the analytical response functions of the microtremor spectral ratios by applying the appropriate
constraints, ie., geotechnical parameters should be determined on -site.
5.
Characterise the area to be zoned by obtaining shear wave velocity profiles, eg, by the Seismic Cone Penetration
test (SCPT) method.
6.
Use earthquake ratios recorded at representative sites in each city zone to provide a control on the frequency
information and provide better estimates of site amplification.
7.
Determine of the natural building responses of characteristic buildings in each city zone, either through SDA and
SRD or another method.
8.
Obtain a reliable compilation of historic earthquakes for the region surrounding the city so as to achieve the degree
of input required to determine the acceleration response of soft soils (such as the SvE method) for each city zone
in Apia.
9.
Determine the acceleration response of each zone in Apia by using the SvE method (of Shapira and Van Eck
1993) and undertake a comprehensive earthquake shaking risk analysis of the city area.
10. Undertake detailed site investigations in all seismic microzones in Apia prior to any planned developments.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[38]
6. REFERENCES
Bell, F.G., 1993. Engineering Geology. Blackwell Scientific Publications, Oxford.
Borcherdt, R. D., 1994. Estimates of Site- Dependent Response Spectra for Design (Methodology and Justification). Earthquake
Spectra, 10(4): 617 -653.
Cooper, P., and Kroenke, L.W., 1993. Deep Seismicity in the North Fiji Basin. In Kroenke L.W., and Eade, J.V., Editors, 1993,
Basin Formation, Ridge Crest Processes, and Metallogenesis in the North Fiji Basin. Houston, Texas, Circum- Pacific Council
for Energy and Mineral Resources, Earth Science Series, Vol. 15, Springer -Verlag, New York.
Computerboards, 1994. Instacal Version 5.02. Analogue to Digital Converter Calibration Software, by Computerboards, Inc.,
Middleboro, USA.
Craig R.F., 1987. Soil Mechanics, Fourth edition. Van Nostrand Reinhold (International), London.
Crouse, C.B., and McGuire, J.W., 1996. Site Response Studies for Purpose of Revising NEHRP Seismic Provisions. Earthquake
Spectra, 12(3): 407 -439.
Berkman, D.A., and Ryall, W.R., (Eds) 1976. Field Geologists' Manual, Monograph Series 9. The Australian Institute of Mining and
Metallurgy, Parkville, Australia.
Department of Statistics, 1991. Report of the Census of Population and Housing 1991. General Report of the Census of Population
and Housing. Department of Statistics, Government of Western Samoa, Apia, Western Samoa.
Fepuleai, A., 1997. Petrology of the Fagaloa Formation, Northeast Upolu Island, Western Samoa. Masters Thesis, Department of
Geology, University of Auckland.
Joyner, W.B., 1977. A FORTRAN Program for Calculating Nonlinear Seismic Response. USGS Open -File Report, 77 -671
(Unpublished).
Lermo, J., and Chávez- Garciá, F.J., 1994. Are Microtremors Useful in Site Response Evaluation? Bulletin of the Seismological
Society of America, 84(5): 1350 -1364.
Hamburger, M.W., and 'sacks, B.L., 1993. Shallow Seismicity in the North Fiji Basin. In Kroenke, L.W., and Eade, J.V., Editors,
1993, Basin Formation, Ridge Crest Processes, and Metallogenesis in the North Fiji Basin. Houston, Texas, Circum- Pacific
Council for Energy and Mineral Resources, Earth Science Series, Vol. 15, Springer -Verlag, New York.
Hedge, C.E., Peterman, Z.E., and Dickinson, W.R., 1972. Petrogenesis of lavas from Western Samoa. Geological Society of
America Bulletin, 83: 2709 -2714.
Kear, D., and Wood, B.L., 1959. The Geology and Hydrology of Western Samoa. New Zealand Geological Survey Bulletin, n.s. 63,
92 pages.
Nakamura, Y., 1989. A Method for Dynamic Characteristics Estimation of Subsurface using Microtremor on the Ground Surface.
Quarterly Report of Japan Railways Technical Research Institute, 30(1): 25 -33.
NEIC, 2000. National Earthquake Information Centre of the United States Geological Survey. http: / /wwwneic.cr.usgs.gov.
-
Nzam, 1990. Aerial Set Id: Sn 7819, Photo Nos. 11/159 - 13/157 Run F; 8/211 - 9/210 Run N; 10/173 - 13/170 Run H; 13/184
14/183 Run L; and 17/244 - 19/242 Run J. Western Samoa, Upolu, Apia. Scale 1:6000; Altitude 7,000 Ft. New Zealand Aerial
Mapping. Post Cyclone Ofa (February 1990).
Saldevar, J.H.,1994. Apia Urban. Map Drawn by Joelo H. Saldevar (Unv) for Apia Urban Youth Survey 1994. Department Of
Statistics, Government of Western Samoa, Apia, Western Samoa.
Shorten, G., Shapira, A., Teakle, G., Regnier, M., Biukoto, L., Swamy, M., and Vuetibau, L., 1999. Site Specific Earthquake Hazard
Determinations in Capital Cities in the South Pacific. SOPAC Technical Report 300.
Shapira, A., and Hofstetter, A., 1993. Source Parameters and Scaling Relationships of Earthquakes in Israel. Tectonophysics, 217:
217 -226.
Shapira, A., and Avirav, T., 1996. PC -SDA Operation Manual, Version 2.2. IPRG Document Z1/567/79 (110c).
Shapira, A., and Van Eck, T., 1993. Synthetic Uniform -Hazard Site Specific Response Spectrum. Natural Hazards, 8: 201 -215.
Shapira, A., 1999. Seismic Microzoning in Capital Cities in the South Pacific. The Geophysical Institute of Israel, Final Report,
Submitted to the US Agency for International Cooperation - CDR Program, USAID Grant No.: TA- MOU- 95 -C13 -024.
Singh, A., Stephenson, B., and Hull, A., 1998. Assessment for Amplification of Earthquake Shaking by Soft Soils in Suva. Mineral
Resources Department Report 71, Suva.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[39]
Stephenson, W.R., and Baguley, D.E., 1996. Assessment for Amplification of Earthquake Shaking by Soft Soils in Wanganui.
Institute of Geological and Nuclear Sciences Client Report 1996/43662b.10, Prepared for Wanganui District Council
(Confidential).
Stephenson, W.R., Barker, P.R. and Mew, G., 1990. Report on Resonant Alluvium Conditions for Part of Porirua Basin, (Part 3 Of
1989 Study), Wellington Regional Council, Regional Natural Disaster Reduction Plan - Seismic Hazard. Contract 9a9101
Between Division of Land and Soil Sciences (Contract 90/5) and Wellington Regional Council. Division of Land and Soil
Sciences, Department of Scientific and Industrial Research, 1990.
Whitlow, R., 1995. Basic Soil Mechanics, Third Edition. Longman Scientific and Technical, Essex, England.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[40]
APPENDIX 1A
Data Sources - Borehole Information
Borehole information was obtained and summarised (see table below) mainly from the records of Geotechnical
Investigation Drilling of the Meteorology Division, Ministry of Agriculture Forests Fisheries and Meteorology, PO Box
3020, Apia, Samoa. The authors obtained the majority of the records from the Apia Observatory. Most of the borehole
logs did not contain detailed geological information, and information provided was inconsistent (for example penetration
resistance was often calculated incorrectly). It was not possible to undertake a detailed geotechnical analysis and
interpretation using the borehole information that was made available. Due to scant information, development of reliable
geological cross -sections within the project area was not possible. The fields left blank either have no known
information or the information obtained was too ambiguous for interpretation. The "New Drillhole Designation" is a
number assigned by SOPAC for ease of reference, and does not necessarily reflect the numbering system obtained
from the individual borehole records of Geotechnical Investigation Drilling in the Meteorological Division.
Summary of Borehole Information
Name
New Drillhole
Designation
Original Drillhole Date
Designation
Completed
Lelata Bridge
ABOO1
BH1
7/29/98
ABOO2
BH2
7/29/98
AB003
BH1
ABOO4
BH3
11
ABOO5
BH4
6.5
ABOO6
BH5
9
ABOO7
BH1
10
ABOO8
BH2
7
2
ABOO9
BH3
8
0.7
AB010
BH1
26
AB011
BH3
32
30
AB012
BH5
31
28
AB013
BH7
30
23
AB014
BH9
15
AB015
Leone Bridge
Tuaefu Bridge
ACB
Total Length of
Drillhole (m)
Depth to
Bedrock
Surface (m)
5
3.5
9
3.45
10.5
7.5
BH10
30
25
Catholic Church AB016
BH1
15
2.8
Mobil Oil
AB017
BH1
6
AB018
BH2
6
AB019
BH3
6
Alafua Campus AB020
BH1
4.5
3.4
Vaitele Uta
AB021
BH1
4.45
2
AB022
BH2
3
2
AB023
BH3
2.5
AB024
BH4
5
McDonalds Bldg AB025
ACB2
BH1
3/30/81
4.8
AB026
BH2
3/31/81
4.6
AB027
BH3
4/1/81
4
AB028
BH4
4/2/81
3.8
AB029
BH1
4/17/99
13
AB030
BH3
4/21/99
13
[SOPAC Preliminary Report 126 - Teakle & Swamy]
1
[41]
APPENDIX 1B
Data Sources - GIS Information
The secondary (available) GIS Data used for Apia mapping in this study was obtained from the SOPAC Regional Data Centre. For more
information contact the SOPAC Information Technology Unit. SOPAC's website address is http: / /www.sopac.org.fj
GIS data of Western Samoa in Mapinfo format has been provided by the Samoan Department of Lands, Surveys and Environment (DLSE) in
Mapinfo format. The original data is from the Samoan Department of Agriculture, which was produced in Arclnfo format under an ADB funded
project. The conversion was carried out in Australia and DLSE have added layers such as Met stations, Land Parcels and Control Points.
There are several files corresponding to the Mapinfo file sets which include those with extension DAT, ID, MAP, TAB while there may be an
optional text file (DOC) which describes the attributes. The extent of the microzonation project area included Apia city and environs and the data
was extracted from the following list of data.
FILE NAME
DESCRIPTION
SOURCE
CONTROL.TAB
Control points
SOPAC data centre
U_COAST.TAB
Upolu Coastline
SOPAC data centre
U_CONT.TAB
Upolu Contours
SOPAC data centre
U_NAMES.TAB
Upolu Names
SOPAC data centre
U_PARCEL.TAB
Upolu Parcels
SOPAC data centre
U_RIVER.TAB
Upolu Rivers
SOPAC data centre
U_ROAD.TAB
Upolu Roads
SOPAC data centre
SOURCE
FILE NAME
DESCRIPTION
Aconto_prj.TAB
Apia Contours
SOPAC data centre
Arivers_prj.TAB
Apia Rivers
SOPAC data centre
AZone1 -5.TAB
Apia Microzone Zone 1
SOPAC HAU
AZone2 -2.TAB
Apia Microzone Zone 2
SOPAC HAU
AZone3 -2.TAB
Apia Microzone Zone 3
SOPAC HAU
Azonlg_zonelegend.TAB
Apia Microzonation Map Legend SOPAC HAU
Census_coast_2.TAB
Apia Coastline
Census_roads.TAB
Apia Roads
Digitised and corrected from coastline
map "Apia Urban" (Saldevar, 1994), with
permission from the Department of
Statistics, Government of Samoa.
MicrozonationSites2_prj.TAB
Digitised and corrected from roads map
"Apia Urban" (Saldevar, 1994), with
permission from the Department of
Statistics, Government of Samoa.
Apia Microzonation Site Location SOPAC HAU
ABposition3.TAB
Apia Borehole Positions
SOPAC HAU
AGgcpWSIG.TAB
Apia Control Points
SOPAC HAU
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[42]
APPENDIX 1C
Data Sources - Projection and Coordinate System
The old mapping series was based on UTM zone 2 and found to have errors of the order of 800 m. The new maps are based on the Western
Samoa Integrated Grid co- ordinate system and this requires an entry in MAPINFOW.PRJ file.
MAPINFOW.PRJ as provided by DLSE has following additions:
" - -- Western Samoa - - -"
"UTM Zone, Western Samoa (WGS 72) ", 8, 103, 7, -171, 0, 0.9996, 500000, 10000000
"WSIG ", 8, 103, 7, -172, 0, 1.0000, 700000, 7000000
The parameters for WSIG and UTM Zone 2 South are listed in the following table:
WSIG
UTM Zone 2 South
Datum
WGS72
WGS72
Projection
Transverse Mercator
First Eccentricity Squared
Semi Major Axis
0.006694317778 or
Central Meridian
Central scale factor
Origin
False Origin
Zone Width
172 °W
172 °W
1.000000
0.9996
0° 172 °W
700 000 mE 7 000 000 nN
0° 171 °W
500 000 mE 10 000 000 nN
2°
10°
Transverse Mercator
298.26
6378135
0.006694317778 or
6378135
[SOPAC Preliminary Report 126 - Teakle & Swamy]
298.26
[43]
APPENDIX 2
Equipment
Listed in the table below is the equipment used in Apia Microtremor Survey. Note, the components written in italics are the items purchased for
the seismic acquisition system upgrade. Note also that redundant parts were also taken (ie, 2 sets of seismometers. cables and blue boxes) in
case of equipment failure.
MICROZONATION ACQUISITION EQUIPMENT
SOURCE
Seismometer Mark Products
2 x Horizontal No 1109 and No 1108
1 x Vertical No 9427
Seismometer Mark Products
2 x Horizontal No 945 and No 941
1 x Vertical No 939
Cable
1 x Seismometer cable to amplifier -filter box
1 x Acquisition cable, PC to amplifier -filter box
2 x Amplifier -filter box
6 x Amplifiers Teledyne Geotech
(3 mounted in each box)
1 x Calibration Card (mounted inside amplifier -filter box)
1 x Power cable connected to amplifier -filter box and other
end with batter clips to 12V battery
Suva (MRD)
Vanuatu (Geology and Mines)
Vanuatu (Geology and Mines)
Vanuatu (Geology and Mines)
Vanuatu (Geology and Mines)
Vanuatu (Geology and Mines)
Suva (MRD)
Vanuatu (Geology and Mines)
Vanuatu (Geology and Mines) adapted by HAU
1 x A/D converter
Computer Boards,
PCM -DAS 16S/16 A/D Converter
1 x PCM- C37/33 Connector Cable
SOPAC HAU
Purchased from Novatech Controls (Aust) Pty Ltd
AU$995
1 x CIO- Mini37 Terminal Board
SOPAC HAU
Purchased from Novatech Controls (Aust) Pty Ltd
AU$125
lx Invertec Sinus Sine Wave Inverter,
S/N IS1200500N0310
x Autohelm Compass, T019
SOPAC Lab
TOOL BOX AND OTHER BACKUP EQUIPMENT
SOURCE
1 x 1 Amp Battery Charger, P/N 87001
1 x 10V Transformer, Mod PK -03
1 x 12V Battery Bag carrier
SOPAC Lab
2 x levels
Sensor alignment set square
1 x Tool Box + contents
1 x Multimeter
1 x Soldering Iron
1 x bush knife
1 x torch
1 x tape measure
2 x screw driver set (medium and small)
1 x wire cutter
1 x large wrench
5 x 10m extra wiring
1 x 2m solder wire
1 x large battery clips
2 x small battery clips
1 x bottle 2 -26 electrical parts cleaner
4 x fuse for amplifier -filter box
SOPAC HAU
1 x CD Gil Seismic software
1 x A/D card software
1 x Compaq Armada 1750 Notebook
1 x mouse
SOPAC ITU
1 x long 240V power extension cord
SOPAC ITU
SOPAC HAU
Purchased from Novatech Controls (Aust) Pty Ltd
AU$65
SOPAC Lab
1
SOPAC Lab
SOPAC Lab
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
Apia Observatory
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC HAU
SOPAC Lab
SOPAC HAU
SOPAC HAU
SOPAC HAU
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[44]
1 X MULTI -ADAPTER POWER BOARD
SOPAC ITU
5 x 10 -box 3 1% inch floppy disks
SOPAC HAU
1 x 12V 50Amp /hr Battery
Located in engine of in- country field vehicle
GPS EQUIPMENT
6 x Geo Explorer II
plus accessories
SN: 00100043KO, 0010003WMY, 00100043JW,
00100043J P, 0010003WMF
2 x 4600LS Antennae
SN: 0220112136, 0220112137
2 x Tripods
SOURCE
SOPAC HAU
SOPAC HAU
SOPAC HAU
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[45]
APPENDIX 3
Catalogue of Large Shallow Earthquakes around Apia, 1970 - 2000
For more information on particulars of the Earthquake Database comprising this catalogue see web -site
http: / /wwwneic.cr.usgs.gov /neis /epic /database.html (NEIC, 2000).
Circle Search: Earthquakes for Apia
File Created: Thu May 11 15:57:10 2000 & Thu May 11 16:14:36 2000
Catalogue Used: PDE (Preliminary Determination of Epicenters) and NOAA (National
Oceanographic and Atmospheric Administration)
Data Selection: Historical & Preliminary Data & Significant Earthquakes World Wide (NOAA)
Circle Centre Point:
Radius:
Magnitude Range:
Depth Range:
Date
Origin Time
Latitude
Latitude:
Longitude:
250 km
5.0 -9.0
0 -70 km
Longitude
(UTC)
13.815S
171.781W
Longitude
(E)
Depth
(km)
Magnitude
Solution*
Recorder
Epicentral
Distance
from Apia
(km)
26- Jun -17
054900.00
-15.50
-173.00
187.00
25
8.7
Ms
NOAA
227
14- Apr -57
191800.00
-15.50
-173.00
187.00
60
7.6
Ms
NOAA
227
16- Feb -73
045101.40
-15.31
-173.32
186.68
50
5.8
mb
GS
233
24-Feb -73
010611.30
-15.41
-173.28
186.72
33
5.3
Ms
GS
239
24-Feb -73
035643.50
-15.17
-173.44
186.56
33
5.2
mb
GS
232
10- Mar -73
095000.30
-15.18
-173.52
186.48
8
5.6
mb
GS
240
20- Apr -73
003310.10
-14.83
-173.14
186.86
49
5.3
mb
GS
184
27- Jul -73
192641.90
-15.54
-173.09
186.91
33
5.6
Ms
GS
236
22- Sep -73
173547.60
-15.04
-173.71
186.29
10
5.1
mb
GS
247
30- Dec -73
162129.30
-15.32
-173.08
186.92
33
6.2
Ms
GS
217
31- Dec -73
034140.00
-15.07
-172.65
187.35
33
5.0
mb
GS
167
18- Mar -74
105612.40
-14.93
-172.83
187.17
27
6.0
Ms
GS
167
6- May -74
113819.70
-15.20
-173.42
186.58
13
5.7
mb
GS
233
3- Jun -74
061438.50
-15.41
-173.33
186.67
33
5.4
mb
GS
242
6- Jun -74
200045.00
-15.24
-173.47
186.53
33
5.2
mb
GS
239
20- Jun -74
165222.30
-15.24
-173.51
186.49
31
5.4
mb
GS
244
18- Jul -74
110443.20
-15.22
-173.59
186.41
33
5.9
mb
GS
248
19- Aug -74
051414.20
-15.34
-173.39
186.61
13
5.0
mb
GS
241
11- Sep -74
161750.00
-15.02
-172.96
187.04
33
5.2
mb
GS
184
8- Nov -74
133435.30
-15.57
-173.22
186.78
12
5.3
mb
GS
248
2- Jan -75
171452.90
-15.28
-173.26
186.74
33
5.1
mb
GS
227
19- Mar -75
134221.00
-15.84
-172.04
187.96
33
5.2
mb
GS
224
10- Apr -75
181201.60
-15.58
-171.94
188.06
15
5.1
mb
GS
195
20- May -75
194028.40
-15.06
-173.35
186.65
2
5.2
mb
GS
217
1- Sep -75
203853.30
-15.23
-172.89
187.11
25
5.3
mb
GS
196
23- Oct -75
045043.10
-14,88
-172.71
187.29
38
5.4
mb
GS
154
9- Dec -75
091440.60
-14.79
-173.00
187.00
33
6.4
UK
PAS
170
26- Dec -75
190650.50
-15.20
-172.34
187.66
33
5.2
mb
GS
163
27- Dec -75
233259.80
-15.34
-171.91
188.09
33
5.5
mb
GS
168
27- Dec -75
233438.40
-15.16
-171.92
188.08
33
5.6
mb
GS
148
28- Dec -75
001033.30
-15.20
-171.92
188.08
18
5.0
mb
GS
154
30- Dec -75
022940.90
-15.68
-172.54
187.46
69
5.2
mb
GS
221
2- Jan -76
021155.80
-15.47
-172.02
187.98
28
5.3
mb
GS
185
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[46]
14- Jan -76
155633.10
-15.23
-173.51
186.49
33
5.7
mb
GS
243
16- Jan -76
071056.70
-14.98
-173.03
186.97
33
5.0
mb
GS
186
26- Jan -76
214613.40
-15.95
-172.51
187.49
36
5.5
Ms
GS
248
11- Feb -76
214355.40
-15.26
-172.27
187.73
33
6.0
UK
BRK
167
29- Feb -76
193327.30
-15.19
-173.62
186.38
33
5.2
mb
GS
249
30- Mar -76
225837.70
-15.49
-172.44
187,56
33
5.1
mb
GS
197
1- Apr -76
061822.80
-14.98
-173.71
186.29
33
5.3
mb
GS
244
5- Apr -76
094820.60
-15.24
-173.29
186.71
33
5.5
mb
GS
226
3- Jun -76
210349.20
-15.28
-172.30
187.70
13
5.3
mb
GS
171
1- Nov -76
062033.70
-15.31
-172.96
187.04
38
5.1
mb
GS
208
13- Mar -77
173348.30
-15.02
-172.33
187.67
13
5.0
mb
GS
145
15- Mar -77
041151.20
-15.18
-173.21
186.79
32
5.3
Ms
GS
215
23- Mar -77
185736.60
-15.01
-173.25
186.75
33
5.1
mb
GS
205
187.20
33
5.1
Ms
GS
191
16- Jun -77
041735.70
-15.24
-172.80
24- Jul -77
062251.30
-15.34
-173.15
186.85
33
6.3
UK
PAS
223
21- Aug -77
044432.30
-14.77
-173.47
186.53
33
5.1
Ms
GS
210
115618.40
-15.29
-173.24
186.76
18
5.4
mb
GS
226
11- Sep -77
140804.60
-15.39
-173.21
186.79
33
5.6
Ms
GS
232
11- Sep -77
141229.90
-15.40
-173.29
186.71
33
5.6
Ms
GS
238
243
3- Sep -77
13- Sep -77
002152.60
-15.45
-173.29
186.71
33
6.2
UK
PAS
13- Sep -77
035944.80
-15.47
-173.03
186.97
33
5.1
mb
GS
227
13- Sep -77
095900.40
-15.30
-172.76
187.24
4
5.1
mb
GS
194
2- Nov -77
011927.90
-15.39
-173.32
186.68
30
5.3
mb
GS
240
14- Dec -77
101833.20
-15.31
-173.14
186.86
33
5.0
mb
GS
220
22- Jan -78
003351.80
-15.35
-173.49
186.51
33
5.1
mb
GS
249
14-Feb -78
022432.00
-15.82
-172.51
187.49
33
5.2
mb
GS
234
1- Mar -78
203339.60
-15.44
-173.26
186.74
33
5.4
mb
GS
239
3- Mar -78
104811.10
-15.29
-173.52
186.48
33
6.2
Ms
GS
247
13- May -78
164112.10
-15.24
-173.39
186.61
33
5.4
mb
GS
234
30- May -78
185742.20
-15.27
-172.90
187.10
38
5.1
mb
GS
200
11- Jun -78
142755.30
-15.26
-173.56
186.44
33
6.2
Ms
GS
249
27- Jul -78
134104.30
-15.47
-173.14
186.86
33
5.4
Ms
GS
233
27- Jul -78
182508.50
-15.58
-173.04
186.96
33
5.8
Ms
GS
237
1- Aug -78
202942.90
-15.28
-173.33
186.67
33
5.5
Ms
GS
232
4- Aug -78
011702.00
-15.67
-173.05
186.95
33
5.3
mb
GS
245
6- Sep -78
232623.60
-15.36
-173.33
186.67
33
5.4
mb
GS
238
30- May -79
164450.00
-15.14
-173.52
186,48
33
5.1
mb
GS
237
1- Jun -79
021004.50
-15.45
-173.12
186.88
33
5.1
mb
GS
230
18- Sep -79
205539.10
-15.67
-172.90
187.10
33
5.4
mb
GS
237
4- Nov -79
144412.10
-15.60
-173.08
186.92
27
5.5
mb
GS
241
27- Dec -79
074525.60
-15.45
-173.14
186.86
33
5.4
Ms
GS
232
21- Jan -80
023851.60
-15.58
-172.82
187.18
33
5.2
Ms
GS
224
23- Feb -80
012952.50
-15.20
-173.43
186.57
33
5.2
mb
GS
234
8- Mar -80
010026.90
-15.10
-173.61
186.39
33
5.7
Ms
GS
242
9- Jun -80
181842.20
-15.39
-173.15
186.85
33
5.8
Ms
GS
228
2- Jul -80
154808.30
-15.18
-173.61
186.39
33
5.7
mb
GS
248
13- Jul -80
221335.40
-15.26
-173.52
186.48
33
5.4
mb
GS
245
17- Sep -80
053642.60
-15.34
-173.38
186.62
33
5.4
mb
GS
240
9- Oct -80
161938.20
-15.38
-173.42
186.58
33
6.2
UK
BRK
246
4- Jan -81
200457.90
-15.37
-173.26
186.74
33
5.2
mb
GS
234
6- Feb -81
054259.80
-15.39
-173.25
186.75
33
5.3
mb
GS
234
26- Feb -81
061100.20
-15,47
-173.30
186.70
33
5.4
mb
GS
244
25- Jun -81
030848.69
-15.22
-173.50
186.50
50
5.1
mb
GS
241
6- Jul -81
010225.59
-15.29
-173.46
186.54
33
5.6
mb
GS
243
1- Sep -81
072302.18
-15.14
-173.29
186.71
33
5.8
mb
GS
218
1- Sep -81
092931.54
-14.96
-173.09
186.91
25
7.9
Ms
BRK
188
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[47]
1- Sep -81
095932.26
-15.15
-173.26
186.74
33
5.6
mb
GS
217
1- Sep -81
105903.71
-15.02
-173.36
186.64
33
5.2
mb
GS
215
1- Sep -81
152436.29
-15.23
-173.07
186.93
33
5.1
Ms
GS
209
1- Sep -81
183847.48
-15.31
-173.30
186.70
33
5.7
mb
GS
232
1- Sep -81
235545.19
-15.23
-173.18
186.82
33
5.6
mb
GS
216
2- Sep -81
084421.19
-15.47
-172.98
187.02
33
5.6
Ms
BRK
223
2- Sep -81
103052.88
-14.91
-173.68
186.32
33
5.3
mb
GS
237
13- Oct -81
033117.66
-15.28
-173.14
186.86
33
5.4
mb
GS
218
25- Nov -81
190147.91
-15.25
-173.31
186.69
33
5.9
Ms
BRK
227
17-Jun -82
114604.83
-15.32
-173.45
186.55
65
5.3
mb
GS
244
27- Jun -82
181925.34
-15.18
-173.49
186.51
33
5.2
mb
GS
237
2- Aug -82
203311.32
-15.39
-173.05
186.95
33
5.1
mb
GS
221
3- Sep -82
233939.09
-15.30
-173.09
186.91
33
6.3
Ms
GS
215
4- Sep -82
020809.03
-15.43
-173.06
186.94
33
6.3
Ms
GS
225
8- Sep -82
054146.65
-15.23
-173.51
186.49
33
5.2
Ms
GS
243
8- Sep -82
101717.31
-14.65
-173.34
186.66
33
5.1
mb
GS
191
16- Sep -82
082312.44
-15.71
-172.74
187.26
37
5.8
Ms
BRK
233
27- Sep -82
115107.72
-14.65
-173.26
186.74
26
5.0
Ms
GS
184
30- Sep -82
015401.66
-15.01
-173.09
186.91
63
5.2
Ms
GS
192
30- Sep -82
142359.24
-14.96
-173.15
186.85
33
5.6
Ms
GS
194
13- Nov -82
102627.29
-15.46
-173.31
186.69
33
5.1
mb
GS
245
8- Jan -83
112129.54
-15.39
-173.33
186.67
33
6.5
Ms
BRK
241
1- Jun -83
105844.84
-15.69
-172.81
187.19
33
6.5
Ms
BRK
235
29- Jun -83
032900.73
-15.39
-172.89
187.11
33
5.2
mb
GS
211
3- Dec -83
012355.30
-15.24
-172.91
187.09
33
6.0
mb
GS
199
29- Feb -84
102311.93
-15.19
-172.36
187.64
33
5.4
mb
GS
163
22- Mar -84
141316.17
-15.23
-172.19
187.81
42
5.4
mb
GS
162
28-Apr -84
153647.09
-14.43
-172.36
187.64
33
5.1
mb
GS
92
15- Aug -84
062204,10
-14.82
-173.53
186.47
33
5.2
mb
GS
218
19- Oct -84
012820.73
-14.85
-171.20
188.80
33
5.1
mb
GS
129
18- Jan -85
175618.37
-14.90
-173.43
186.57
33
5.2
Ms
GS
214
23- Jan -85
204440,34
-15.02
-173.14
186.86
33
5.4
Ms
GS
197
3- Jun -85
120621.13
-15.29
-173.52
186.48
33
7.0
Ms
BRK
247
6- Jun -85
012239.21
-15.49
-173.10
186.90
33
5.2
mb
GS
232
13- Aug -85
222821.52
-15.15
-173.46
186.54
33
5.0
mb
GS
233
23- Oct -85
171617.79
-15.22
-173.34
186.66
33
5.2
mb
GS
228
19- Dec -85
042302.29
-15.18
-173.62
186.38
33
5.3
mb
GS
248
12- Mar -86
140623.56
-15.12
-173.32
186.68
33
5.0
mb
GS
219
12- Apr-86
040431.76
-15.42
-173.22
186.78
33
5.5
mb
GS
235
12- Apr-86
202046.36
-15.18
-173.33
186.67
33
5.6
Ms
GS
224
21- Apr-86
011129.02
-15,20
-173.35
186.65
33
5.2
mb
GS
227
24- May -86
104335.93
-15.57
-173.05
186.95
33
5.8
Ms
GS
237
1- Jul -86
004907.18
-15.56
-172.45
187.55
33
5.8
Ms
BRK
206
1- Jul -86
010646.89
-15.81
-172.72
187.28
33
5.7
Ms
GS
242
1- Jul -86
051015.76
-15.64
-172.85
187.15
33
5.2
Ms
BRK
231
1- Aug -86
221738.99
-15.33
-173.26
186.74
33
5.4
mb
GS
231
26- Aug -86
215226.67
-15.19
-173.48
186.52
20
5.8
Ms
GS
237
26- Aug -86
221139.04
-15.10
-173.47
186.53
33
5.7
Ms
GS
230
4- Feb -87
044113.01
-15.19
-172.93
187.07
33
5.2
mb
GS
195
12- Mar -87
015744.66
-15.66
-172.66
187.34
33
5.1
mb
GS
224
1- May-87
064637.12
-15.34
-173.42
186.58
33
5.3
mb
GS
244
27-Jun -87
162916.35
-15.63
-173.07
186.93
33
5.1
mb
GS
243
2- Aug -87
031256.90
-14.97
-173.78
186.22
33
5.0
mb
GS
249
2- Oct -87
201548.21
-15.22
-173.49
186.51
33
5.0
mb
GS
240
10- Dec -87
092446.43
-15,50
-173.21
186.79
33
5.3
mb
GS
241
21-Feb-88
052017.29
-15.81
-172.78
187.22
63
5.0
mb
GS
245
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[48]
21- Feb -88
083418.78
-15.18
-173.24
186.76
33
5.1
mb
GS
218
2- Apr -88
142629.04
-15.45
-173.08
186.92
33
6.3
Ms
BRK
228
2- Apr -88
145658.55
-15.28
-172.72
187.28
33
5.0
mb
GS
190
2- Apr -88
220014.16
-15.42
-173.24
186.76
33
5.1
mb
GS
236
16- Apr -88
010822.17
-14.53
-173.05
186.95
33
5.1
mb
GS
157
11- Jun -88
121727.04
-14.99
-173.47
186.53
35
6.2
Ms
BRK
223
20- Aug -88
195243.15
-15.14
-173.51
186.49
33
5.2
mb
GS
236
26- Aug -88
093623.45
-15.39
-172.71
187.29
33
5.4
mb
GS
200
27- Aug -88
163016.90
-15.86
-172.07
187.93
27
6.0
mb
GS
228
5- Dec -88
160532.76
-15.26
-173.52
186.48
40
6.3
Ms
GS
246
16- Dec -88
092955.49
-15.33
-173.01
186.99
33
5.2
mb
GS
213
16- Mar -89
102315.21
-15.29
-173.44
186.56
33
5.1
Ms
GS
241
8- Apr -89
030601.59
-15.74
-173.00
187.00
32
5.7
Ms
GS
249
4- Jan -90
053221.04
-15.40
-172.85
187.15
53
6.4
mb
GS
209
20- Jan -90
072021.83
-15.26
-173.38
186.62
33
5.9
Ms
BRK
234
6- Apr -90
060903.08
-15.15
-172.13
187.87
33
5.7
Ms
BRK
152
6- May -90
235342.10
-14.96
-173.51
186.49
33
5.2
Ms
GS
225
10- Jul -90
141910.96
-15.03
-173.70
186.30
33
5.0
mb
GS
246
3- Aug -90
083630.59
-14.67
-173.39
186.61
36
5.1
mb
GS
197
9- Aug -90
024208.75
-15.64
-172.60
187.40
33
5.0
Ms
GS
219
15- Sep -90
225204.54
-15.10
-173.49
186.51
33
5.2
mb
GS
232
23- Sep -90
065033.09
-14.99
-173.63
186.37
33
5.8
Ms
GS
237
23- Sep -90
082945.46
-14.27
-173.86
186.14
33
5.2
mb
GS
229
26- Sep -90
201041.93
-14.74
-173.37
186.63
33
5.0
mb
GS
199
24- Oct -90
121816.41
-15.79
-172.11
187.89
32
5.3
mb
GS
221
11- Dec -90
144142.55
-15.46
-173.12
186.88
12
6.1
Ms
GS
231
11- Dec -90
160931.01
-15.37
-173.05
186.95
33
5.4
Ms
GS
219
26- Jan -91
085602.08
-15.04
-173.17
186.83
33
5.3
Ms
GS
201
24- Feb -91
110415.87
-15.12
-173.38
186.62
33
5.4
Ms
GS
225
30- May -91
191813.14
-15.23
-172.81
187.19
33
5.4
Ms
GS
191
1- Sep -91
072157.64
-15.14
-173.29
186.71
10
5.4
Ms
GS
218
30- Oct -91
103541.44
-15.31
-173.19
186.81
17
6.4
Ms
GS
223
17- Dec -91
114035.08
-15.14
-173.55
186.45
33
5.0
mb
GS
239
24- Jun -92
090018.19
-15.42
-173.40
186.60
30
5.2
mb
GS
248
003413.81
-15.29
-173.13
186.87
23
6.4
Ms
GS
218
25- Jan -93
144609.98
-15.21
-173.50
186.50
37
5.7
Mw
HRV
240
16- Feb -93
091616.25
-15.47
-173.29
186.71
39
5.4
Mw
HRV
244
25- Apr -93
000708.73
-15.68
-172.96
187.04
33
6.0
Mw
HRV
241
3- May -93
231506.66
-14.40
-173.02
186.98
23
5.4
Mw
HRV
148
16- May -93
214448.94
-15.29
-173.33
186.67
21
6.8
Ms
BRK
232
23- May -93
195221.53
-15.32
-173.45
186.55
33
5.0
mb
GS
244
20- Aug -93
112358.08
-15.25
-173.19
186.81
22
5.5
Mw
HRV
219
23- Aug -93
210259.81
-15.58
-172.86
187.14
33
5.3
Mw
HRV
226
9- Oct -93
033723.84
-15.29
-173.54
186.46
10
5.1
mb
GS
249
3- Dec -93
122217.68
-15.68
-171.99
188.01
10
5.3
Mw
HRV
207
16- Dec -93
234111.23
-15.46
-173.36
186.64
33
5.4
Mw
HRV
248
16- Jan -94
194850.86
-15.05
-173.61
186.39
28
5.2
Mw
HRV
239
4- Apr -94
013702.81
-15.47
-173.01
186.99
24
5.8
Mw
HRV
225
7- May -94
124413.66
-15.30
-173.36
186.64
57
5.5
mb
GS
235
23- May -94
073211.17
-15.38
-172.32
187.68
36
5.6
Mw
HRV
182
29- Jun -94
122102.25
-15.28
-173.49
186.51
33
5.1
mb
GS
245
30- Aug -94
101430.35
-15.25
-173.34
186.66
33
5.7
Mw
HRV
230
12- Sep -94
224350.76
-15.45
-172.99
187,01
15
6.0
Mw
HRV
222
12- Sep -94
224908.61
-15.53
-172.93
187.07
33
5.4
mb
GS
226
12- Sep -94
233454.45
-15.12
-172.99
187.01
33
5.5
Mw
HRV
194
5-Oct-94
183501.35
-15.40
-173.31
186.69
33
5.3
mb
GS
239
24- Dec -92
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[49]
28- Oct -94
180533.01
-15.03
-171.72
188.28
33
5.1
mb
GS
133
18- Jan -95
160725.34
-15.84
-172.72
187.28
33
5.2
mb
GS
245
7- Apr -95
220656.89
-15.20
-173.53
186.47
21
8.1
Ms
BRK
242
8- Apr -95
010208.06
-15.14
-173.55
186.45
33
5.0
mb
GS
239
8- Apr -95
012008.76
-15.21
-173.42
186.58
37
6.1
Ms
GS
234
8- Apr -95
142837,85
-15.22
-173.32
186.68
32
5.5
mb
GS
226
8- Apr -95
163535.94
-15.30
-173.41
186.59
31
5.5
Ms
GS
240
8- Apr -95
171254.55
-15.35
-173.36
186.64
33
5.5
Mw
HRV
240
13- Apr -95
065637.51
-15.16
-173.41
186.59
33
5.0
Ms
GS
229
5- May -95
160811.69
-15.82
-172.80
187.20
33
5.6
Mw
HRV
247
5- May -95
185158.21
-15.28
-173.37
186.63
33
5.0
Ms
GS
235
7- May -95
223828.10
-15.42
-173.27
186.73
20
6.0
Mw
HRV
238
25- May -95
054451.71
-15.32
-173.42
186.58
33
5.0
mb
GS
242
12- Jul -95
043000.68
-15.44
-172.82
187.18
27
5.0
mb
GS
211
26- Sep -95
014942.05
-15.13
-173.44
186.56
33
5.4
Mw
HRV
230
13- Nov -95
073842.63
-15.11
-173.47
186.53
10
6.0
Mw
GS
231
13- Nov -95
075438.86
-14.99
-173.50
186.50
33
5.5
Ms
GS
226
16- Feb -96
113431.11
-15.28
-173.22
186.78
33
6.0
Mw
HRV
223
24- Jul -96
210033.46
-15.14
-173.49
186.51
33
5.3
mb
GS
235
5- Aug -96
020858.25
-15.27
-173.13
186.87
41
6.8
Ms
BRK
216
5- Aug -96
170105,25
-15.29
-173.33
186.67
33
5.4
Mw
HRV
233
27- May -97
080029.05
-15.21
-173.33
186.67
14
6.4
Mw
HRV
226
27- May -97
092712.84
-15.40
-173.06
186.94
33
5.6
Mw
HRV
222
29- May -97
134647.97
-15.33
-173.42
186.58
33
5.2
Ms
GS
243
14- Dec -97
231003.97
-15.57
-173.17
186.83
33
5.6
Mw
HRV
244
1- Apr -98
210815.84
-15,28
-173.44
186.56
33
5.2
Ms
GS
241
16- Aug -98
044814.64
-15.31
-173.35
186.65
33
5.4
Mw
HRV
236
9- Oct -98
053752.62
-15.39
-173.39
186.61
33
5.5
Mw
HRV
245
20- Jan -99
060222.04
-14.95
-173.18
186.82
33
5.4
Mw
HRV
195
26- Jul -99
060358.14
-15.28
-171.68
188.32
33
5.1
Mw
HRV
162
27- Jul -99
233020.00
-15.20
-173.32
186.68
33
5.5
Mw
HRV
225
10- Oct -99
012804.90
-15.46
-173.23
186.77
33
5.0
mb
GS
239
3- Nov -99
235024.95
-15.76
-172.54
187.46
33
5.4
Ms
GS
229
5- Nov -99
213204.26
-15.62
-172.77
187.23
33
5.0
mb
GS
226
14- Jan -00
184444.50
-15.28
-173.52
186.48
33
5.2
Ms
GS
248
7- May -00
084742.00
-14.68
-173.56
186.44
33
5.7
Mw
GS
213
*The following magnitude "solution" explanations can be found on the NEIC (2000) website,
http: / /wwwneic.cr.usgs.gov /neis /epic /code magnitude.html
Surface -wave (Ms): Magnitudes are computed for earthquakes that are located at distances between 20 and 160 geocentric degrees from
the recording station, seismic -wave period between 18 and 22 seconds, and depth is less than 50 km (generally Ms magnitudes are not
computed for depths greater than 50 km).
Ms = Average NEIS surface -wave magnitude (if given, Z = vertical component, H = horizontal component).
Body -wave (mb): Magnitude values are computed based on the seismic -wave period greater than or equal to 0.1 and less than or equal to
3.0, and distance is greater than or equal to 5 degrees.
mb = Average NETS body -wave magnitude.
Moment Magnitude (Mw): The magnitude is computed from a long- period body- and mantle -wave moment tensor inversion method; it is
also related to the product of the area of the earthquake fault, multiplied by the average fault slip over that area and by the shear modulus of
the fault rocks. The Mw value is approximately the same as the Ms magnitude value.
Unknown Magnitudes (UK): The computational method was unknown and could not be determined from published sources.
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[50]
APPENDIX 4
Apia Site Microtremor Field Notes and Raw Data File Names
Site Number
Site Name
File Name
Location Description
Field Comments
Mt Vaea RadioPhone
08231156.src
Back Radio Phone
See topo
2
Mt Vaea
Our Landing
08231617.src
3
Mt Vaea Mango Peak
08231716.src
4
Leading Light Mt Vaea
08271624.src
Turn in Mt Vaea Rd near
Bit of soil cover.
clearing on right, 2/3rd way
up to radio phone. See topo
Top of Mango peak, half way
down Mount Vaea at a local
topographical high,
See topo.
120 m East of leading light
(08231258.src Didn't work)
5
Marist Sports Ground
08252116.src
6
Government Buildings
08241123.src
7
Mataututai
09011204.src
8
Vaipuna CCCS
08261042. src
1
SouthSide Marist Sports
ground, see street map
Government building
reclaimed area, sea -side,
see topo
Close to house
Bit of soil cover, weathered material.
Big problem with E- component, lump
every 6 seconds
Airphoto J18
Lots of domestic house noise, not a
good site, maybe on fill
House next to church ground In mangrove swamp. May have' 1 m of fill coral etc
backyard
Volcanic boulders and soil close by
RCCattle Farm Moamoa
09021030.src
08271225.src
11
Savalalo
Behind F/M Station
Mobil Oil
12
Apia Observatory
08251814.src
Next to Doppler trig point
13
Fale Fono
08301008.src
Arial Photo H12
14
Teuila Hotel
09021250.src
15
Magiagi Inland
09021423.src
See topo
16
Adjacent Samoa College
09021337.src
See topo
17
Mt Vaea Club
08271003.src
Backyard of Mt Vaea Club
18
Fungalei Home
08301121.src
Aerial Photo J7
19
Tufuiopa Cemetary
08271119.src
Outside cemetery backyard
20
AOG Leone Church
08281904.src
See sketch
21
08252229.src
Ministers Back yard, 150 m
from John Williams Building
22
John Williams
Building
Magiagi Primary School
23
Moata'a CCCS
08261349.src
In front of church 15 m from
sea wall (rock pile)
24
Apia Park
08241230.src
25
Royal Samoa Golf Club
08271522.src
26
Motootua National Hospital
08261144.src
South side, near fence, see
topo
See street map, ie off golf
course entrance
Behind the Hospital Center
yard between acute 7/8 and
children's hospital
27
Papauta Girls School
09011008.src
Close to factory with power station
28
RobertLuis
Museum
08311217.src
Volcanic Breccia
29
Avele College
08311109.src
Lava Domes nearby
30
St Mary's College
09011340.src
31
Fish Pond
08241740.src
32
Old St Joseph's College
08241612.src
9
10
Behind Radio station near
mangrove flood plain
09011617.src
High frequency 08300944.src
Next door to the Teuila Hotel, close
to road and close to runoff area.
Inverter broke down
Z- component producing 2 sec period
frequency
Last 2 mins chickens walking past
Software problems, the first recording
did not record
08301313.src
20 m off from site on beach seems
like N/E sloping sandstone. Very
Windy!!
On mangrove area, opposite Airphoto
the Fale Fono, see topo
North side of sports field, see
topo
[SOPAC Preliminary Report 126 - Teakle & Swamy]
[51]
33
USP, Alafua Campus
08241502.src
34
Chinese Cemetery
08251058.src
35
Vailoa CCCS
08251155.src
36
Vaigaga Primary School
08301719.src
37
Faotoia
09011107.src
See topo
38
Papase'ea Sliding Rock
08251636.src
At hill edge near sliding rock
tourist entrance, walkway
down hill 25 m south
39
Tuaefu, Tupua's House
40
Lepeka Hotel
08311646.src
41
Vaivaseuta
08301356.src
42
Vailele Rd
08241335.src
South side of fence in back
yard, see topo.
43
Fagalii CCCS
08271415.src
Back of Church near
Mangrove flood zone
44
Toamua
08311335.src
45
Pasefika Inn
08231554.src
Back of hotel on river
deposits, clock set FJT
46
ACB Building
08252002.src
Opposite RSA
47
Warf Reclaimed area
08260942.src
48
ORUM Catering
08281807.src
Near borehole, down by
Nipon importers
See air photo L13
49
Sinamoga
08301208.src
Front Johnys house
50
Taufusi
08282022.src
Opposite Mings and Hanna
Supermarket
Elisefou, off Vaiusu Rd
Behind the church near the
back fence
08251512.src and Ex -PM's house
08251532.src
51
VaiusuutA
08311449.src
52
Alamagato CCCS Ministers
House
Leififfi School
09011443.src
Magiagi Cemetary
08261250.src
55
Abandoned Quarry
08301553.src
56
Vailoa Tai
09091052.src
57
Lepea
09091146.src
53
Back of Staff house #10,
west side of campus, see
topo
Half way up Chinese
cemetery
See sketch
See sketch
North side of house front garden of
the x -pm's house, up from 20 m from
Tuefu bridge, (drop)
Lameko's House
Noisy site, cars etc
09011443.src
Centre of cemetery
54
Lost Voltage on the E component, no
rock response, soft weathered layer?
Use inbuilt computer battery
58
VaimosoMM
09091248.src
H10 Aerial Photo, back of
Niko's house
Near Fuluasou river bed
below 50 ft contour
100 m South of MM church
59
VaimosoHouse
09091504.src
In front of Taito's house
60
VaitoloaPS
09091551.src
A.Photo H /10, Centre school
front field
Similar signal to Tafusi, rather noisy,
61
Adj Tusitala Hotel
09091649.src
Back of house adj and west
of Tusitala
First 4 mins very E is flat, then very
noisy. Voltage drop at 1703 on E,
and at 1707 on N
Use mains power
Use computer battery
Similar signal to Tafusi, rather noisy,
1/2 second
1/2 second
[SOPAC Preliminary Report 126 - Teakle & Swamy]