1FOCAL MECHANISM STUDIES OF EARTHQUAKES ALONG MAIN MANTLE THRUST (MMT) IN PART OF THE HINTERLAND ZONE OF

HIMALAYAN FOLD BELT. PAKISTAN

2Azam A. Khwaja, 3MonaLisa, & 4Syed Kazim Mehdi

ABSTRACT

Main Mantle Thrust (MMT) marks the northern coll isional boundary of Indo-Pakistan plate with Kohistan Island Arc. It has been the source of moderate to large earthquakes. An attempt has been made to study the seismological characteristics of MMT in relation to the tectonics of the area. For this purpose the earthquakes with magnitude 4.0 Mw from both international seismological networks as well as the local seismic observatories have been used to prepare seismicity map of the area for the period of 1904-2002. The epicentral distribution shows that the area is active. A total of fourteen focal mechanism solutions (FMS) of earthquakes that occurred in the vicinity of MMT have been investigated. Out of these fourteen, eight are reverse/thrust and the remaining 6 are strike slip solutions. Based upon the FMS, it is believed that the events (focal depths ranging from 47 to 68 km) located near MMT and having their P-axis orientations in the NE direction are related to the intraplate activity due to steep bending of the Indo-Pakistan plate. Probable cause is imbricate thrusting, breaking and thickening of the crust. The 6 strike sl ip solutions are inferred to be a result of uplift of the Nanga Parbat-Haramosh massif and the Besham dome.

1 Islamabad Journal of Sciences, Quaid-i-Azam University, Islamabad, Pakistan ISSN 0304-5218, Vol. 15, No.1 (2005-2006). 2 Vice Chancellor, Quaid-i-Azam University, Islamabad. Pakistan. azam_khwaja@yahoo.com 3 Assistant Professor, Quaid-i-Azam University, Islamabad, Pakistan. lisa_qau@yahoo.com 4 Deputy Director, Seismic Studies Program, WAPDA Seismic Observatory, Mangla, Pakistan.

sspkazim@hotmail.com

1. INTRODUCTION

A large number of workers have contributed towards the understanding of the geology/tectonics and seismological characteristics of the MMT [1, 2, 3, 4, 5]. However, in the discussion to follow the focal mechanism solutions (FMS) have been incorporated along with the tectonics of the area to highlight the active nature of the area. It is hoped that that this study would be helpful to understand the ongoing collisional process of the Indo-Pakistan plate with the Kohistan Island Arc. 2. GEOLOGY/TECTONICS OF THE AREA The Main Mantle Thrust (MMT) forms the northern boundary of NW Himalayan Fold-and-Thrust Belt Pakistan [6]. This fold and thrust belt is seismically one of the active regions of the world and forms an area between the MMT and Salt Range Thrust (SRT) with its westward extensions (Surghar, Marwat, Bhittani and Manzai ranges). According to [7] and later workers, the southern sides of these ranges are also marked by thrusts. The tectonic domains of Hazara-Kashmir Syntaxis and the Nanga Parbat Haramosh Massif comprise its eastern boundary. The western limit is not clearly defined. Besides the Kurram Fault in the southwestern portion, series of thrusts beyond the borders of Pakistan (like the Sarobi Fault in Afghanistan) are considered to be delineating this boundary. In this nearly 250 km wide and 560 km long fold and thrust belt, the Panjal-Khairabad fault (Fig.1) divides it into a northern hinterland zone and the southern foreland zone. The hinterland zone is also referred to as the Hazara Crystalline Zone [8] and Himalayan Crystalline Zone [9]. In the hinterland zone, mostly crystalline rocks represented by Proterozoic to Mesozoic metamorphic and igneous rocks occur. Shearing and imbrication has resulted in a complex deformation pattern. Basement is also involved in thrusting. [10] identified six nappe zones (Mohmand-Swat nappe, Besham nappe, Hazara nappe, Banna nappe, Kaghan nappe and Nanga Parbat-Haramosh massif) separated from each other by prominent shears and thrust faults. The stratigraphy and degree of metamorphism varies in them. All show inverted metamorphism (i.e. higher grade rocks in hanging walls instead of the footwalls). According to [6], uplift rates in different blocks of the nappe zones also vary, and in the area between Mansehra Thrust and MMT, crustal shortening of more than 470km has taken place. Recent work of [11] suggests that in the nappe zone of [10], named the Nanga-Parbat Haramosh Massif, an area of oblique convergent margin, the most recent structural changes is the east-west alignment of the antiformal (domal) structures. Prior to that the sequence of folding recognized consists of isoclinal structures followed by large E-W kink folds and formation of large N-S anticlines [9]. It is an active tectonic feature, characterized by uplift rates of 7mm/year [12], high seismicity, this study and active faulting [1, 10, 13].

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Main Mantle Thrust (MMT): The name proposed by [14] marks the northern boundary of the NW Himalayan Fold and Thrust Belt (and the Hinterland Zone) which here is mostly represented by a metamorphic and magmatic terrain characterized by thick stacks of nappes, thrust sheets and mylonitised shear zones [6]. It also marks the northern collisional boundary of the Indo-Pak plate with the Kohistan Island Arc and following [15] is also known as the Indus Suture. Seismicity map (Figs.2a and 2b) shows different segments of this major fault to be active. It is a complex fault zone with width varying upto several tens of kilometers [14] and comprising of a number of thrust sheets [6] that dip between 350 and 500 towards the north [16]. Mostly it separates the mafic and ultramafic rocks of the Kohistan Island Arc from the sialic rocks of the Indo-Pakistani plate. Metamorphism has affected the rocks to variable degree with high-pressure

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metamorphic rocks (blueschists) locally found associated with the MMT. According to [17] different structures related to obduction-subduction can be observed at different sites along the MMT. Deformation phases are still being unraveled. In the east, the MMT forms a loop around the Nanga Parbat Haramosh Massif. The structure is more complex here. According to [13], on the western side of the massif the MMT is offset by the right lateral Raikot Fault [10], has recognized a complex system of faults and shears in this part between Raikot and Sassi. Two recent focal mechanism solutions [1] and discussed in this study also indicate right lateral strike slip faulting in this part. At the same time, a complex system of faults referred to as the Stak Fault Zone occur in a 3-5km wide zone along the northeastern margin of the massif.

3. FOCAL MECHANISM STUDIES The focal mechanism solutions (FMS) of fourteen earthquakes have been selected amongst the earthquakes reported by USGS, ISC and the local network with magnitudes 5.0 Mw in the area during the period of 1965-2004. These FMS are shown in Fig.2 and their football diagrams are represented in the Fig.3 while the parameters for the epicenters and the focal mechanism solutions are listed in Table-1& Table-2. With the help of a computer program PMAN [18] that required input of geographic coordinates, magnitude, focal depth and P wave polarity the focal mechanism solutions have been drawn and interpreted and the parameters like azimuthal angle and take-off angle are determined by the software AZMTAK [18]. It should be noted that the P wave polarity data has been taken from all the maximum possible sources i.e. both the local and international seismological observatories. The events considered to be suitable for the determination of FMS with the condition that is they must have first motion data recorded at least 8 stations. Events have been numbered 1-14 considering their date of occurrence with number 1 being the oldest and number 14 the last event to have occurred in this time period. Starting from east to west, these FMS are discussed according to their location within MMT.

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FMS 11 and 12: Both of these events are situated in the Nanga Parbat Haramosh massif (the north-eastern most part of MMT) as shown in Fig.2a. Focal depth is 23 and 45 km respectively (Table.1). The fault-bounded massif has a complex geology. According to [19], the tectonic history (plutonism, metamorphism and deformation) of the Indo-Pakistan plate in the massif is markedly different from other parts. On the western side of the massif, near the epicentral locations, a nearly N-S trending MMT/Raikot-Sassi fault zone occurs. Both thrusting and dextral strike slip faulting has been documented from this part with the latter more prominent in the northern portion. It is considered to be part of an ongoing later phase of deformation. FMS 11 (Fig.3) obtained is of strike slip faulting with some normal component. From the two nodal planes, the one trending in the NNE-SSW direction (Table-2) is considered to be the rupture plane. This inference has been made due to the reason that it is similar to the dominant trend of faults in the area including that of the major suture (MMT). Further, a number of fault splays parallel to the MMT occur in the vicinity. Probably one of the splays was activated. The trend (NNE-SSW) of the rupture plane indicates right lateral sense of motion that is in agreement with such type of movement in the area.

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Table 1: Source parameters of fourteen earthquakes whose FMS has been determined in the present study

FMS Nos. Date

D/M/Y Time

H: M: S Latitude (N) Longitude

(E) Depth (Km)

Magnitude (Mw)

1. 08-11-65 21:23:09 34.6 73.3 65 5.1 2. 06-04-66 1:51:53 34.91 73.06 54 5.6 3. 27-12-71 20:59:39 34.98 73.02 55 5.7 4. 27-09-72 20:24:56 35.07 72.91 49 5.3 5. 28-12-74 12:11:46 35.06 72.91 15 6.4 6. 28-12-74 22:38:53 34.99 73.1 68 5.3 7. 07-04-75 6:41:02 34.91 72.97 53 5.5 8. 28-12-84 16:28:01 34.61 73.61 47 5 9. 16-03-91 3:57:42 34.52 72.66 33 5 10. 17-07-00 5:26:00 3459 72.89 10 5.5 11. 01-11-02 22:09:29 35.62 74.66 23 5.8 12. 20-11-02 21:32:00 35.51 74.68 45 6.7 13. 14-02-04 11:56:58 34.81 73.19 10 5.4 14. 14-02-04 10:30:22 34.828 73.255 10 5.5

Table 2: Parameters obtained from the fourteen focal mechanism solutions FMS)

Fault Plane (FP) Auxiliary Plane

(AP) P-Axis T-Axis FMS

No. Nature of

FMS Strike Dip Strike Dip Strike Plunge Strike Plunge

1. THRUST 142O 33 ONE 289 O 62 O 31 O 15 O 164 O 69 O 2. THRUST 308 O 7 ONE 40 O 90 O SE 136 O 44 O 303 O 45 O 3. THRUST 331 O 12 ONE 87 O 85 O S 186 O 39 O 345 O 49 O 4. THRUST 320 O 13 ONE 154 O 78 OSW 242 O 33 O 68 O 57 O 5. REVERSE 348 O 52 ONE 105 O 60 OSW 225 O 5 O 321 O 53 O 6. THRUST 329 O 21 OE 135 O 70 OS -131 O 24 O 37 O 65 O 7. THRUST 7 O 11 ONE 175 O 79 ONW -93 O 34 O 82 O 56 O 8. THRUST 284 O 34 ONE 162 O 70 O -129 O 21 O 108 O 56 O 9. LLSS 42 O 88 ONE 312 O 85 ONW 177 O 2 O 267 O 4 O 10. RLSS 312 O 85 ONE 42 O 88 ONW 177 O 2 O -267 O 4 O 11. RLSS 209 O 87 ONW 113 O 27 OSW 93 O 42 O -38 O 37 O 12. RLSS 211 O 58 ONW 121 O 90 ONE 71 O 22 O 171 O 22 O 13. RLSS 256 O 84 ONE 88 O 71 OSW 43 O 9 O 310 O 18 O 14. RLSS 356 O 84 ONE 88 O 71 OSW 43 O 9 O 310 O 18 O

FMS 12 is for an event having magnitude of 6.7Mw (Table.1) and is the strongest of the earthquakes to have affected the area. This event occurred after about 19 days of event number 11, in the southerly part of the same fault zone. According to [6], the gneisses of the Indo-Pakistan plate have been thrusted on the recent alluvium in the vicinity of the epicentre and the southeast dipping thrust is named as the Liachar Thrust (Raikot Fault). The MMT is also situated nearby. A number of right lateral strike slip faults trending in the NNE-SSW direction have also been recognized in this area of thrusting. As such, in the focal mechanism solution (Fig. 3), the NNE-SSW trending nodal plane that indicates right lateral sense of motion is inferred to be the rupture plane.

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[1] has obtained similar solutions for both the events. Most likely, complex convergence of movement of the plate, differential but rapid rates of uplift that are being accommodated along the fault zone, structural alignment of domes in an E-W direction within the massif singly or in combination may be contributing towards this type of activity. It may be mentioned that for both these events Harvard CMT Catalogue shows normal faulting and did not incorporate local data. Similarly [20] also determined the FMS of these two events and obtained normal faulting. However they themselves committed that although the strike of both the solutions is similar to the Raikot fault but nature of solutions i.e. normal is not appropriate to the local structure. FMS 13 and 14: These events are situated near the MMT (Fig.2b) that forms the northern boundary of the study area and is also the collisional boundary where the Indo-Pakistan plate underthrusts the Kohistan Island Arc. Another important thrust of the area lying immediately south of the MMT is the Banna Thrust. In this part of the study area, on the basis of seismicity, a wedge shaped NW trending structure has been recognized. [21] named it as the Indus Kohistan Seismic Zone (IKSZ). Later workers [22, 23] have also confirmed the presence of this NW trending 100 km long feature between the Hazara-Kashmir Syntaxis and the MMT. This nearly 50km wide zone of seismicity has a nearly horizontal upper surface and a lower NE dipping surface. [23], on the basis of relocated hypocentres, have identified two seismic zones within the IKSZ i.e. a shallow zone extending from the surface to a depth of 8km and a more pronounced midcrustal zone lying at depths of 12 to 25km. The upper boundary at a depth of about 12km is considered to be representing a decollement surface that decouples the sediments and metasediments from the basement. Focal depth of these events i.e. 10km (Table. 1) suggests that the cover rocks within the Banna nappe zone were affected by the earthquake activity. These rocks comprising of schists, slates, phyllites and marbles have undergone both ductile and brittle deformation and have been emplaced on the mylonites, gneisses and schists of the Precambrian Tanawal Formation. In both the solutions (Fig.3), the nodal planes trending in the NW-SE direction are considered to be the rupture planes. Their dip direction towards the NE (Table.2) is in agreement with the general dip direction of the area. A right lateral sense of motion indicated may be due to their location in a shear zone. The FMS in Harvard CMT Catalogue also show strike slip solutions with some thrust component. The ongoing basement uplift of the Besham domal structure [24, 10] may be the reason for generating right lateral strike slip faults in this part of the study area.

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FMS 5: This event like the previous two events is also located near the MMT (Fig.2b) and with a magnitude of 6.4 Mw (Table.1) is the second largest earthquake to have affected the study area. Another important fault located near the epicentre is the left lateral Puran Fault. As compared to the earlier two events, the focal depth for this event is 15km. From the earlier description, it is known that the IKSZ occurs at depths ranging from 12 to 25km. According to [23] most IKSZ events are deeper than 12 km in which the shallow events are associated with the reactivated parts of MMT, while the deeper earthquakes may be related to the underthrusting of the Indo-Pakistan plate beneath the IKSZ. Lower IKSZ (12-25 km) represents a major thrust zone [21, 23]. Various workers named this event as the Pattan earthquake of 1974, due to its location near Pattan village, and determined its FMS. Different workers [24, 25] have inferred thrust faulting with the rupture plane trending in the NW direction and dipping towards NE except the solution of [5] that shows dip in the NW direction. Composite fault plane solutions obtained from the IKSZ show reverse faulting along NW striking planes dipping towards the NE or more steeply towards the SW [21,23], or strike slip solutions or a mixture of both [21]. Solution obtained in the present study (Fig.3) is of reverse faulting. A similar NW trend dipping towards the NE is obtained. Dip is 520NE (Table.2). However, like [24], there is a right lateral strike slip component. Finally the solution supports the contention that a major active thrust fault zone (IKSZ) underlies the decollement. FMS 1, 2, 3, 4, 6, 7 and 8: All of these events are situated near or along the MMT (Fig.2b) that forms the northern boundary of the study area. The other important active faults in this area are the right-lateral Thakot and left-lateral Puran faults except in the case of event number 8. This has its epicentral location on the northeastern side of HKS (Fig.2) between the MBT and Batal Thrust. The latter fault is also considered to be representing the Main Central Thrust [19], a major suture that separates the Lesser Himalayas from the Higher Himalayas. Focal depth for these events varies from 47 to 68 km (Table.1) thereby indicating seismic activity below the earlier described IKSZ. Publications of [21, 22] have documented seismic activity from these deeper levels. According to [20], a zone of diffuse seismicity occurs below the IKSZ till a depth of 50km and indicates intraplate activity in the Indo-Pakistani plate. [21] recognized seismic activity till a depth of 70km. Earthquake activity is inferred to be within the basement of the plate or in the uppermost mantle due to the steep bending of the plate. Most workers including those basing their interpretation on gravity data [27] agree on the presence of the Indo-Pakistan plate and its underthrusting beneath the Himalayas.

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Most agree that thrusting is the major deformational process operating at these levels. Focal mechanism solutions of these events support this contention as all solutions show thrusting (Fig.3). All trends in the NW-SE direction and have dips ranging from 70 to 340 in the NE direction (Table.2). P-axis orientations of these events are NE-SW directed. Following [21], it is believed that imbricate thrusting, breaking and thickening of the crust to a depth of 60 to 70 km is producing thrusts with mostly shallow dips in an overall steeply dipping seismic pattern. Thus, they are considered to be intraplate events affecting the lower crust. In the case of event number 8, it supports the contention of [28] that a similar style of deformation prevails in this part of the area also. FMS 10: This event is located on the nearly N-S trending, right-lateral strike slip fault (Fig.2b). It is considered to be the surface expression of the Tarbela Seismic zone. This zone lying between depths of 8-18km and 8 to 15km according to [23] comprises of faults with strikes in both the NW and NE directions [22 23]. According to [23], the strike of faults is in the E-W direction and can be correlated to the surface mapped thrusts and strike slip faults. Composite fault plane solutions of microseismic data indicate steeply dipping faults with reverse or strike slip motions [21]. In the present case, the nodal plane trending in the NW-SE direction with nearly vertical dip (Table.2) is considered to be the rupture plane (Fig.3). The sense of motion indicated is of right lateral slip. Thus, the FMS although having a slightly different trend, but with similar sense of motion as the Thakot Fault is considered to be representing it. It also supports the contention of [24] that it is a basement fault. FMS 9: This event is located in the Mohmand-Swat crystalline nappe zone of [6] about 10 kms west of the Puran Fault (Fig.2b). It is high angle, left lateral shear that marks the western boundary of the Besham nappe zone. No other mapped fault is situated at the epicentral location. Focal depth given by ISC is 33km (Table.1) and according to them this number should be treated with caution. From amongst the two nodal planes in this strike slip solution (Fig.3), the one trending in the NE direction is considered to be the rupture plane. This trend is not only similar to the trend of the Puran Fault, but also gives steeper dip (880) and indicates left lateral sense of motion (Table.2). It is believed that the continued uplift of the Besham antiformal structure is responsible for left lateral strike slip faulting on its western side. Thus the shear zone accommodating the uplift may be broader than previously envisaged and is most likely extending at least upto this part of the study area.

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4. DISCUSSION

Based upon the present work the MMT can be considered as the source of moderate to large earthquakes. Most recently, Pattan (1974), Bunji (2002) and Batgram (2004) earthquakes badly affected the areas surrounding MMT. The maximum magnitude event that occurred within the NW Himalayan fold-and-thrust belt is of 6.7 Mw in the Raikot-Sassi Fault Zone (FMS 12 discussed in the present work), which is the maximum magnitude earthquake ever recorded in the area. Looking at the study area in the regional perspective, a number of interpretations by different workers are available to explain the collisional tectonics between the northward moving Indo-Pakistan and the Eurasian plates. All agree that the Indo-Pakistan plate is being underthrusted at the suture referred to in this study as the MMT. In the present work, prevalence of a compressional regime is reflected by the thrust (8 out of the 14 FMS). All of these events (focal depths ranging from 47 to 68 km) are located near this convergent boundary (maximum epicentral distance is about 40 km away from the MMT) and have their P-axis orientations in the NE direction. In the earlier discussion they have been interpreted as indicative of intraplate activity due to steep bending of the Indo-Pakistan plate. Thus, following [21], it is believed that imbricate thrusting; breaking and thickening of the crust are taking place. Similarly two tectonic features, namely the Nanga Parbat-Haramosh massif and the Besham dome are believed to be influencing the development of strike slip faulting in the hinterland zone. Some of the world's most rapid rates of uplift have been recorded from this fault bounded massif. This uplifting could explain the formation of right lateral and left lateral strike slip faulting on its eastern and western sides. Thus, FMS 11 and 12 (Fig.2a) have been related to the stresses being generated by the massif. Similarly the uplift documented in the Besham dome [1] is most likely generating left (FMS 9) and right lateral (FMS 10, 13, 14) strike slip on its western and eastern sides respectively.

Acknowledgments We wish to acknowledge the anonymous reviewers from their useful criticism and suggestions. Also the work is partially financed from the Quaid-i-Azam University Research Fund.

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ABSTRACT1. INTRODUCTION