Focal mechanism solutions and nature of plate movements in Pakistan

JOURNAL OF GEODYNAMICS 5, 331--351 (1986) 331

F O C A L M E C H A N I S M S O L U T I O N S A N D N A T U R E OF PLATE M O V E M E N T S IN PAKISTAN

R. K. VERMA and CH. C H A N D R A SEKHAR

Indian School of Mines Dhanbad, lndia

(Received May 29, 1985; revised version accepted February 12, 1986)

ABSTRACT

Verma, R. K. and Chandra Sekhar, Ch., 1986. Focal mechanism solutions and nature of plate

movements in Pakistan. Journal of Geodynamics, 5: 331-351.

From the seismic point of view, the territory of Pakistan which lies between latitude 2 3 37' N and

longitude 61 75' E is one of the most active zones in the world. The importance of this area lies in

terms of movements of the Indian plate with respect to Eurasia on the west. Seismicity, as well as focal-

mechanism solutions, throws a considerable light on the nature of forces acting in the area. All the

available solutions, along with 12 new ones, have been considered for the present study. Their

relationship to major faults in the area is discussed. The majority of the solutions in the central and

northern parts show strike-slip faulting with a left-lateral sense of motion, followed by thrust faulting;

few show normal faulting. This suggests that the Indian plate is moving with respect to the Eurasian

plate along the Chaman fault, Quetta transverse zone, Sulaiman Ranges and the Hazara thrusts region

joining the Hazara /Kashmir syntaxis. The orientations of P and T axes have been studied. It is seen that

in a large number of cases compressive stress is acting nearly in NNW-SSE to N-S directions. The

Hazara thrust region appears to be the most complex. Here, the influence of the Himalayan thrust front

is evident to a large extent.

The nature of faulting along the C haman fault and Quetta tranverse zone is to some extent similar to

that of the San Andreas fault system of California. So far as the energy release is concerned, the

maximum energy is being released in the form of strike-slip movements close to the Chaman fault and

Quetta transverse ranges.

I N T R O D U C T I O N

The area lying approximately between latitudes 23 ° 37 ° N and longitudes 61 ° 75 ~ E, comprising most of the territory of Pakistan, forms the western part of the India-Pakistan subcontinent and is one of the most active zones in SW Asia from the seismic point of view. The importance of the area lies in the fact that the process of the collision of the Indian plate with Asian plate is taking place there at present. The area is bounded by the alluvial- covered peninsular shield of India on the east, the great Himalayan arc

0264-3707/86/$3.00 (t) 1986 Geophysical Press Ltd.

332 VERMA AND CHANDRA SEKHAR

towards the northeast, the Pamir and Hindukush mountains to the north, the central Afghan mountains to the northwest, the Zagros folded belt on the west and the Arabian Sea on the south. All the major structures in the area, such as the Chaman Fault, Kirthar Ranges, Sulaiman Ranges, Salt Range, Potwar Plateau, Hazara-Kashmir Syntaxis, Nanga Parbat- Haramosh massif, the Main Boundary fault and the Hazara thrusts owe their origin to the processes of collision that have taken place since Early Tertiary times. Figure 1 shows the tectonic setting of the area and the major structures. Geological and seismic evidence has been put forward to show that the Zagros folded belt extends along the Makran coast of Pakistan to the Chaman fault (Jacob and Quittmeyer, 1979). The area forms the western boundary between the Indian and the Eurasian plates, along which large-scale movements have taken place during Tertiary times and are still taking place today, as evidenced by seismicity and focal-mechanism solutions (Abdel Gawed, 1971; Armbruster el al., 1978; Chandra, 198l~ Kazmi, 1979; Jacob and Quittmeyer, 1979; Nowroozi, 1972: Seeber and Jacob, 1977; Seeber and Armbruster, 1979; Verma et al., 1980: and Verma and Chandra Sekhar, 1985).

A number of fault-plane solutions for epicentres along the length of the Himalayan ranges and the Baluchistan arc have been studied by Armbruster e ta l . (1978), Chandra (1978, 1981). Nowroozi (1972) and Quittmeyer et al. (1979, 1984). Recently, Chandra (1981) has summarised the results of focal-mechanism solutions for the eastern Alpide belt, including the Iranian folded belt, the Baluchistan arc, the Himalaya, Assam and Burma. The results of these studies have suggested the underthrusting of the Indian plate along the Main Boundary Thrust (MBT) and the Main Central Central Thrust (MCT) of Himalaya in a northeasterly direction towards the Tibetan Plateau. Another major zone of underthrusting exists at present in the Zagros folded belt of Iran which, according to Quittmeyer and Kafka (1984), extends along Makran coast to the Ornach-Nal zone. Between these two major thrust belts lies the territory of Pakistan, where the nature of faulting appears to be quite different.

According to Armbruster e ta l . (1978), the Main Boundary Thrust (MBT) of the Himalaya extends further in a northwesterly direction towards Hindukush and forms a well-defined seismic zone known as the Indus Kohistan Seismic zone (IKSZ, see Fig. 2). According to their studies, the Hazara thrust system is relatively inactive at present. To the west of the Hazara syntaxial bend, they have found several faults which are very active. These are generally oriented in NE-SW and NW-SE directions. Their composite focal-mechanism solutions suggest that these faults have thrust or strike-slip nature, supporting N-S compression as a result of convergence between the Indian and Eurasian plates.

PLATE MOVEMENTS IN PAKISTAN

65" 7 0 °

~ . . . . .

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T A R I H

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~ PROMINENT STRIKE - SLIP FAULT /4~PROMINENT THRUST } M A J O R TREND

INDUS SUTURE ALLUVIUM / / , MASSIF / / /

Fig. l. Tectonic setting, major geological structures and faults of the area surrounding the territory of

Pakistan, modified after Sarwar and De Jong (1979). Abbreviations: B-Bannu, CF-Chaman fault, GAF-Gant, Alichur fault, HF-Herat fault. HKS-Hazra Kashmir syntaxis, HT-Hazara thrust, K-Karachi, KK-Khuzdar Knot, KP-Kelat Plateau, KPF-Kohistan Patan fault, MBH-Mari Bukti Hills, MMT-Main Mantle Thrust, NPF-Nor th Pamir fault, ONF-Ornach Nal fault, P-Peshawar, PKF-Pamir Karakorum fault, Q-Quetta, RPF-Rushan

Pshart fault, SPF-South Pamir fault, VAF-Vanch Akbaital fault, Z-Ziarat.

334 V E R M A A N D C H A N D R A S E K H A R

6 9 * 71" 73 ° 75"

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PLATE MOVEMENTS IN PAKISTAN 335

Twelve new focal-mechanism solutions have been obtained. The existing data have been interpreted in terms of movements along major faults/blocks which are taking place at present as a result of collision between the Indian and Eurasian plates. It appears that different parts of Pakistan arc moving relative to each other as a result of northward movement of the Indian peninsula.

PHYSIOGRAPHY OF THE AREA

The Himalayan mountains, which have a general NW-SE trend, take a sharp bend near the north of Islamabad and west of Kashmir, forming the Hazara mountains with a general NE-SW trend. This sharp bend is known as the Kashmir or Hazara Syntaxis. To the west of the Hazara mountains lie the Potwar Plateau and the Salt Range. To the south of Peshawar, the Hazara mountains merge with the Sulaiman Ranges which extend for a few hundred kilometers in north-south direction (see Fig. 1). These ranges bend sharply towards the west, near latitude 30 ° N and longitude 70 ° E south of Quetta, formig the Quetta transverse ranges of Sulaiman arc. These ranges further take a sharp bend towards the south and continue along the Kirthar Ranges in a general north-south direction. The Kirthar Ranges again take a sharp bend in east-west direction along the Makran Coast, near about latitude 25°-26° N and longitude 66 ° E. The Makran coast extends upto 58 ° E and merges with the Zagros folded belt west of Hormuz.

SEISMICITY AND ITS RELATIONSHIP TO TECTONICS

Before discussing the focal-mechanism solutions, a brief introduction to the seismicity of Pakistan is presented. A seismicity map of Pakistan, prepared for the period 1951-1975 using the data obtained from National

Fig. 2. Epicentral map of the Pakistan region for the period 1951 1975, based on data from NOAA, U.S.A. Locations of major faults are after Kazmi (1979). Explanation for index: 1. Major faults, active or likely to have been active, 2. Major lineaments observed on aerial photographs, 3. M~ajor thrusts, 4. International boundary of Pakistan. Abbreviations: A-Abbottabad, B-Bela, C-Chaman, DGK-Dera Hhazi Khan, DK-Dera Ismail Khan, IS-Islamabad, J-Jhelum, K-Kalat, M-Multan, MB-Muslimbagh, N-Nakkundi, P-Peshawar, Q-Quetta, S-Sibi, SR-Sulaiman ranges. Faults: CF-Chaman fault, CMF-Chukan Manda fault, CTF-Chiltan Takhatu fault, CWF-Choudhwan fault, DF-Domanda fault, DBF-Dalbandian fault, GF-Gardez fault, GZF-Ghazaband fault, HBF-Hoshab fault, HTS-Hazara thrusts system, IKSZ-Indus-Kohistan seismic zone, JLF-Jhelum fault, KF-Kunar fault, KGF-Kingri fault, KHF-Khalifat fault, MBT-Main boundary thrust, MT-Murree thrust, OF-Ormara fault, ONF-Ornach-Nal fault, PF-Pinjgur fault, SKF-Safed-Koh fault, ZF-Zhob fault.

JOG 5;3-4-6


Oceanic and Atmospheric Administration, U.S.A., is shown in Fig. 2. The epicentres of earthquakes are plotted according to their magnitude on a tectonic map of Pakistan based on Kazmi (1979) and other published literature. For earthquakes the magnitude of which was not available, it was assumed to be 4.0. This is the minimum detectability level for observatories located in the area.

From Fig. 2, it is evident that the entire area of Pakistan is seismically very active. Along the Makran Coast the general tectonic trend is nearly E-W, while the Kirthar Ranges strike nearly in a N-S direction. The east- west trend of the seismicity along the Makran Coast can be explained in terms of subduction of the Arabian plate beneath the Afghan-Lut block of the Eurasian plate (Quittmeyer and Kafka, 1984). The seismicity appears to be scattered along the Kirthar ranges. The Ornach-Nal fault zone, which is located in the central part of the Kirthar Ranges and probably forms the southward continuation of the Chaman fault (Kazmi, 1979), is seismically quite active. The seismicity continues almost up to 31 ° N, 66.7 ° E along the Chaman fault and is oriented nearly in a NNE-SSW direction. This direction changes to almost E-W, following the trend of the Quetta transverse zone (QTZ). This zone has one of the highest concentrations of seismicity in the entire area of Pakistan. In the eastern part of the QTZ, the seismicity changes in direction sharply to nearly N-S, following the direction of strike of the Sulaiman Ranges. North of these ranges lie the Salt Range, which appears to be relatively inactive. The seismicity is concentrated in nearly NE-SW and NW-SE directions along the Hazara thrusts region and along the Main Boundary Thrust (MBT) and its extension follows the tectonic trend in these areas. The NW-SE oriented seismicity continues further along the strike of the Indus Kohistan Seismic Zone (IKSZ), as named by Armbruster et al. (1978). Pocket-type seismicity is observed in the area near latitude 36 ° N and longitude 73-74 ° E, which lies NNE of the IKSZ.

In the Gardez-Kunar fault region, the seismicity follows the strike direction of well-known faults and is oriented in a NE-SW direction. North of this lies a region of very high seismicity which is attributable to the southern part of the Hindukush mountains.

FOCAL MECHANISM SOLUTIONS

For the determination of focal-mechanism solutions, the earthquakes reported by the USGS with magnitudes (mb) near 5.0 and above occurring in the area of Pakistan during the period 1965 to 1978 were considered. Twelve new focal-mechanism solutions (see Fig. 3 and Table 1) were determined from the P-wave first-motion directions, as reported in the Inter- national Seismological Summary Bulletins. These solutions were interpreted


along with those obtained by Nowroozi (1972), Chandra (1978), Quittmeyer etal . (1984) and the composite fault-plane solutions of Armbruster et al. (1978).

The parameters for the epicentres and the focal-mechanism solutions of all the earthquakes (excluding composite fault-plane solutions) are listed in Table 1.

INTERPRETATION OF FOCAL MECHANISM SOLUTIONS

Altogether, 34 earthquake mechanisms and 17 composite fault-plane solutions have been considered for the analysis. The earthquake events listed in Table 1 occurred during the period 1962-1977. Most of the events in the present study have shallow focal depths. For the detailed analysis of focal-mechanism solutions, the entire territory of Pakistan was divided into five parts:

1) Makran Coast and Kirthar Ranges

2) Chaman fault and Quetta transverse zone (QTZ)

3) Bakran region and Sulaiman Ranges

4) Bazara thrust region

5) Gardez-Kunar fault region

The schematic orientations of nodal planes of all the earthquake solutions studied are shown in Figs. 4, 5 and 6. Of the two nodal planes, the one which strikes parallel or nearly parallel to the strike of a nearby fault or a fracture zone was chosen to be the fault plane. At each epicentral location, the horizontal projection of the P or T axis or the strike-slip motion has been represented. An interpretation of the solutions obtained for various areas is discussed below.

MAKRAN COAST AND KIRTHAR RANGES

In this area, four mechanisms have been determined, three of which are shown in Fig. 4. Solutions JQ1 and JQ2 (determined by Jacob and Quittmeyer, 1979) lie nearly 250 km north of the Makran coast, both show normal faulting.

Solution P1 belongs to an earthquake which occurred near the Makran coast, while P2 is for an event which was located to the east of the Ornach- Nal (ONF) fault region. Both the solutions have been obtained in the present study. They suggest normal faulting, the first one with appreciable strike-slip component. The general trend of faults in the area close to P1 is NE-SW. This event may be ascribed to the Ras Malan fault (RMF) or the


a N

1 •

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N

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MAR- 3, 1968 3/,,. 7°N

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. . . : . .

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N

FEB- 2 , 1966 3&.0eN

0 . 9 : 2 0 : 0 9 73-0 ° E

• C O M P R E S S I O N • D I L A T A T I O N

Fig. 3. Focal-mechanism solutions for a few earthquakes in Pakistan obtained during the present study. P and T refer to compression and tension axis. Parameters for solutions are listed in Table I. Com- pressions are indicated by solid circles, dilatations by triangles.

P L A T E M O V E M E N T S IN P A K I S T A N 339

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05:33 : /+7 69"00W

• C O M P R E S S I O N • D I L A T A T I O N

Fig. 3 (continued).

340 VERMA AND C H A N D R A SEKHAR

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INDEX /ACTIVE FAULT

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72°E

Fig. 4. Fault-plane solutions of earthquakes in the areas of the Makran Coast and Kirthar Ranges.

Lineaments and faults are after Kazmi (1979). Prominent faults in the area are: CF-Chaman fault,

HBF-Hoshab fault, KRF-Kir thar fault, NRF-Nai rud fault, ONF-Ornach-Nal fault, PJF-Pinjgur fault.

Ornach-Nal fault. It may be noticed that the sense of motions due to strike- slip component is left lateral.

The event for P2 shows normal faulting, with nodal planes striking in NE-SW and NNW-SSE directions. The latter is nearly parallel to the trend of faults and lineaments in this area. The direction of the tension axis for this event is nearly NW-SE. Jacob and Quittmeyer (1979) have attributed normal faulting (as shown by JQ1 and JQ2) along the Makran coast in terms of down-up tension existing along the subducting lithosphere. The tension axis for their solutions are oriented in a N-S direction, the depth of foci for their solutions is 65-74 km. For solutions P1 and P2, located in Ornach-Nal region, the depths of foci are shallow (less than 30 km). This suggests that normal faulting is taking place in the southern part of the Kirthar ranges. This area has had a history of sedimentation and mountain building during Tertiary times. The events suggest continuation of the same activity up to the present time.

C H A M A N FAULT AND Q U E T T A TRANSVERSE Z O N E

Solution Q1 belongs to an earthquake which occurred in the Chaman fault zone, while solutions CH1, P3, P4, P5, P6 and Q2 belong to events


660 B7 ° Sl l ° B 9 ° 7 0 a 71 °

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[ ~ STR,KE SLIP FAULT,NG ~ THRUST FAULTING

Fig. 5. Fault-plane solutions of earthquakes in the area adjoining the Chaman fault, the Quetta trans-

verse zone, the Bakran region and Sulaiman Ranges. Faults and Lineaments are after Kazmi (1979). Explanation: 1. Lineaments observed on aerial photographs or Landsat imagery, 2. Lineaments observed on Landsat imagery.

Faults discussed in the text: BF-Bakran fault, CF-Chaman fault, CTF-Chiltan Takhatu fault, CWF-Chowdhwan fault, GZF-Gazband fault, HF-Harnai fault, KGF-Kingri fault, KHF-Khalifat fault, KF-Kohlu fault, LF-Loralai fault, MTF-Mekhtar fault, SRFZ-Sulaiman ranges fault zone, TF-Tatra fault.

which have taken place in the QTZ. The location of the epicentres of the above events and the corresponding mechaisms are shown in Fig. 5. Solution P5 suggests thrust faulting, while the rest indicate predominantly strike-slip faulting.

The epicentre of event Q1 is located very close to the Chaman fault, having a strike direction of NNE-SSW. This solution (after Quittmeyer et al., 1979) indicates strike-slip faulting along the fault. The nature of faulting shows that the Chaman fault is a left-lateral fault, which is consistent with the available geological evidence and Landsat Imagery (Abdel-Gawad, 1971). The Chaman fault is known to have ruptured in a large earthquake in 1892, with a left-lateral displacement of 0.75 m (Griesbach, 1893). The length of rupture for this event was nearly 30 km. Events for solutions CH 1, P3, P4, P5, P6 and Q2 are located in the central part of the QTZ. In this region, several east-west trending faults, such as the Khalifat fault (KHF), Kohlu fault (KF) and Harnai fault (HF) meet with the NNW-SSE-trending


69 ° 70 ° 71 o 72" 73o 74.° 75~'E 3 6 ° N

35 °

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69 ° 70 ° 71"

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T H R U S T , - - . . . . INTERNATIONAL

BOUNOARY

33 ° ,72 ° P t t~3o - - 7to 75 °

1 , - - - - - 2 , . . . . . . . . . . . 3

SLIP SOLUTION SOLUTION

Fig. 6. Earthquake mechanisms and the nature of faulting in the area of Gardez, the Kunar and the Hazara thrust region. A1, A2, A3,..., A17 are referred to the composite fault-plane solutions determined

by Armbruster et al. (1978). Tectonic features based on Kazmi (1979). Explanation: 1. Faults, 2. Lineaments observed on serial photographs, 3. Lineaments inferred from composite fault-plane solutions (Armbruster etal. , 1978). Faults discussed in the text: AF-Attock fault. CF-Chaman fault, CBF-Campbellpur fault, GF-Gardez fault, KF-Kunar fault, NF-Nowshera fault,

PBFZ-Peshawar Basin fault zone, SFZ-Shinkiari fault zone, SKF-Safed-Koh fault, TFZ-Tarbela fault

zone.

Loralai lineament (LF). Considering the uncertainty involved in the location of epicentres, the strike-slip motion could be parallel to either of the lineaments/faults mentioned above. However, it is quite probable that the Loralai lineament represents a left-lateral strike-slip fault, as suggested by solutions CH1 in the northern part and P6 in the southern part of this feature. P axis for these events are oriented in a NW-SE direction.

Solutions P3 and P4 could be attributed to either the Khalifat fault or the Kohlu fault, which appear to be strike-slip in nature. The solution P4 is for an event of Aug. 1, 1966 for which Quittmeyer et al. (1984) have obtained a thrust solution (QT-1) from analysis of Rayleigh-wave. However, the


P-wave first-motion data suggest a strike-slip solution, as shown in Fig. 3. It may be noted that the suggested nature of movement along these faults is left-lateral. These faults have probably been developed during late Tertiary times, as a result of movement of the Indian peninsula with respect to Afghanistan, resulting in the formation of the Baluchistan arc (see Fig. 1 ). The event for P5 is located near the intersection of two major lineaments discussed above and represents a thrust solution with the P axis in a NW-SE direction (see Table 1). The significance of this solution is not very clear at present.

Solution Q2 possibly represents a strike-slip motion along an E-W-trending nodal plane, nearly parallel to the Tatra fault (TF), as suggested by Quittmeyer et al. (1979). The P-axis for this event is also direc- ted in a NW-SE direction.

BAKRAN REGION AND SULAIMAN RANGES

In the eastern part of the QTZ, the strike direction of faults and lineaments changes sharply from east-west to NE-SW, as evidenced by the Bakran (BF) and its associated faults. However, several prominent faults, such as the Kingri fault (KGF) and the Chaudhwan fault (CWF), trend in a NNW-SSE direction.

Events for N1, CH2, CH3 are located in the Bakran region. N1 is after Nowroozi (1972) and shows predominantly strike-slip faulting. For this event, the NE-SW trending nodal plane is most likely the fault plane and the nature of movement is left lateral. Events for CH2 and CH3 are located near the same epicentre, on two different dates (as given in Table 1), and indicate thrust faulting along NE-SW-trending nodal planes. Similar is the case for solution CH4, the event for which is located close to BF. P-axis for these events are in a NW-SE direction, almost at right angles to the trend of the major lineaments. It appears that the eastern part of the QTZ is experiencing compression, in contrast to the central and western parts which are experiencing mostly strike-slip movement. The event for P7 is located close to the Kingri fault (KGF). The focal-mechanism solution suggests major strike-slip movement along this fault in a right-lateral sense. This is, however, inconsistent with the direction of motion along the Kingri fault as known geologically (Kazmi, 1979). Alternatively, it is possible that some of the northeasterly trending faults located to the west of the KGF may extend towards the east and the event may be related to one of these faults.

TA

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Foc

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or t

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the

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s.

Sol.

No.

Dat

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1 P

ole

2 P

ole

B A

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P A

xis

T A

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Ori

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at °

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AZ

P

L

AZ

P

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AZ

P

L

AZ

P

L

AZ

P

L

Typ

e of

R

efer

ence

(km

) fa

ulti

ng

2

CH

I A

ug.

1, 1

966

21:0

3:01

CH

2 Ja

n. 2

4, 1

966

07:2

3:10

CH

3 Fe

b. 7

, 19

66

04:2

6:11

CH

4 Fe

b. 7

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1972

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46

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6 M

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63

1:34

:00

JQ1

Nov

, 7,

196

9 18

:34:

04

JQ2

Aug

. 2,

1968

13

:30:

23

N1

Feb.

7,

1966

07

:23:

35

P1

Sep.

1,

1962

15

:00:

58

P2

Oct

. 4,

197

4 22

:24:

33

P3

Aug

. 2,

196

6 05

:41:

33

P4

Aug

. 1,

196

6 20

:30:

57

P5

Aug

. 1,

196

6 19

:09:

55

P6

Mar

, 24

, 19

75

05:3

3:47

3 4

5 6

7 8

9 10

11

12

13

14

15

16

17

18

19

20

30.1

0 68

.60

33

6.0

29.9

0 69

.70

26

5.6

29.9

0 69

.70

10

6.0

30.3

0 69

,90

11

5.8

34.7

0 70

.40

63

5.3

34.6

0 70

.90

22

5.6

33.9

74

.7

44

5.1

27.8

60

.02

74

6.5

27.5

4 60

.92

65

5.7

29.8

0 69

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HAZARA THRUST REGION

The Hazara thrust system lies in between the rivers Jhelum and Indus, and is characterised by a number of thrusts striking nearly in NE-SW to ENE-WSW directions (Fig. 1). Deformation within this zone has been primarily as a result of decollement processes associated with collision between the Indian and Eurasian plates (Gansser, 1964). Some of the major faults and lineaments (such as Attock and Cambellpur) strike nearly in an east-west direction. The northern part of this region is characterised by N-S oriented lineaments and are named as the Tarbela fault zone (TFZ) and the Shinkiari fault zone (SFZ), which lie on the western and eastern sides of the river Indus, respectively (Fig. 6). South of the TFZ (to the west of the river Indus), is the Peshawar Basin fault zone (PBFZ) which is characterised by a number of faults and lineaments oriented in northeasterly direction. The Nawshera fault is a part of this zone. Armbruster e t al. (1978) have studied the microseismicity of the area as a part of the Tarbela dam site investigations. They have shown that microseismicity is associated with steeply dipping faults/lineaments. They have also obtained 17 composite fault-plane solutions from the recorded data. The solutions (numbered A1 to A17) and the faults associated with them are shown in Fig. 6.

It is interesting to note that most of the lineaments/faults trend in a northwesterly direction, while some trend in a northeasterly direction. The composite fault-plane solutions show mostly thrust in the northern part and strike-slip in the southern part. Besides composite fault plane solutions, several solutions have been obtained from teleseismic data. These are discussed below.

The event for solution P9 is located at the northeastern end of the Safed- Koh fault and suggests strike-slip faulting. The NE-SW oriented nodal plane, which is nearly parallel to the Safed-Koh fault, is most probably the fault plane, suggesting left-lateral motion. The direction of the P-axis corresponding to this solution is nearly NW-SE. Solution PN1 is located in the Indus Kohistan seismic zone (IKSZ), north of the Tarbela dam site and corresponds to the Pattan earthquake of 1974. It shows thrust faulting along either of the WNW-ESE-oriented nodal planes (Pennington, 1979). The general trend of seismicity in this area is NW-SE (see Fig. 2). The P-axis for this solution is oriented in NNE-SSW direction, suggesting that the IKSZ is under compression from this direction at present. This solution, along with A4 and A14, supports the extension of the Himalayan thrust front along the Main Boundary Fault of the western Himalaya beyond the Hazara syntaxis in a nothwesterly direction (Armbruster et al., 1978).

Events for solutions P10 and P12 are located in the northern part of the Hazara thrust region and suggest strike-slip faulting. These two events can


be attributed to the lineaments striking in NNE-SSW and E-W directions as mapped by Armbruster et al. (1978) from microseismicity studies. The sense of motion for these events is left-lateral. Solutions P11 and SA1 are identical and can be explained in terms of strike-slip movement along faults located in the southern part of the Hazara thrust region.

It appears that the pattern of energy release in the Hazara thrust region is very complex. Faults/lineaments oriented in both NE-SW and NW-SE directions are active and energy is being released through thrust as well as strike-slip mechanisms. The Peshawar Basin fault zone, located between the Safed-Koh fault and the river Indus, appears to be under compression, while the region south of the Indus is experiencing strike-slip movements, mostly with left-lateral sense.

GURDEZ AND KUNAR FAULTS REGION

The Gardez and Kunar faults (GF and KF, respectively) region is located to the east of Kabul and is particularly active in the southwestern part. Both the faults trend in a northeasterly direction. The Gardez fault passes through the Nuristan massif, while the Kunar fault passes to the east of it. A number of focal mechanisms (CH5, P8, PR6D, PR10D, PR13D, Ri26) show thrust faulting along the GF and KF and the region in-between. However, events for the two solutions, PR14D and PR8B, show strike-slip faulting, predominantly with left-lateral sense. Solution PR2B, for an event close to the Chaman fault, supports left-lateral movement, as discussed earlier.

It appears that the Gardez-Kunar fault region is also experiencing compression in a NW-SE direction at present. This direction is nearly at right angles to that along the Himalayan Main Boundary Thrust, as shown by solution CH7 for an event in Kashmir.

The direction in which the maximum stress in the area is acting can be seen from Fig. 7 in which the horizontal projection of the maximum stress axis is plotted at the location of the epicentres. Major faults and lineaments are after Kazmi (1979). It may be seen that for most of the solutions (strike- slip as well as thrust), the predominant direction of the P axis is NW-SE. However, near the Hazara syntaxial region, the NE-SW trend is more predominent. This direction prevails all along the northwestern part of the Himalayan arc (including Kumaun, Panjab and Kashmir Himalaya) and extends up to the IKSZ (Verma and Kumar, 1986). This trend is also visible in several focal-mechanism solutions of the Hindukush-Pamir region (Verma and Chandra Sekhar, 1985).

348 VERMA A N D C H A N D R A SEKHAR

S C A L E

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ABBREVIATIONS

2~, ° C- CHAMAN, J-JHELUM

K- KALAT , P-PESHAWAR

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M- MULTAN

Fig. 7. Horizontal projection of P axis for all focal-mechanism solutions discussed in the text. It may be noted that over a large part of the territory, the direction of the Paxis is NW-SE, Only in the northeastern part (near Hazara syntaxis), is the NE-SW orientation predominant, on account of the influence of Himalaya thrust front• Approximate directions of plate movements are shown by large arrows.

PLATE M O V E M E N T S IN PAKISTAN 349

S U M M A R Y AND C O N C L U S I O N S

The nature of the faulting taking place at present in Pakistan is determined by the movement of the Indian plate with respect to the Lut block on the west and continent-continent collision in the north. It is well known that the latter is responsible for the creation of the Himalaya thrust front and the folded structures of Hindukush-Pamir and the Karakorum region. Although the entire territory of Pakistan has experienced thrust movements during Tertiary times, resulting in the formation of the Kirthar-Sulaiman Ranges, the Salt Range, the Hazara thrusts and the Hazara Syntaxis, the movements taking place at present are not predominantly of thrust type. Strike-slip faulting appears to be predominant. This may be due to relative movement of different blocks and is quite in contrast to the faulting along the Himalayan front.

Briefly, the results are summarized as follows:

.

.

.

Strike-slip faulting is predominant over the entire territory of Pakistan. To a lesser extent, thrust faulting takes place, particularly in the eastern part of the QTZ and the Hazara syntaxial region. Although a major part of the Indian plate is probably moving along the Chaman fault, this movement is being accommodated to a lesser extent along the QTZ, the Sulaiman ranges and the Hazara thrust region ter- minating near the Hazara syntaxis. The nature of faulting along the Chaman fault and the QTZ is largely similar to that of the San Andreas fault system of California. So far as the energy release is concerned, maximum energy is being released close to the Chaman fault and along the Quetta transverse ranges, mostly through strike-slip mechanisms.

To the west as well as east of Pakistan there lie two major thrusts, the Zagros thrust in Iran and the Main Boundary Thrust in the Himalaya in the Kashmir region. Thrust faulting is predominant along both these thrusts, as shown by focal-mechanisms solutions (Nowroozi, 1972; Chandra, 1981; Verma and Kumar, 1986). The nature of plate movement in the territory of Pakistan is quite in contrast to these two regions.

Movement of the Indian plate along its eastern margin is marked by the Arakan-Yoma seismic belt, while on the western side it is marked by the Chaman fault, the QTZ, the Sulaiman ranges and the Hazara thrust region. The orientation of P axis along the eastern margin is mostly east-west to ENE-WSW (Verma and Kumar, 1986), while on the western side it is mostly NW-SE. Only in the northeastern part of


Pakistan does the orientation change to NE-SW, where the influence of the Himalayan thrust front to a large extent is present.

4. The influence of northwestward compression is felt over the entire territory of northern Pakistan, as far west as the Gardez and Kunar faults region.

REFERENCES

Abdel-Gawad, M., 1971. Wrench movements in the Baluchistan arc and relation to Himalayan-Indian Ocean tectonics. Geol. Soc. Am. Bull., 82: 1233-1250.

Armbruster, J., Seeber, L. and Jacob, K. H., 1978. The northwestern termination of the Himalayan Mountain Front: active tectonics from microearthquakes. J. Geophy. Res., 83: 269- 282.

Chandra, U., 1978. Seismicity, earthquake mechanisms and tectonics along the Himalayan Mountain Range and Vicinity. Phy. Earth and Planet. Int., 16:109 131.

Chandra, U., 1981. Focal mechanism solutions and their tectonic implications for the Eastern Alpine- Himalayan region. In: Zagros-Hindukush-Himalaya Geodynamic Evolution, H.K. Gupta and F. M. Delany, editors; Geodynamics Series, Vol. 3; Am. Geophy. Union; Washington.

Desio, A., 1979. Geologic evolution of Karakorum. In: Geodynamics of Pakistan, A. Farah and K.A. Dejong, editors. Pub. by Geol. Surv. of Pakistan, Quetta, pp. 111 124.

Griesbach, C. L., 1893. Notes on the earthquake in Baluchistan on 20th Dec. 1892. Rec. Geol. Surv. India, 26(Pt. 2): 57-61.

Jacob, K. and Quittmeyer, R., 1979. The Makran region of Pakistan and Iran: Trench-Arc system with active plate subduction. In: Geodynamics of Pakistan, A. Farah and K. A. Dejong, editors. Pub. by Geol. Survey of Pakistan, pp. 305-317.

Kazmi, A. H., 1979. Active fault systems in Pakistan. In: Geodynamics of Pakistan, A. Farah and K. A. Dejong, editors. Pub. by Geol. Surv. Pakistan, Quetta, pp. 285 294.

Nowroozi, A. A,, 1972. Focal mechanism of earthquakes in Persia, Turkey, West Pakistan and Afghanistan and plate tectonics of the Middle East. Bull. Seis. Soc. Am., 62(3): 823-850.

Pennington, W. D., 1979. A summary of field and seismic observations of the Pattan earthquake--28 December, 1974. In: Geodynamics of Pakistan, A. Farah and K.A. Dejong, editors. Pub. by Geol. Surv. Pakistan, Quetta, pp. 143-147.

Quittmeyer, R. C., Farah, A. and Jacob, K. H., 1979. The Seismicity of Pakistan and its relationship to surface faults. In: Geodynamics of Pakistan, A. Farah and K. A. Dejong, editors. Pub. by Geol. Surv. of Pakistan, Quetta, pp. 271-284.

Quittmeyer, R. C. and Kafka, A. L., 1984. Constraints on plate motion in Southern Pakistan and Northern Arabian Sea from the focal mechanisms of small earthquakes, J. Geophy. Res., 89: 2444--2458.

Quittmeyer, R. C., Kafka, A. L. and Armbruster, J. B., 1984. Focal mechanisms and depths of eartquakes in Central Pakistan: A tectonic interpretation. J. Geophy. Res., 89: 2459-2470.

Sarwar, G. and Dejong, K. A., 1979. Arcs, Oroclines, Syntaxes: the curvatures of mountain belts in Pakistan. In: Geodynamics of Pakistan, A. Farah and K. A. Dejong, editors. Pub. by Geol. Survey of Pakistan, Quetta, pp. 341 349.

Seeber, L. and Jacob, K. H., 1977. Microearthquake survey of northern Pakistan: Preliminary results and tectonic implications. In: Proc. CNRS Colloquium on the Geology and Ecology of the Himalaya, Paris, December 1976, pp. 347 360.


Seeber, L. and Armbruster, J., 1979. Seismicity of the Hazara arc in northern Pakistan: Decollement vs. Basement faulting. In: Geodynamics of Pakistan, A. Farah & K. A. Dejong, editors. Pub. by Geol. Surv. Pakistan, Quetta, pp. 134-142.

Verma, R. K., Mukhopadhyay, M. and Bhanja, A. K., 1980. Seismotectonics of the Hindukush and Baluchistan arc. Tectonophysics, 66: 301-322.

Verma, R. K. and Chandra Sekhar, Ch., 1983. Seismicity of Pakistan and its relationship to faults and lineaments. Geophy. Res. Bull., 21(3): 209-221.

Verma, R. K. and Chandra Sekhar, Ch., 1985. Seismotectonics and focal mechanisms of earthquakes from Pamir-Hindukush regions. Tectonophysics, 112: 297-324.

Verma, R. K. and Kumar, G. V. R. K., 1986. Seismicity and the nature of plate movement along the Himalayan arc, NE India and Arakan-Yoma. Proc. IASPEI Assembly at Hyderabad, Tectono- physics (under publication).

JOQ 5/3-4-7

Focal mechanism solutions and nature of plate movements in Pakistan

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