The current state of seismic monitoring in Puerto Rico
John F. Clinton1,3, Georgia Cua2,3, Víctor Huérfano2, Christa G. von HillebrandtAndrade2, José Martínez Cruzado1
A. Introduction
The US Commonwealth of Puerto Rico has a population of 3.8 million (2000
Census), a higher population density than any US state. The island, approximately 160km
from east to west by 50km from north to south, is bounded by off-shore active faults on
all sides. Numerous local and regional events in the recorded history with M>7.0, some
of which have generated tsunamis, have caused extensive damage to local infrastructure;
though the last significant ground motions were felt on-shore in 1918. The USGS hazard
maps (Mueller et al., 2003) indicate that the seismic hazard is similar to the Basin and
Range province in the Western USA, and the island is assigned Seismic Zone 3 in the
current standard Building Code in Puerto Rico, the 1997 UBC. The significant hazard,
combined with the large population and untested infrastructure, result in a potentially
devastating combination for Puerto Rico. Efficient emergency response in the event of a
large earthquake will be crucial to minimising the resultant loss of human life and
disruption of lifeline systems. The first step in providing an appropriate response to such
a disaster is a timely knowledge of the magnitude, location and expected ground shaking
and damage patterns from a large earthquake. This requires a modern and dense seismic
network, capable of not only recording the earthquake ground motion without saturation,
but also doing so in real-time and then providing data for near-immediate analysis, which
can be made available to the emergency services and community at large.
The seismicity of the island, as well as the northeastern Caribbean region in general
(including the US and British Virgin Islands), is monitored jointly by the Puerto Rico
1
Puerto Rico Strong Motion Program, Department of Civil Engineering and Surveying,
University of Puerto Rico at Mayagüez
2
Puerto Rico Seismic Network, Geology Department, University of Puerto Rico at
Mayagüez
3
now at Swiss Seismological Survey (SED), Institute of Geophysics, ETH Hönggerberg,
Switzerland
1
Seismic Network (PRSN) and the Puerto Rico Strong Motion Program (PRSMP), both
operating within the University of Puerto Rico at Mayagüez. They currently acquire,
analyse and archive broadband, short-period and strong-motion seismic data in
continuous real-time format, using both Antelope and Earthworm software, from stations
on Puerto Rico and its surrounding islands, the US and British Virgin Islands, and the
Dominican Republic. In addition, PRSN imports and processes data from various
regional stations operated by other networks. The PRSMP also operates dial-up and
stand-alone installations. Stations range from state-of-the-art seismic vaults for event
location and magnitude determination, to urban free-field installations and structural
arrays for engineering studies and structural health monitoring. In the near future, the
networks expect to additionally monitor continuous streams of data from tidal gauges and
buoys (for tsumani warning), GPS and digital weather stations.
B. Tectonic background
The island of Puerto Rico has a long history of damaging earthquakes and
tsunamis. The island is bounded on all sides by major tectonic fault lines (Figure 1).
Major earthquakes have produced damaging ground motions in Puerto Rico in 1615,
1670, 1751, 1776, 1787 (~M8.0 Puerto Rico Trench), 1867 (~M7.3 Anegada Passage)
and 1918 (~M7.3 Mona Passage). Large events in 1943 (~M7.5) and 1946 (~M7.8) also
caused much damage in the neighbouring Dominican Republic (PRSN Historical
Catalogue, http://temblor.uprm.edu/~victor/PRSN/history/; Shepherd and Lynch, 1992).
The 1867 and 1918 events were accompanied by destructive tsunamis (Reid and Taber,
1919; Mueller et al, 2003).
Puerto Rico is located on a microplate sandwiched between the obliquely subducting
North American and Caribbean plates (Figure 1). Puerto Rico accommodates
approximately 16.9mm/yr of deformation relative to North America, and 2.4mm/yr
relative to the Caribbean plate (Jansma et al., 2000; Jansma and Mattioli, 2005), primarily
by left-lateral strike-slip motion along east-west striking faults. The main sources of
seismic activity in the region are at the supposed boundaries of the microplate; the
subduction zones to the north (the Puerto Rico Trench, which is the location of the largest
2
gravity anomaly on earth) and south (the Muertos Trough: Carbó et al., 2005), and zones
of extension at the Anegada Trough to the east, and the Mona Canyon region to the west.
All regions are capable of producing events greater than M7.0, and all have evidence of
having done so in the recorded history of the island (Asencio, 1980; Moya and McCann,
1992; Macari, 1994). On average Puerto Rico is strongly shaken with Modified Mercalli
Intensity (MMI) >VII once every hundred years, and MMI > VI is experienced on the
island once every 50years.
In addition to these offshore sources, recent trenching shows evidence of 2 surfacerupturing events on the inland South Lajas fault in southwestern Puerto Rico (Prentice et
al., 2000, Prentice and Mann, 2005), predominantly along a normal fault with a
component of strike-slip motion, both within the last 5000 years. This 50-km long inland
fault segment can produce M7.0 events (LaForge and McCann, 2003), and potentially
could be part of a longer fault zone that extends towards Ponce, the second largest city on
the island (population 186,000, 2000 Census). Other shallow faults, mainly with E-W
trends, are interspersed across the island. These include the Great Northern and Southern
Puerto Rico Fault Zones. These structures have unknown potential for large magnitude
events, as yet there is no evidence of Holocene rupture. Tectonic models (Prentice and
Mann, 2005) and GPS studies (Jansma and Mattioli, 2005) indicate there are active faults
on-shore to accommodate some extension across the island.
The most recent large event to cause widespread damage across the island occurred in the
Mona Passage in 1918, with MS7,3 (Pacheco and Sykes, 1992). This event caused
substantial structural damage to the large towns of Mayagüez and Aguadilla on the West
of the island. The associated tsunami had a run-up of 6m at Aguadilla, and 2m at
Mayagüez (reaching over 1km inland) (Mercado and McCann, 1998), which killed over
100 people. Widespread liquefaction was observed along the unconsolidated floodplains
in the municipalities of Añasco and Mayagüez (Capacete et al., 1972; Moya and
McCann, 1992), areas now with pockets of dense population. With the island now having
a far greater density of population and infrastructure – infrastructure that has not yet been
3
tested by strong-motions since this event in 1918 – a repeat of such grounds motions
would lead to a far more severe loss of life and infrastructure.
The tsunami risk in the north-east Caribbean is also very real. Aside from the potential
for large magnitude events (Huérfano, 2003), recent bathymetry studies have shown
numerous large landslide scarps and cliffs near the Puerto Rico Trench as well as the
Muertos Trough (Grindlay et al., 2005a; Grindlay et al., 2005b; ten Brink, 2004). The
off-shore bathymetry is particularly severe to the North of the island, where the Puerto
Rico Trench drops to a depth of over 8.3 km just 180 km north of the island. The interior
of the island is mountainous, resulting in much of the population being concentrated in
the at-risk low-lying coastal flood plains and alluvial basins.
C. Seismic Networks in Puerto Rico
The Puerto Rico Seismic Network (PRSN) and the Puerto Rico Strong Motion
Program (PRSMP) jointly monitor seismic activity in the northeastern Caribbean region.
The primary duty of the PRSN is to identify and provide information on local, regional,
and teleseismic earthquakes. PRSN is also the host of the Emergent Tsunami Warning
System for Puerto Rico and the Virgin Islands. PRSN is the reporting authority for event
locations and magnitudes, and maintains the authoritative event catalogue. The PRSN
operate vault stations primarily in low noise locations. The objective of the PRSMP is to
record on-scale ground motions from earthquakes affecting the island with as high a
quality and station density as possible. The PRSMP maintain a dense urban network with
both free-field stations and structural arrays. Both networks share the common goal of
providing high-quality data and information in response to needs of the emergency
management, engineering, and scientific communities, as well as the general public.
The PRSMP and PRSN run complimentary operations and share all continuous data in
real-time (see Figure 2). This exchange provides an essential level of redundancy and
robustness that will be important in ensuring timely and accurate information following a
large earthquake.
4
D. Puerto Rico Seismic Network (PRSN)
The Puerto Rico Seismic Network (PRSN) has historically provided locations and
magnitudes for earthquakes in the Puerto Rico and Virgin Islands (PRVI) region. PRSN
is the reporting authority for the region bounded by latitudes 17.0N to 20.0N, and
longitudes 63.5W to 69.0W (Figure 1). The network has been operating since 1974, when
the U.S. Geological Survey installed several short-period stations for the Puerto Rico
Electric Power Authority that were used to evaluate local seismicity in response to
concerns regarding the planned construction of two nuclear power plants. In 1982 the
network was transferred to the University of Puerto Rico at Mayagüez, where it has been
operating under the jurisdiction of the Department of Geology since 1987.
The main objective of the PRSN is to record, process, analyze, provide information and
research local, regional and teleseismic earthquakes, providing high quality data and
information to be able to respond to the needs of the emergency management, academic
and research communities, and the general public. It operates 13 broadband stations and
10 short-period stations throughout Puerto Rico and the US and British Virgin Islands
(Figure 3). Six of these broadband stations also jointly house strong-motion sensors. The
broadband (BB) stations consist of Guralp CMG 40T, CMG 3ESP and CMG 3T
seismometers recording onto 24-bit Refraction Technology (130 and 72A), NetDas DAQ
or Quanterra Q330 digitizers, all sampling at 40 sps. The strong-motion (SM) sensors are
all Kinemetrics EpiSensors, sampling at 100 sps. The broadband sensors are linked to the
central data collection center in Mayagüez via Monitron UHF digital radios, DDS 56K
telephone lines, spread-spectrum radios and Internet service. The analogue short-period
(SP) seismic stations consist of Teledyne S-13 and Mark L-4 seismometers and the data
is digitized at 16-bits at 100 sps at the offices of the PRSN.
The PRSN runs Earthworm software (Johnson et al, 1995) to acquire and write
waveforms to disk for permanent archival. Automatic locations and alerts are generated
5
for events in Puerto Rico, the Intra America Seas, and the Atlantic by the EarlyBird
system (Whitmore and Sokolowski, 2002), which monitors PRSN stations as well as
some 35 additional stations run by networks operating in North, Central and South
America and other sites in the Caribbean (Figure 2). PRDANIS (Puerto Rico Data
Analysis and Information System) software, developed by PRSN, supports manual
locations and analyst review of automatic locations of events within the PRSN area of
responsibility (AOR), using all the broadband, strong-motion and short-period
waveforms. The location algorithm is based on the inversion schema HYPOINVERSE2000 (Klein, 2002), using the crustal model of Huérfano and Bataille (1994). Currently
all magnitudes reported by PRDANIS are Md, duration magnitude. All phase arrival
times, locations, amplitudes and magnitudes are reviewed by a seismic analyst, usually
on the same day, and always within one week. The continuous waveform data is
permanently archived at PRSN in GSE2.1 format, though all broadband waveforms are
continuously exported to IRIS (Incorporated Research Institutions for Seismology,
http://www.iris.edu), where they are archived in SEED format. Full dataless SEED
volumes are available for all stations. Phase data is saved in HypoInverse 2000 format.
The PRSN hosts a web-site with a searchable event database, felt event reports and
general information about seismicity in the area of responsibility, which can be accessed
from http://redsismica.uprm.edu/english/.
The PRSN is also producing ShakeMaps (Wald et al., 2000) for all felt events in Puerto
Rico and the Virgin Islands, and provides QDDS (Quake Data Distribution System) data
to the USGS ‘Did You Feel It?’ tool (http://pasadena.wr.usgs.gov/shake/prt/).
E. Puerto Rico Strong Motion Program (PRSMP)
Strong-motion instrumentation efforts in Puerto Rico began in the 1970’s and
have continued at a steady pace due to the activities of the Puerto Rico Strong Motion
Program (PRSMP). In the immediate aftermath of a large earthquake, the mission of the
PRSMP is to provide timely information regarding the distribution of peak ground
shaking to emergency management officials to guide response and recovery efforts. In
6
the long term, the PRSMP aims to provide the relevant strong-motion data to the
scientific and engineering communities to facilitate the construction of earthquakeresistant structures. The PRSMP focuses on (1) deploying and maintaining
instrumentation for recording large earthquakes that will produce damaging ground
motions on the island with as high a data quality and station density as possible; (2)
providing timely and appropriate information to the local authorities, seismologists and
engineers; and (3) supporting and conducting research activities associated with these
records, in particular the understanding of the effects of earthquakes in Puerto Rico, and
the mitigation of potential damage from future earthquakes.
Currently the PRSMP maintains a network of (1) 78 3-component 18-bit ETNA urban
free-field stations; (2) 6 24-bit EpiSensors alongside broadband sensors (jointly operated
with the PRSN) at remote sites with vault conditions, and (3) 8 monitored structures with
multi channel 19-bit K2 instrumentation (Figure 4). The density of strong-motion
instrumentation across the island is comparable to Greater Los Angeles, and greatly
exceeds the density deployed in California as a whole.
Station sites for the urban free-field network are selected based on the proximity to highly
populated urban areas and regions of high seismicity (i.e. regions exposed to the greatest
seismic risk). Local cultural noise, site geology, ease of access for maintenance,
availability of Internet or telephone lines, and security are additionally important
considerations. Of the 78 stations, 9 have a real-time continuous Internet communication
with PRSMP. These Internet stations are primarily located on different University of
Puerto Rico campuses throughout the island. Of the remaining ETNA stations, 42 have
dial-up telephone communications, and 27 have no communications (‘stand-alone’
stations that must be visited to gather data). For reasons of accessibility, security, and
ease of obtaining site permissions, a majority of the dial-up and stand-alone stations are
installed in fire stations or on Catholic Church property.
In addition to the ETNA stations, PRSMP monitors 8 structures across the island with
multi-channel sensors. These sites are all equipped with 19-bit Kinemetrics K2
7
dataloggers and Kinemetrics EpiSensors or FBA-23/FBA-11 sensors. The critical
infrastructures monitored are: high rise buildings (Plaza Immaculada Apartment Complex
and Minillas Government Center, San Juan), concrete dams (Lucchetti, La Plata and
Carraizo), and bridges (Ponce Bypass, Mayagüez Viaduct and Paso del Indio bridge). All
structural installations are currently stand-alone.
The PRSMP also shares the operation and maintenance of the 6 strong-motion sensors
co-located with PRSN broadband sensors (Figure 3), ensuring the entire seismic
frequency and amplitude bandwidth is covered at these select stations.
The Antelope Real-Time System (ARTS, http://www.brtt.com; Malone, 2000; Pavlis et
al., 2005) facilitates communication with the PRSMP ETNA stations as well as with the
strong-motion and other data collected at PRSN. Antelope software also provides
automatic and reviewed location and magnitude determinations (Richter Magnitude (M L)
for local events; Body Wave Magnitude (mb), and Surface Wave Magnitude (MS) for
regional and teleseismic events), which run as backup to PRSN operations. Data are
archived in a temporary ring buffer of 15 days in SEED format, with permanent archiving
of all data from all triggered events in SAC format. Event information, and selected event
data, is available on-line at http://www.uprm.edu/prsmp.
Data from 9 strong-motion stations that recorded the recent 3 march 2006 Md5.3 event
off Anegada was submitted to Cosmos, and is available from their website
(http://db.cosmos-eq.org).
F. Brief Description of the PRSN Catalogue
The PRSN generates and maintains the earthquake catalogue for Puerto Rico and the
Virgin Islands. Prior to July 2004, the PRSN catalogue was compiled using the PRHYPO earthquake location code with both local (Richter Magnitude, ML) and duration
magnitude (Md) computed using the equations of von Hillebrandt and Bataille (1994). In
July 2004, the location schema HYPOINVERSE2K was implemented and magnitudes
have been calculated from short-period and broadband data using the Eaton (1992) “full
8
digital velocity” equations. The PRSN now calculates a near-real time moment
magnitude based on a Regional Moment Tensor (RMT) inversion code (Randall et al.,
1995) and the ASPO search grid algorithm of Zahradnick et al. (2001).
Figure 5 shows the epicenters of the analyst-reviewed events in the PRSN local reporting
region (AOR) between January 1986 and March 2006. There are over 14,100 events in
the catalogue, 9 with M>5.0 (6 of these large events were shallow with depths > 35km).
The largest event was an Md5.6, occurring at 67km depth, just south of Anegada, BVI, on
7 December 1998. The most recent large event was an Md5.3 event, 25km deep located
100km NE of Anegada, on 3 March 2006. Shallow seismicity is concentrated in regions
along the Mona Canyon, the region between the Puerto Rican Trench and the 19o N fault
zone, the Sombrero fault zone, and across the south of Puerto Rico. The oblique
subduction of the North American Plate under the Puerto Rican microplate appears to be
the source of much of the deep seismicity, with events concentrated in the Dominican
Republic and along northern Puerto Rico.
Short-period sensors dominated the network until 2000, when broadband sensors were
first introduced, although the station density has remained relatively constant. Figure 6(a)
shows the number of events binned into periods of 3 months. It is clear that the level of
recorded seismicity increased by about 20% around the turn of the century once the
broadband sensors were introduced to the network. The spike in events in the bin from
January-March 2006 is due to the M5.3 indicated in Figure 5. This anomalously large
number of events reflects significant recent improvements in automated event-picking
algorithms using both Earthworm and Antelope modules. In Figure 6(b) and 6(c), the
magnitude and depth distribution of the catalogue is analysed. From Fig. 6(b), the
average magnitude of completeness, MC, for the entire catalogue can be estimated to be
about Md 3.5. The majority of events are shallow (Figure 6(c)), though the spike in event
depth at 25km is due to this being the initial default depth for events up until 2004. Figure
6(d) shows the Gutenberg-Richter distribution (Gutenberg and Richter, 1944) for the
catalogue, with a b-value of 1.44. It is not clear whether this high b-value is real, and it
could be due to magnitude compression of the Md calculations – this scale saturates for
9
large events, so direct comparisons between the PRSN magnitude and independent
magnitudes from global networks are not possible even for the infrequent large events.
No other regional networks provide magnitudes from this region.
The spatial distribution of magnitude of completeness for the catalogue across the PRSN
reporting region is shown in Figure 7. At best, the magnitude of completeness, MC, is
Md2.0, and is at least Md3.3 across the island of Puerto Rico. MC is highest in the
northeast of the island, where high gain sensor coverage is poorest (Figure 3). In the
corners of the PRSN reporting region, which are the areas most distant from land (and
thus stations), the MC degrades to about Md3.8. Some degradation is inevitable, an
unavoidable consequence of the island geography, but the problem is significantly
compounded by the frequent loss of communication with the most remote sites: the
Virgin Islands to the east, and Desecheo and Mona Islands to the west of Puerto Rico.
The ongoing efforts to secure high quality broadband data with good communications at
the more isolated sites, such as Anegada and Mona, coupled with expected installations
in the Dominican Republic, will reduce the magnitude of completeness in the extreme
regions. The density of seismicity is shown in Figure 8. Although this data must be
interpreted within the context of the spatial distribution of MC presented in Figure 7, it is
clear that that there are pockets of increased seismicity in the region. The seismicity is
dominated by 3 regions: (1) north of the 19o N fault zone directly north of Puerto Rico;
(2) the Sombrero Seismic zone north of Anegada island; and (3) on-land Puerto Rico, in
the southwest of the island, south of the Great Southern Puerto Rico Fault Zone.
In addition to the difficulties in detecting off-shore events, exact earthquake location is
also difficult in the region as most of the stations lie in a band between 18 oN and 18.5oN,
with poor azimuthal constraint on events lying to the north and south (Mendoza and
Huérfano, 2005).
G. Current Network Initiatives
1. ShakeMap for Puerto Rico
10
ShakeMaps are ground shaking and intensity maps that combine instrumental
measurements of shaking with information regarding local geology and earthquake
location and magnitude to estimate shaking variations throughout a geographic area
(Wald et al., 2005). The ShakeMap software is distributed and maintained by the USGS,
with local network operators responsible for customizing the distributed software to their
particular network environment. The PRSN has the capability to manually generate
ShakeMaps within 1.5 hours following a significant felt event, after which they are
posted on the website - an example for the recent 2 March 2006 M5.3 event is at
redsismica.uprm.edu/spanish/sismos/sismarzo.php. PRSN ShakeMaps are constrained by
peak ground motion parameters obtained from real-time broadband and strong-motion
instruments operated by PRSN and PRSMP; the initial ShakeMaps can be updated as
strong-motion data from PRSMP dial-up stations become available. Site amplification
effects are approximated by assigning Vs30 values on a uniformly-spaced 1.5 km grid
throughout the island. A set of 14 Vs30 measurements (Odum et al., 2006) sampling
representative geologic units throughout mainland Puerto Rico was used to determine
how to assign average Vs30 values to geologic units.
The efficiency and performance of the ShakeMap system in Puerto Rico can be improved
by: 1) automating the transfer of data from the PRSN earthworm data acquisition system
to ShakeMap, 2) updating the current site conditions map to take into account detailed
microzonation studies available for the San Juan Metropolitan Area (Aponte et al., 2000),
Ponce, and Mayaguez (Llavona, 2004), the three largest urban centers on the island, and
3) educating potential users on how to use ShakeMap products to guide post-earthquake
response and recovery efforts.
2. Caribbean Tsunami Warning System
The PRSN has been working towards establishing a local Tsunami Warning Center for
the region since 2000. The original goal was to provide a platform for tsunami warnings
to Puerto Rico (von Hillebrandt and Huérfano, 2005), but this has now evolved into an
initiative to create a Caribbean Tsunami Warning Center, which would be a fundamental
11
component of the Tsunami and other Coastal Hazards Warning System for the Caribbean
and Adjacent Regions (UNESCO, 2005). Efforts to lay the foundation for this center
included the installation of the Early Bird System (Sokolowski, 2002) at PRSN for the
detection and reporting of potentially tsunamigenic earthquakes. This system monitors
the seismic stations of the PRSN and 35 other stations in and around the Caribbean that
are available in real time through the GSN or bilateral agreements with regional seismic
networks. Earthworm and Seiscomp are the utilities that are used for the real time
exchange of seismic data. During 2006 the network will integrate additional broadband
stations into the system: the 9 GSN quality stations the USGS will be installing in the
region (McNamara et al., 2006) and stations to be installed in Dutch territories and the
Cayman Islands. The goal is to be able to timely and precisely detect all earthquakes of at
least magnitude 5 in the Caribbean region. The Early Bird system not only automatically
locates earthquakes and provides different magnitudes for the events, it notifies personnel
of the PRSN once specific regional dependant thresholds have been exceeded. In 2006, as
part of the warning system, the PRSN will also be installing six FEMA funded tsunamiready tide gauge stations in Puerto Rico in addition to a GOES satellite receiver at
Mayagüez to gather data from these and other regional tide gauges. The data from five
DART buoys that were deployed in the spring of 2006 in the Caribbean Sea and Adjacent
seas will also be incorporated into the monitoring system.
To achieve a true Tsunami Warning System, the PRSN complements improved
monitoring capabilities with a series of research and education and outreach initiatives.
These activities include tsunami inundation modeling, seismic source characterization,
protocol development, improved dissemination techniques, production of audiovisual
materials, workshops, talks and drills. In May 2006, the City of Mayagüez was declared
by NOAA as the first TsunamiReady community in Puerto Rico and the Caribbean. The
TsunamiReady program has proven to be an excellent venue to promote and validate
tsunami readiness and has been widely accepted by local public officials and the media.
All of these monitoring and complementary activities are being coordinated together with
other local, regional and international institutions, including the Pacific Tsunami Warning
12
Center, the institution presently responsible for providing tsunami warning guidance for
the region.
3. Focal Mechanisms and Moment Tensors
With the support of SeaGrant, the PRSN is evaluating available seismological methods
for the rapid identification of earthquake source parameters using the broadband seismic
waveforms recorded by the PRSN and the development of automated procedures for the
emergent Tsunami Warning System in the Puerto Rico-Virgin Islands region. This work
involves adaptation of existing codes for the numerical inversion of both local and
regional broadband waveforms to allow for rapid implementation within the PRSN
seismic-detection and information system. The results will be incorporated to the
broadcast schemas as part of the tsunami protocol in the PR-VI region
The implementation of near-real time moment magnitude estimation based on the
Regional Moment Tensor (RMT) inversion code of Randall et al. (1995), and regular
calculation of first motion focal mechanisms are products of this work. For the recent 2
March 2005 (25km deep), a moment magnitude of MW5.5 was determined within 1 hour
of event initiation. The focal mechanism has a strike/dip/rake of 83.2o/52.4o/53.6o, a
thrust fault with some strike-slip component reflecting the compressional dynamics
typical of subduction zones, and is likely be related with the plate interface. This is
consistent
with
the
Harvard
Quick
CMT
solution
(http://www.seismology.harvard.edu/CMTsearch.html), which indicates an MW5.5 high
angle thrust event with 103o strike.
4. Structural Monitoring
The PRSMP currently monitors 8 structures, all stand-alone, with multi-channel 19-bit
K2 systems, with ETNA free-field sensors. These are all located on critical infrastructure
around the island; 2 high-rise buildings in San Juan, 3 bridges on the main road around
the island, and 3 concrete dams. The instrumentation is summarised in Table 1. Currently
13
all these stations are stand-alone, although cable Internet is being installed at the Plaza
Inmaculada building. Nine of the 15 channels at this site will be continuously monitored
at PRSMP.
The PRSMP is also in the process of installing dense instrumentation at the 19-story El
Castillo apartment complex in Mayagüez, a component of the Advanced National
Seismic System (ANSS) Structural Response Monitoring System. This structural
monitoring test-bed will consist of at least 27 channels of 24-bit strong-motion data
recording on Quanterra Q330 dataloggers.
At these test-bed locations, the natural frequencies and modes of the monitored building
systems will be determined in real-time. In addition to the response of the structures to
earthquakes, severe wind loading conditions from hurricanes can be expected over the
duration of the instrumentation. The detection of significant and permanent changes in
the dynamic properties of the system during a heavy loading event is a key parameter for
the remote identification of structural damage (Clinton, 2004; Clinton et al, 2006).
5. Network Expansion and Improvement

Upgrade at remote sites and new installations
Existing network infrastructure at Mona Island in Puerto Rico and Anegada in the British
Virgin Islands (Figures 1, 3) will be upgraded by installing 24-bit broadband and strongmotion sensors and dataloggers. Satellite telemetry will be used to transfer data to PRSN.
Both of these stations will also provide data to the emergent USGS earthquake and
tsunami monitoring system for the Caribbean (McNamara, 2006).
In addition, the
currently non-operational short-period station at CSB (Figure 3) is likely to be moved to
an area near San Juan, which will improve the sensitivity of the network in the NE region
of the island.

Installations in the Eastern Dominican Republic
14
Two sites have been selected and permitted in the Eastern Dominican Republic, at
Samana and Punta Cana (Figure 1), where the PRSN and PRSMP will jointly install and
operate 24-bit collocated broadband and strong-motion stations, with real-time
continuous internet data transmission.

Strong-Motion Monitoring in the British Virgin Islands
The PRSMP has begun a 3-year contract with the Department of Disaster Management in
the British Virgin Islands to install a suite of free-field and structural strong-motion
sensors at critical local infrastructure. The airport control tower on Beef Island and the 3story Government Building in Road Town, Tortola will have 12-channel K2
instrumentation with both free-field and structural sensors. Both lie on reclaimed fill.
Free field sites on hard rock in Road Town on Tortola, and Spanish Town on Virgin
Gorda are in the course of being instrumented. All instrumentation will be transmitted in
real time by Internet to PRSMP.
Acknowledgements
We would like to thank the PRSN and PRSMP personnel, technicians, administrative
staff and students, who have made immense contributions to the success of both
networks. The figures in this paper were made with the Generic Mapping Tools (GMT)
software (Wessel and Smith, 1991), and ZMAP (Wiemer, 2001). We thank Stefan
Wiemer for his comments and suggestions. Sue Hough provided helpful comments that
greatly improved the quality of the manuscript. The authors gratefully acknowledge GNU
software developers. The development of moment tensor solutions at PRSN was funded
by the SeaGrant Program (R-122-2-04).
References
Aponte, A., R. Ramos, and J. Martinez-Cruzado (2000). Mapping of soil classification of
the San Juan Metropolitan Area. Department of Civil Engineering and Surveying,
University of Puerto Rico at Mayagüez.
Asencio, E. (1980). Western Puerto Rico Seismicity, USGS Open File Report 80-192.
Capacete, J. L. and A. Herrera, (1972). The 1918 earthquake; An Engineering Study.
CIAA, Vol. XXII, No. 1. p41-48. Puerto Rico.
15
Carbó, A., D. Córdoba, J. Martín Dávila, U. ten Brink, P. Herranz, C. von Hillebrandt, J.
Payero, A. Muñoz Martín, A. Pazos, M. Catalán, J. L. Granja, and M. Gómez (2005).
Survey Explores Active Tectonics in Northeastern Caribbean. Eos, Vol. 86, No. 51.
Clinton, J. F., (2004). Modern digital seismology - instrumentation, and small amplitude
studies in the engineering world. PhD Thesis, California Institute of Technology.
http://resolver.caltech.edu/CaltechETD:etd-05202004-225044 .
Clinton, J. F., S. C. Bradford, T. H. Heaton and J. Favela (2006). The Observed
Wandering of the Natural Frequencies in a Structure, Bull. Seism. Soc. Am., v. 96;
no. 1; p. 237-257.
Eaton, J. P. (1992). Determination of amplitude and duration magnitudes and the site
residuals from short-period seismograph in Northern California, Bull. Seism. Soc.
Am., v. 82. no 2. pp 533-579.
Huérfano, V. (2003). Susceptibilidad de Puerto Rico ante el efecto de maremotos locales,
PhD Thesis, Univ. of Puerto Rico-Mayagüez, Marine Sciences Department.
Huérfano V., and K. Bataille (1995). Crustal Structure and stress regime near Puerto
Rico, Seismic Bulletin: Preliminary locations of earthquakes recorded near Puerto
Rico, Jan-Dec 1994, PRSN, pp 15-19.
Grindlay, N. R., M. Hearne, and P. Mann (2005a). High Risk of Tsunami in the Northern
Caribbean: Research Focuses on Active Plate Boundary Faults and Potential
Submarine Landslides, Eos Trans. AGU, 86(12), 121.
Grindlay, N., P. Mann, J. Dolan and J. P. van Gestel (2005b). Neotectonics and
subsidence of the northern Puerto Rico-Virgin Islands margin in response to the
oblique subduction of high-standing ridges, in Mann, P., editor, Active Tectonics and
Seismic Hazards of Puerto Rico, the Virgin Islands, and Offshore Areas, Geological
Society of America Special Paper 385, p. 31-60.
Gutenberg, B. and C. F. Richter (1944). Frequency of earthquakes in California, Bull.
Seism. Soc. Am. 34, 185-188.
Jansma, P.E., G.S. Mattioli, A. Lopez, C. DeMets, T.H. Dixon, P. Mann, and E. Calais
(2000). Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean,
from GPS geodesy, Tectonics, v. 6, 1021-1037.
Jansma, P. E., and G. S. Mattioli (2005). GPS results from Puerto Rico and the Vigin
Islands: Constraint on tectonic setting and rates of active faulting, in Mann, P., ed.,
Active tectonics and seismic hazards of Puerto Rico, the Virgin Islands, and offshore
areas: Geological Society of America Special Paper 385, pp. 13-30.
16
Johnson, C.E., A. Bittenbinder, B. Bogaert, L. Dietz, and W. Kohler (1995). Earthworm:
a flexible approach to seismic network processing, IRIS Newsletter, 14, 1-4.
Klein, F.W. (2002). User's Guide to HYPOINVRSE-2000, a Fortran Program to solve for
Earthquake Locations and Magnitudes, USGS, Open File Report 02-171, Version 1.0.
LaForge, R.C., and W.R. McCann (2003). A seismic source model for Puerto Rico, for
use in probabilistic ground motion hazard analysis, in Mann, P., ed., Active tectonics
and seismic hazards of Puerto Rico, the Virgin Islands, and offshore areas: Geological
Society of America Special Paper 385, pp. 223–248.
Llavona, A. (2004). Clasificación de Suelos (UBC-97) del Municipio de Mayagüez,
Master of Science thesis, University of Puerto Rico, Mayagüez Campus, Mayagüez.
Macari, E. J. (1994). A Field Study in Support of the Assessment for Liquefaction and
Soil Amplification in Western Puerto Rico, Puerto Rico Earthquake Safety
Commission.
McNamara, D., J. McCarthy and H. Benz (2006). Improving earthquake and tsunami
warnings for the Caribbean Sea, the Gulf of Mexico, and the Atlantic coast, USGS
Fact Sheet FS2006-3012 http://pubs.er.usgs.gov/usgspubs/fs/fs20063012 (last
accessed April 2006).
Malone, S. (2000). Electronic seismologist : Network operations transitions to Antelope
at the Nevada Seismological Laboratory, SRL, .71, n. 4, pp 444-448.
Mendoza C., and V. Huérfano (2005). Earthquake Location Accuracy in the Puerto RicoVirgin Islands Region, SRL, v. 76, n. 3, pp 356-363.
Mercado, A., and W. R. McCann (1998). Numerical simulation of the 1918 Puerto Rico
tsunami, Natural Hazards, v.18, 57-76.
Moya, J. C. and W. R. McCann (1992). Earthquake Vulnerability Study of the Mayagüez
Area, Western Puerto Rico, Comision de Seguridad Contra Terremotos.
Mueller, C., A. D. Frankel, M. D. Petersen, and E. V. Leyendecker (2003).
Documentation for 2003 USGS Seismic Hazard Maps for Puerto Rico and the U. S.
Virgin
Islands
http://earthquake.usgs.gov/hazmaps/products_data/Puerto-RicoVI/prvi2003doc.html (last accessed Jan 2006).
Odum, J., R. Williams, W. Stephenson, C. von Hillebrandt, E. Ascencio, and A. Cameron
(2006). Near-surface S- and P-wave velocities of geological formations within urban
areas and PRSN stations, Puerto Rico. United States Geological Survey Open File
Report (in preparation).
Pacheco, J. F., and L. R. Sykes (1992). Seismic moment catalog of large earthquakes,
1900 to 1989, Bull. Seismol, Soc. Am., v. 82, no. 3, pp. 1306-1349.
17
Pavlis, G. L., F. L. Vernon, D. Harvey, and D. Quinlan (2004). The generalized
earthquake location (GENLOC) package: A modern earthquake location library,
Computers in Geosciences 30:1079-1091.
Prentice, C. S., P. Mann, and G. Burr (2000). Prehistoric earthquakes associated with a
Late Quaternary fault in the Lajas Valley, southwestern Puerto Rico, EOS,
Transactions of the American Geophysical Union, v. 81, F1182 (abstract).
Prentice, C. S. and P. Mann (2005). Paleoseismic study of the South Lajas fault: First
documentation of an onshore Holocene fault in Puerto Rico, in Mann, P., ed., Active
tectonics and seismic hazards of Puerto Rico, the Virgin Islands, and offshore areas:
Geological Society of America Special Paper 385, pp. 215–222.
PRSN Historical Catalogue,
accessed March 2006).
http://temblor.uprm.edu/~victor/PRSN/history/
(last
Randall, G. E., C. J. Ammon and T. J. Owens (1995). Moment tensor estimation using
regional seismogams from a Tibetan Plateau portable network deployment, GRL, 22,
pp 1665-1668.
Reid, H.F. and L. Taber (1919). The Porto Rico Earthquake of 1918 with descriptions of
earlier earthquakes. Report of the Earthquake Investigation Commission, Document
No. 269, U.S. House of Representatives, 66th Congress, 1st Session.
Shepherd, J.B. and L.L. Lynch (1992). An earthquake catalogue for the Caribbean Part I,
the pre-instrumental period 1502-1900. Report submitted to the Executive Committee
of the Latin American and Caribbean Program of Seismic Vulnerability, 59pp.
Sokolowski, T. J. (2002). Automatic earthquake processing at the US West Coast/Alaska
Tsunami Warning Center, Abstract, Cordilleran Section, AGU.
ten Brink, U.S. and J. Lin (2004). Stress interaction between subduction earthquakes and
forearc strike-slip faults: Modeling and application to the northern Caribbean plate
boundary, J. Geophys. Res., v. 109.
UNESCO, 2005. Workshop Report No. 199, International Conference for the
Establishment of a Tsunami and Other Coastal Hazards Warning System for the
Caribbean and Adjacent Regions, Intergovernmental Oceanographic Commission.
von Hillebrandt, C. and V. Huérfano (2006). Emergent tsunami warning system for
Puerto Rico and the Virgin Islands, in Caribbean Tsunami Hazard, A. MercadoIrizarry and P. Liu, eds., World Scientific, pp. 231-243.
18
von Hillebrandt, C. and K. Bataille (1994). PRSN formulas for the calculation of
magnitude; Puerto Rico Seismic Network Bulletin: Preliminary Location of
earthquakes recorded near Puerto Rico, Jan-Dec 1993, pp 36-37.
Wald, D. J., B. C. Worden, V. Quitoriano and K. L. Pankow (2005). ShakeMap manual:
technical manual, user’s guide, and software guide: U.S. Geological Survey
Techniques and Methods, book 12, section A, chap. 1, 132 p.
Wessel, P. and W. H. F. Smith (1991). Free software helps map and display data, EOS
Trans. AGU, 72, 441 & 445-446.
Wiemer, S. (2001). A Software Package to Analyze Seismicity: ZMAP. Seismological
Research Letters, Vol. 72, 373-382, 2001.
Whitmore, P.M. and T.J. Sokolowski (2002). Automatic earthquake processing
developments at the U.S. West Coast/Alaska Tsunami Warning Center, in Recent
Research Developments in Seismology, Transworld Research Network, Kervala,
India, 1-13.
Zahradnick, J., J. Jansky and K. Papatsima, (2001). Focal mechanisms of weak
earthquakes from amplitude spectra and polarities, Puer and Appl. Geophysics., 158,
pp 647-665.
19
Figure 1: Map of northeast Caribbean, showing major tectonic structures and
approximate locations for damaging earthquakes in recent history. GPS
displacement vectors are from Jansma et al. (2000). [USVI: US Virgin Islands; BVI:
British Virgin Islands; Cul Is.: Culebra, PR; Vq Is.: Vieques, PR; PC: Punta Cana;
SM: Samana; SLF: South Lajas Fault; GSPRFZ: Great Southern Puerto Rico Fault
Zone; GNPRFZ: Great Northern Puerto Rico Fault Zone]
20
Figure 2: Data Acquisition and Processing at PRSN / PRSMP
21
Figure 3: Stations with real-time continuous monitoring at PRSN. All are
operated by PRSN except the IRIS station SJG and the PRSMP 18-bit strongmotion stations. 3 joint BB and SM stations will be installed in locations
outside the map: on Mona Island, and at Punta Cana and Samana in the
Dominican Republic (see Fig 1). Currently short-period stations IDE and CSB
are out of operation. [BB: broadband sensor; SM: strong-motion sensor]
22
Figure 4: Strong-Motion stations in Puerto Rico monitored by PRSMP. Additional
details of the dense instrumentation in three largest towns in Puerto Rico, San Juan,
Ponce and Mayagüez, is shown.
23
Figure 5: Seismicity within PRSN local reporting region between January 1986 and
March 2006. The largest event in this time period was a M5.6 located near Anegada,
BVI. There were 9 events with M>5.0 (blue stars). The most recent large event was a
M5.3 on 2 March 2006, about 100km NE of Anegada. Black stars indicate large
historical events (labeled in Figure 1).
24
(a)
(b)
(c)
(d)
Figure 6: Analysis of the PRSN Catalogue (Jan 1986 - March 2006) (a) number of
events over time, in 3month bins; (b) number of events in 0.1 magnitude bins; (c)
number of events in 5km depth bins; (d) Gutenberg-Richter relation.
25
Figure 7: Geographical distribution of the magnitude of completeness, MC, for the
PRSN Catalogue for the reporting region (Jan 1986 – Mar 2006). Colorbar units are
magnitude of completeness, MC. MC is below Md2.5 for a significant portion of the
center and southwest of the island, and is at least Md3.3 throughout the island of
Puerto Rico. As distance from the islands increases, the magnitude of completeness
degrades, in some regions being greater than Md3.7.
26
Figure 8: Density of seismicity in the reporting region, PRSN catalogue, January
1986 – March 2006. Colorbar is in units of log(earthquakes per km2). Local
seismicity is concentrated in southwest Puerto Rico (south of the Great Southern
Puerto Rico Fault Zone); the region between the Puerto Rico trench and the 19o
Fault Zone; and the Sombrero seismic zone (see Figure 1 for fault locations).
27
San Juan
Station
ID
B02L
Etna Free
field
SJ05
# K2
sensors
15
Minillas Government
Center
San Juan
B01L
SJ05
6
Highway Overpass
Mayagüez Viaduct
Mayagüez
B01R
MY09
24
Highway Overpass
Ponce By-Pass
Ponce
B02R
PN10
12
Highway Bridge
Paso del Indio
Vega Baja
B03R
VGB1
24
Dam
Lucchetti
Yauco
D01M
YAC1
12
Dam
Carraizo
Trujillo Alto
D02M
TA02
9
Dam
La Plata
Toa Alta
D03M
TOA2
12
Structure
Name
Location
High Rise Apartment
Plaza Inmaculada
High Rise Building
Table 1: Summary of Instrumented Structures Operated by PRSMP. Locations for
the sensors can be found in Figure 4.
28