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 plates has been and continues to be accommodated by slip on a

_______________________________________________ 1 Department of Earth Sciences, Quaid-i-Azam University,

Islamabad. 2 College of Earth & Environmental Sciences, University of Punjab,

Lahore.

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 the earthquake events within this region are extracted from

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

66 Seismotectomics and Seismic Hazard Analysis in Selected Area of Margala Hills

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.

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).

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:

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.

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.

Gokina

Forest BoundaryNilan NMargala

Kotla

Pir Suhawa

Study Area

. 4484

.4525

.4174

Sara

Road

N

Scale

0 1000 m

Figure 1- Location map of the study area.

Zulfiqar et al. 67

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.

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.

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 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

Figure 3- Earthquake activities since 1990 to present.

Figure 4- Seismic activities in 2005.

Zulfiqar et al. 69

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.

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.

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.

SEISMIC HAZARD ANALYSIS

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 of

P EAK G R O U N D V ELO C IT Y R O C K

0

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 0 0

R e tu rn P e rio d (Ye a rs )

PGA

(Gal

s)

Figure 5- Peak ground acceleration of soil.

Figure 6- Peak ground acceleration of rock.

020406080

100120140160180200

10 100 1000 10000 100000Return Period (Years)

PGV (cm /se

Figure 7- Peak ground velocity of soil.

PEAK G RO UND VELO CIT Y RO CK

0

20

40

60

80

100

120

140

10 100 1000 10000 100000

R etu rn P erio d (Years )

PGA

(Gal

s)

Figure 8- Peak ground velocity of rock.

70 Seismotectomics and Seismic Hazard Analysis in Selected Area of Margala Hills

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:

Building Height Typical Natural Period

2 story 0.2 seconds 5 story 0.5 seconds 10 story 1.0 seconds 20 story 2.0 seconds 30 story 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.

MAXIMUM CREDIBLE EARTH (MCE) ANALYSIS

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

Figure 9- Reoccurrence intervals.

0

200

400

600

800

1000

1200

1400

1600

1800

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Fundamental Period of Structure (Seconds)

Peak

Gro

und

Acc

eler

atio

n (P

SA)

PSA1 200 yPSA2 500 yPSA3 1000 yPSA4 2000 y

Figure 10- Response spectra of soil.

0

200

400

600

800

1000

1200

1400

1600

1800

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Fundamental Period of Structure (Seconds)

Peak

Gro

und

Acc

eler

atio

n (P

SA)

PSA1 200 yPSA2 500 yPSA3 1000 yPSA4 2000 y

Figure 11- Response spectra of rock.

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 R PGA PGV(Rock) PGV(Soil)

Low High PGV PGA Low High Low High Low High (km) (km) (km) (g) (g) (cm/sec) (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 17 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 80.4 0.029 0.038 2.095 3.682 3.098 5.446

Maximum Credible Pseudo AccelationResponse Spectrum - Soil Site

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5Period (Sec)

PSA

(g)

PSA(Low)PSA(High)

Figure 12- Maximum credible pseudo acceleration response spectrum of soil site.

Maximum Credible Pseudo AccelationResponse Spectrum - Soil Site

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5Period (Sec)

PSA

(g)

PSA(Low)PSA(High)

Figure 13- Maximum credible pseudo acceleration response spectrum of rock site.

Maximum Credible Pseudo Relative Velocity Response Spectrum - Rock Site

0

50

100

150

200

250

0 1 2 3 4 5Period (Sec)

PSR

V (c

m/s

ec)

PSV(Low)PSV(High)

Figure 14- Maximum credible pseudo velocity response spectrum of rock site.

Maximum Credible Pseudo Relative VelocityResponse Spectrum - Soil Site

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5Period (Sec)

PRSV

(cm

/sec

)

PSA(Low)PSA(High)

Figure 15- Maximum credible pseudo velocity response spectrum of soil site.

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. Literature review together with this study shows that

active or likely to be active faults are located close to the area on regional scale.

2. Figure 4 shows that major epicenter of event magnitude 7.6 occurs in the area.

3. Thickness of alluvium over bedrock at Margalla Hills is shallow and structure will be placed possibly on rock.

4. Seismic design parameters can be used for designing earthquake resistance buildings and structures.

5. 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.1041-1057

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. 87-97. 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).