Pakistan Journal of Hydrocarbon Research
Vol.15, (June 2005), p.65-72, 15 Figs., 1 Table
Seismotectonics and Seismic Hazard Analysis in Selected Area of
Margala Hills, Islamabad
Zulfiqar Ahmad1, Iftikhar Ahmad2, Gulraiz Akhtar and Shazia Asim1
ABSTRACT
Margala Hills near Pakistan's Capital, Islamabad is an
area where lot of recreational and hotelling activities
are planned to construct on its Northern slopes. This
paper investigates seismotectonics and seismic risk
assessment for the future development in the light of
Oct 8, 2005 earthquake that caused a huge damage of
human life and property in the effected areas. It
registered 7.6 on the moment magnitude scale making
it a major earthquake. With the help of recent data,
geological interpretations were carried out and high
seismic areas have been identified.
Computer modeling studies with the use of historic
and recent seismic data provided peak ground
acceleration, peak ground velocity, response spectra
and maximum credible earthquake of the investigated
area. This study indicated that Peak Ground
Acceleration (PGA) for soil and rock is 258 gals (0.26 g),
Peak Ground Velocity (PGV) for soil is 31.70 cm/sec and
for rock 21.69 cm/sec. Response Spectra Acceleration
(200 year return period) for soil site= Max PSA 755 gals
occurred at period of 0.2 secs; for rock site = Max PSA
777 gals occurred at period of 0.15 secs. Maximum
Credible Earthquake (MCE) includes both the low and
high values of Peak Ground Acceleration (PGA) in the
context of Main Boundary Thrust (MBT) are 0.46g and
1.02g respectively. Recommendations were made for
the earthquake resistant design of structures and
buildings in the vicinity of Margala Hills.
INTRODUCTION
Earthquakes are caused by different reasons. They may
be caused by sudden slip on faults or movement/grinding of
tectonic plates or due to volcanic activities. Causes of
earthquakes and active faults in northern Pakistan are
associated with the movement of the Indian Plate
northwardly at a rate of about 40 mm/yr and colliding with
the Eurasian continent. This collision is causing uplift of
mountains. As a result it produces the highest mountain
peaks in the world including the Himalayan, the Karakoram,
the Pamir and the Hindu Kush ranges.
As the Indian plate moves northward, it is being
subducted or pushed beneath the Eurasian plate. Much of
the compressional motion between these two colliding
_______________________________________________
1
2
Department of Earth Sciences, Quaid-i-Azam University,
Islamabad.
College of Earth & Environmental Sciences, University of Punjab,
Lahore.
plates has been and continues to be accommodated by slip
on a suite of major thrust faults that are at the Earth's
surface in the foothills of the mountains and dip northward
beneath the ranges. These include the Main Frontal Thrust,
the Main Central Thrust, the Main Boundary Thrust, and the
Main Mantle Thrust.
OBJECTIVE OF THE STUDY
The destruction caused by the recent earthquake is a
reminder, that development and construction in disregard of
environmental concerns can wreak havoc and cause
immense loss of life and property. The main objective of the
study includes the followings:
 Determine geological and tectonic setting of the area.
 Collect up-to-date historical earth quake data and its
processing with reference to Oct 8 Earthquake.
 Determine historical seismicity.
 Perform Seismic Hazard studies.
 Estimate Maximum credible Earthquake and Peak
Ground Acceleration and velocities.
LITERATURE REVIEW
The earth surface consists of a number of large intact
blocks called plates (Kramer 1996). Historic earthquake
data indicated that most of the earthquakes occur along
plate boundaries. Kramer 1996, presented procedures to
perform seismic hazard analysis. These methods include
deterministic and probabilistic approaches. The theory of
elastic rebound (Reid, 1911) describes process of
successive buildup and release of strain energy in the rock
adjacent to faults. Allen (1975) studied geological criteria for
evaluating seismicity. Joyner and Boore 1991, presented
procedures for estimating strong earthquake ground motion
and engineering design. Seismic design codes and
procedures were described by Berg (1983). Bolt (1989)
presented different aspects of earthquakes with necessary
details. In Pakistan, Geological Survey of Pakisan and
WAPDA carried out seismic studies. WAPDA estimated
acceleration g factor for the design of different large dams.
These analyses are available in the published feasibility
reports (1967-2005) of dams such as Kalabagh, Tarbela,
Mangla Raising and Akhori Dams.
METHODOLOGY
Seismic hazard analysis may be carried out with the help
of Long term historic earthquake data including all major
events in the region. An area of influence usually 150-200
square km would be marked from the point of interest and
66
Seismotectomics and Seismic Hazard Analysis in Selected Area of Margala Hills
the earthquake events within this region are extracted from
the historic data. The tectonic boundaries and fault
alignments, faults lengths and their orientation may be used
in the calculation procedure.
Computer programs SEISM and EQRISK originally
developed by McGuire (1976 & 1985) and modified by GTZ
German Technical Cooperation (1991). Programs have
been used to perform the seismic risk analysis. The seismic
parameters obtained from this analysis can be used in the
man-made facilities constructed in this region to avoid
collapsing of structures during high magnitude earthquakes
and to minimize the loss of life.
Precambrian to Paleozoic rocks
These are exposed in Hazara Mountains situated in the
north part of Main Boundary Thrust (MBT). The main part of
the Dor River is composed of these rocks. Precambrian
rocks consist of low-grade slate with intercalation of
limestone layers. Paleozoic rocks consist of sandstone,
shale, conglomerate, limestone and dolomite.
Mesozoic Rocks and Eocene to Paleocene Series
These are exposed in Hazara and Margala hills
occupying the northern to central part of the area.
They are in general black, hard and compact shale that
are highly eroded along the bedding planes.
The main part of the Haro River basin consists of these
layers.
LOCATION OF STUDY AREA
Study area is located in the Northern slopes of Margallah
Hills and presented in figure 1. It is located about 25 km
from Islamabad and about 2 km from Pir Sohawa. The area
falls in District Haripur of the North Western Frontier
Province (NWFP).
Miocene to Pliocene Series
These sequences cover the southern part of the area.
The rocks in this category consist of alternation of shale and
sandstone. This series can be divided into two groups. One
is Murree group of Miocene age and other is Siwalik group
of Pliocene age. Rocks in these groups are generally weak,
highly weathered and not able to endure erosion well.
Among them, the shale is highly sheared and has turned
into very weak red clay. The Soan river basin is composed
of these layers.
REGIONAL GEOLOGICAL SETTINGS
Margala hills are a part of geosynclinal trough known as
Indo-Gangetie synclinorium with an ENE-WSW axial trend.
In the area various geological formations are exposed and
they widely range from Precambrian to Holocene in age.
STRATIGRAPHY
Based on the previous and present studies, the
stratigraphy of the area is described below:
N
.4174
Kotla
.4525
Study Area
Pir Suhawa
Gokina
Margala
Nilan N
Road
. 4484
Forest Boundary
Figure 1- Location map of the study area.
Sara
0
1000 m
Scale
Zulfiqar et al.
QUATERNARY SYSTEM
Quaternary system that widely distributes on Potwar
Plateau is composed of highly cemented conglomerate
named Lei Conglomerate and unconsolidated Alluvial
deposits. Lei conglomerate is composed of highly
consolidated gravel layers by the deposition of calcium
carbonate. It has no specific horizon and sporadically
distributed with several tens of meter in maximum
thickness. Alluvial deposits consist of unconsolidated silt,
sand, gravels and boulders and they bear main aquifers in
the study area. Their maximum thickness is supposed to be
more than 150 m. Because materials become coarser
toward the mountains, moderate yield aquifers are
distributed along the foot of the mountains. Few of the
shallow (50 m) and deep-seated (122 m) water wells were
reported to be pumping muddy water in the Islamabad
watershed after the event of October 08, 2005 earthquake.
GEOLOGICAL STRUCTURE
The bedrocks in the study area are highly folded, faulted
and over thrusted because of Himalayan uplift during
Pliocene epoch. The deformational axes are running in
ENE-WSW direction. Among the many deformational units,
MBT is the major fault. It has considerably wide fractured
zone accompanied with many derivative faults and
moreover some epicenters of earthquake have
concentrated along certain part of this fault. Figure 2 shows
the tectonic features of the area.







67
Main Boundary Thrust (M.B.T.) in ENE-WSW trend
Margala Fault in ENE-WSW trend
Hazara Thrust
Panjal Thrust
Jhelum Fault in N-S trend
Manshera Thrust
Murree Thrust
SEISMO-TECTONICS FEATURES
Areas in Margalla hills are an intensely deformed and
tectonised belt which along-with the Attock-Cherat range
and Kala Cheta range represents the uplifted southern
margin of Peshawar basin. It is part of active Himalayan
fore land – fold and thrust belt region in the collision zone
between Indo-Pakistan and Eurasian plates. The plate
boundary is characterized both by northward under
thrusting plate margin and southward abduction of upper
crustal rocks and sediments.
This has resulted in dramatic horizontal tectonics and
crustal shortening since initial collision of the Indian Plate
with the Kohistan
Island Arc in latest Cretaceous to middle Eocene
developing local micro faults and thrusts. This zone could
be termed a source of earthquakes.
Continental thrust transferred or distributed southward to
a zone of weakness defined as Main Boundary Thrust
(MBT) that also represents the frontal thrust of Margalla
Hills and Kala Chitta. However, the deformation within
these hills fold and thrust belt predates its thrusting to south
over Kohat-Potwar plateaus. The area constitutes an
Figure 2- Tectonic features of the study area.
68
Seismotectomics and Seismic Hazard Analysis in Selected Area of Margala Hills
allocthonous wedge of deformed sedimentary rocks. The
wedge propagated southwardly rapidly during the Pliocene
and Quaternary times over a lower detachment surface that
is rooted out in Attock-Cherat Range and is responsible for
major regional over thrusting.
SEISMIC HAZARD STUDIES
For the seismic hazard evaluation, the seismotectonic
setting of the investigated area associated earthquake
potential and related ground was studied. The
seismotectonic framework, the motions are described in this
section.
HISTORICAL SEISMICITY
Investigations were made to determine the seismic
conditions at study area with the use of historic seismic
data. Figure 3 shows the earthquake activities since 1990 to
present (Sources USGS website). Several events of 33 km
depth were found near the study area.
Figure 4 shows the seismic activities in 2005. Major
events were found near Islamabad during 2005.
Figure 3- Earthquake activities since 1990 to present.
ESTIMATION OF SEISMIC DESIGN PARAMETERS
The following seismic design parameters have selected
and computer based studies were carried out for their
estimation.
 Peak Ground Acceleration (PGA) – It is highest pulse of
ground acceleration during an earthquake and can be
related to structural design parameters.
 Peak Ground Velocity (PGV) – It can be indirectly used to
evaluate seismic stresses in structural analysis.
 Design Spectra – It is used to calculate the seismic
loading on structures like high rising buildings and one of
the main tools for a final structural design.
 Maximum Credible Earthquake (MCE) – It is an estimate
of upper-bound earthquake in the study area and assumed
to be the worst possible earthquake intensity that can occur
in the area.
INSTRUMENTAL EARTHQUAKE RECORD
The instrumental recording of earthquakes started in
1904. For the present seismic studies, two classes of
instrumental earthquake data have been studied. The first
one is based upon earthquakes recorded by regional
seismic networks and the other is compiled from local
network data catalogue. The regional data was compiled
from earthquake listings of International Seismological
Centre (ISC) England, National Earthquake Information
Services (NEIS) of US Geological Survey (2005) and
Geophysical Centre, Quetta.
SEISMOTECTONIC MODEL
From the tectonic and seismic data, the understanding
about the seismotectonic set up of the study area can be
developed. The main seismogenic features that are
Figure 4- Seismic activities in 2005.
responsible for seismic hazard in Islamabad and Margala
hills include:
 Main Boundary Thrust (M.B.T.) in ENE-WSW trend
 Margala Fault in ENE-WSW trend
 Hazara Thrust
Zulfiqar et al.
Panjal Thrust
Jhelum Fault in N-S trend
Manshera Thrust
Murree Thrust
A few other faults are located in the north but at a greater
distance from the study area.
PEAK GROUND VELOCITY ROCK
140
120
100
PGA (Gals)




69
80
60
40
MAIN BOUNDARY THRUST (MBT)
The Main Boundary Thrust (MBT) is a long feature
extending for several hundred kilometers (about 270 Km)
along the Himalayan front. West of the Hazara-Kashmir
syntaxes, it takes several bends and is concealed under the
alluvial sediments at many places and therefore its
structural continuity cannot be established. It passes at a
closest distance of about 1 km from Margala hills.
20
0
10
100
1000
10000
100000
Return Period (Years)
Figure 5- Peak ground acceleration of soil.
PANJAL- KHAIRABAD FAULT
The Panjal-Khairabad fault is passing north at a distance
of about 26 km.
HAZARA THRUST FAULTS SYSTEM
The three branches of the Hazara thrust fault system are
present in the Margala hills. The nearest trace of this fault is
at a distance of about 15 km from the area. These are
active tectonic features.
It is import to design the buildings/ structures to bear the
shocks of major earthquake without any significant damage.
It requires estimation of seismic design parameters.
Acceleration has this important influence on damage,
because, as an object in movement, the building obeys
Newton' famous Second Law of Dynamics.
F = ma
This states the Force (F) acting on the building is equal to
the Mass (m) of the building times the Acceleration (a).
Therefore acceleration plays an important role to generate
forces on the buildings. The following quantitative
parameters called the seismic design parameters have
been estimated.
The historic long-term instrumental data of earthquake
from 1900 to Oct 2005 have been used to perform seismic
hazard analysis. Joyner and Boore, 1991 attenuation
relationships used to calculate the peak ground acceleration
(PGA), peak ground velocity (PGV), and design spectra
(PSA) at various risk levels. Computer programs SEISM
and EQRISK (McGuire 1976 & 1985) were used to compute
long-term simulations.
Peak ground acceleration is the highest pulse of ground
acceleration during an earthquake and widely used as a
numerical value of the punch of an earthquake. Figures 5
and 6 present the peak ground acceleration [in Gals =
cm/sec2] for different return periods in soil and rock sites
respectively. Peak ground velocity (PGV) is the highest
velocities that can occur by an earthquake. It is used to
evaluate seismic stresses. Figure 7 presents peak ground
velocities in the study area. Similar calculations were made
for rock sites and shown in figure 8. Reoccurrence interval
Figure 6- Peak ground acceleration of rock.
200
180
160
P
140
G
V 120
(c 100
m/ 80
se 60
c)
40
20
0
10
100
1000
10000
100000
Return Period (Years)
Figure 7- Peak ground velocity of soil.
PEAK GROUND VELOCITY ROCK
140
120
PGA (Gals)
SEISMIC HAZARD ANALYSIS
100
80
60
40
20
0
10
100
1000
10000
100000
Return Period (Years)
Figure 8- Peak ground velocity of rock.
70
Seismotectomics and Seismic Hazard Analysis in Selected Area of Margala Hills
of earthquakes of different magnitude explains that how
many times a certain magnitude of earthquake may occur in
a year or what will be the return period of an earthquake.
Reoccurrence interval of earthquake in the study area is
determined and presented in figure 9.
RESPONSE SPECTRA
Spectral acceleration is approximately what is
experienced by a building, as modeled by a particle on a
massless vertical rod having the same natural period of
vibration as the building. The building's natural period is the
inverse of the frequency; whereas the frequency is the
number of times per second that the building will vibrate
back and forth, the period is the time it takes for the building
to make one complete vibration.
This means that a short building with a high natural
frequency also has a short natural period. Conversely, a
very tall building with a low frequency has a long period.
The table given below indicates representative range of
building heights and natural periods:
Typical Natural Period
0.2 seconds
0.5 seconds
1.0 seconds
2.0 seconds
3.0 seconds
Design spectra are used to calculate seismic loading on
structure/buildings. Using the same computer programs
response spectra was computed for soil and rock sites and
shown in figures 10 and 11.
Figure 9- Reoccurrence intervals.
Peak Ground Acceleration (PSA)
(Gals)
Building Height
2 story
5 story
10 story
20 story
30 story
1800
1600
PSA1 200 y
1400
PSA2 500 y
1200
PSA3 1000 y
1000
PSA4 2000 y
800
600
MAXIMUM CREDIBLE EARTH (MCE) ANALYSIS
400
200
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Fundamental Period of Structure (Seconds)
Figure 10- Response spectra of soil.
Peak Ground Acceleration (PSA)
(Gals)
Instead of calculating seismic design parameters for
various return periods or probabilities, it is more pragmatic
to evaluate them with the help of a Maximum Credible
Earthquake (MCE) that may occur in an area. The MCE is
the largest reasonably conceivable earthquake that appears
possible along a recognized fault or within a geographically
defined tectonic framework. It is an upper-bound
earthquake of an area that can be calculated using the
seismic sources present in that area. The MCE is calculated
with the seismic sources and their maximum earthquake
events. On the basis of Maximum Credible Earthquakes,
Peak Ground Acceleration (PGA), Peak Ground Velocity
(PGV) have been calculated using computer program
EQRISK and summary results provided in table 1.
Similarly response spectra have been calculated on the
basis of MCE analysis. Figure 12 through figure 15 shows
Maximum Credible Pseudo Acceleration and Velocity for
soil and rock sites.
Recommended Seismic Design Parameters
Ground motions that will characterize the Maximum
Design Earthquake (MDE) and Operational Basis
Earthquake (OBE) for the site specific area located at
Margalla Hill have been evaluated on the basis of the
historic seismic data and maps. Based on the simulated
results as shown in different graphs and computer outputs,
the following design parameters have been recommended
1800
1600
PSA1 200 y
1400
PSA2 500 y
1200
PSA3 1000 y
1000
PSA4 2000 y
800
600
400
200
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Fundamental Period of Structure (Seconds)
Figure 11- Response spectra of rock.
4.5
Zulfiqar et al.
71
Table 1. Summary results of maximum credible earthquake, peak ground acceleration and peak ground
velocity.
MAXIMUM CREDIBLE EARTHQUAKES
[ PGA & PGV ]
For Source at Islamabad Buildings/structures
Sources
Magnitude
r0
Low High
R
PGA
PGV
PGA Low
(km)
(km)
(km)
(g)
PGV(Rock)
High
Low
(g)
(cm/sec)
PGV(Soil)
High
Low
High
(cm/sec) (cm/sec)
(cm/sec)
MBT
6.7
8.2
1
4.12
8.06 0.460 1.018
64.129
348.382
94.854
515.294
Margala Thrust
6.7
7.7
0.5
4.03
8.02 0.463 0.786
65.629
202.811
97.072
299.980
Hazara Thrust
6.7
7.5
15
15.52
0.206 0.315
15.909
39.231
23.530
58.027
Panjal Thrust
6.8
7.7
26
26.31
27.2 0.128 0.206
9.853
27.199
14.573
40.230
Jhelum Fault
7.0
7.5
45
45.18 45.71 0.075 0.098
6.421
11.288
9.498
16.696
Manshera Thrust
6.5
7.3
70
70.11 70.46 0.032 0.049
2.027
4.999
2.999
7.394
Murree Thrust
6.7
7.2
80
80.1
2.095
3.682
3.098
5.446
17
80.4 0.029 0.038
Maximum Credible Pseudo Accelation
Response Spectrum - Soil Site
Maximum Credible Pseudo Relative Velocity
Response Spectrum - Rock Site
3
250
2.5
PSA(High)
2
PSA (g)
PSRV (cm/sec )
PSA(Low)
1.5
1
200
150
100
PSV(Low)
50
0.5
0
PSV(High)
0
0
1
2
3
4
5
0
1
Period (Sec)
3
4
5
Period (Sec)
Figure 12- Maximum credible pseudo acceleration
response spectrum of soil site.
Figure 14- Maximum credible
response spectrum of rock site.
Maximum Credible Pseudo Accelation
Response Spectrum - Soil Site
pseudo
velocity
Maximum Credible Pseudo Relative Velocity
Response Spectrum - Soil Site
3
400
350
2.5
PRSV (cm/sec)
PSA(Low)
PSA(High)
2
PSA (g)
2
1.5
1
0.5
300
250
200
150
100
PSA(Low)
50
PSA(High)
0
0
0
1
2
3
4
5
Period (Sec)
Figure 13- Maximum credible pseudo acceleration
response spectrum of rock site.
0
1
2
3
Period (Sec)
Figure 15- Maximum credible
response spectrum of soil site.
4
pseudo
5
velocity
72
Seismotectomics and Seismic Hazard Analysis in Selected Area of Margala Hills
on 200 year return period with the consideration of the life of
building and structures of 200 year.
Peak Ground Acceleration
For Soil and Rock sites: 258 Gals (0.26 g)
Peak Ground Velocity
For soil site:
31.70 cm/sec
For rock: site: 21.69 cm/sec
Response Spectra Acceleration (200 year return
period)
Soil site: Max PSA = 755 Gals occurred at period = 0.2
sec
Rock Site: Max PSA = 777 Gals occurred at period = 0.15
sec
Maximum Credible Earthquake (MCE)
The low and high values of Peak ground acceleration at
site (MBT case) are 0.46g and 1.02g respectively.
Similarly, the low and high values of Peak ground velocity
at site (MBT case) are 64 and 348 cm/sec (for rock) and 94
and 515 cm/sec (for soil). For maximum credible pseudo
relative acceleration and velocity response spectrum,
graphs in the previous sections are referred.
Maximum Design Earthquake (MDE)
Peak Bedrock Acceleration: 0.26g
These earthquakes may be used as a basis for selecting
appropriate time histories for use in the design. However, It
is recommended to perform detailed probabilistic seismic
hazard analyses to firm up and optimize these
recommended
parameters.
Presently
adequate
seismological and neo-tectonic data is not available for
deduction of realistic probabilistic peak ground acceleration
(PPGA).
CONCLUSIONS
1.
2.
3.
4.
5.
Literature review together with this study shows that
active or likely to be active faults are located close to
the area on regional scale.
Figure 4 shows that major epicenter of event
magnitude 7.6 occurs in the area.
Thickness of alluvium over bedrock at Margalla Hills is
shallow and structure will be placed possibly on rock.
Seismic design parameters can be used for designing
earthquake resistance buildings and structures.
The recommended seismic design parameters are
based on the seismic historic data and its analysis.
Because of the complexity of geology and tectonic in
the area, and non-availability of adequate seismic data,
errors up to 10-15% may be possible.
ACKNOWLEDGEMENT
Mr. M. Iqbal, Principal Geologist at HDIP Islamabad is
acknowledged for critical technical review and suggestions.
REFERENCES
Allen, C.R., 1975, Geological criteria for evaluating seismicity.,
Bulletin of geological society of America, v.86, no.8. p.10411057
Berg, G.V., 1983, Seismic design codes and procedures,
Earthquake engineering research institute, Berkeley, California.
P. 1-9.
Bolt, B.A., 1989, Earthquakes, W. H.Free -man, New York, p. 8797.
GTZ (Pakistan-German Technical Cooperation Program), 1991,
Report on Seismic Hazard analysis for Neelum Jhelum Hydro
Electric Project, p.1-8.
Joyner W., B, and D. M. Boore, 1991, Strong earthquake ground
motion and engineering design, Geotechnical News, v.9, no.1,
p.21-26.
Kramer S. L., 1996, Geotechnical earthquake engineering, Prentice
Hall, New Jersey. P. 106-142.
McGuire R.K., 1976 and 1985, EQRISK, Evaluation of earthquake
risk to site. A computer program, Open file report, US
Department of Interior, USGS, p.67-76.
Reid H. F., 1911, The elastic rebound theory of earthquakes,
Bulletin of the department of geology, University of Berkley, v.6,
p.413-444.
US National EQ information center (NEIC) 2005, Information and
maps available on Earthquakes
WAPDA, 1967-2005, Feasibility reports of various large dams
(geological / seismic hazard analysis volumes).