Characteristics of October 08, 2005 Kashmir-Hazara Earthquake and

Its Connection with Indus Kohistan Seismic Zone (IKSZ)

Syed Kazim Mehdi Director MSMS WAPDA, Pakistan, sspkazim@hotmail.com

ABSTRACT:

This paper deals with the mega (Mw = 7.7) earthquake that struck the Kashmir Hazara region of Pakistan on October 08, 2005. The Kashmir Hazara

Syntaxis (KHS) is one of the bold tectonic scars that physically isolate the terrain from rest of the Himalaya. Other major seismotectonic features

sculpturing this terrain in the shape of major faults and folds are: Main boundary Thrust (MBT), Main Mantle Thrust (MMT), Indus Kohistan

Seismic Zone (IKSZ), Punjal Thrust Fault (PTF), Hazara Thrust System (HTS) and the Indus Valley Faults (IVF). The study presents Fault Plane

Solutions (FPS) of ten pre (1900 2005) and 15 post (October 2005 to February 2006), Kashmir Hazara earthquakes and their tectonic

interpretation. Aftershock distribution which is more prominent in the crystalline zone defines a more refined orientation of the Indus Kohistan

Seismic Zone (IKSZ). WAPDA Large Dam i.e. Tarbela and two under construction Hydropower Projects viz. Neelum Jhelum and Kohala lie in the

mainshock and aftershocks area of the earthquake that are still occurring till date. Micro Seismic Monitoring Systems (MSMS) should be installed in

Muzaffarabad and Balakot areas for the seismic safety monitoring of Neelum Jhelum and Kohala Hydropower Projects that are under construction in

this seismically active region.

Keywords: Kashmir-Hazara Earthquake, Indus Kohistan Seismic Zone, Fault Plane Solutions.

1. INTRODUCTION

A massive earthquake of Mw = 7.7 occurred in the Pakistans

Kashmir Hazara Zone on October 08, 2005 at 0350 UTC, killing

over 0.10 million people. As per Micro Seismic Monitoring

System (MSMS) of WAPDA, Pakistan, this shallow focused

earthquake (16.2 km) occurred at Lat. 34.53 E & Lon. 73.55 N.

The earthquake resulted from the subduction of the Indian plate

beneath the Eurasian plate, and fault plane solutions indicated

that the earthquake resulted due to thrust faulting. Muzaffarabad

and Balakot where the Modified Mercalli Intensity reached a

maximum of X were the most effected cities.This was the largest

historical earthquake on the Indus Kohistan Seismic Zone

(IKSZ) of Armbruster et al. (1978) and Seeber and Armbruster

(1979). It was the first Himalayan earthquake to be accompanied

by surface rupture, reactivating the BalakotBagh (BB) reverse

fault (Figure-1) and, locally, offsetting the Main Boundary thrust

(MBT). A field investigation by Yeats et al. (2006) and Kaneda

et al. (2008) revealed a surface rupture 70 km long, with up to 7-

m vertical separation, mostly along the pre-existing BB fault.

Northwest of the end of the surface rupture, heavy damage was

sustained northwest to the Indus River, site of the 1974 Pattan

earthquake of Mw = 6.3 (Figure-1). The physiography of the

range, as well as tectonic structure defines a Syntaxis, called the

Hazara Kashmir Syntaxis (HKS), outlined by the hairpin

rotation of the Main Boundary Thrust (MBT). The MBT is a

most important fault bounding the Himalayan range that has

thrust metasediments of the Lesser Himalaya over the Tertiary

molasses. Active deformation in the area results from the 3

cm/yr northward notch of the northwestern Indian Peninsula into

Eurasia. Along the northwestern Himalaya, a fraction of that

junction, estimated to about 1.4 cm/yr, is absorbed by thrusting.

2. KASHMIR HAZARA TERRAIN

The Kashmir Hazara terrain lying in the lesser Himalayan belt

constitutes the NW segment of the Indo Pakistan plate. This part

of the plate has been grilled by two continental accidents; the

earlier one 100-75 my ago when the Indo-Pakistan Plate collided

with Eurasia, overwhelming the Palaeotethys. After the initial

collision, the Indo-Pak plate started under-thrusting the Eurasian

Plate and according to an estimate over 700 km of the Indo-

Pakistan Plate has been consumed under Eurasian plate. The

uplift of the Tibetan Plateau and Himalayan Belt and their

attaining higher elevation is considered as a result of under-

thrusting the Indo-Pak Plate underneath Eurasia. Even now the

northward subduction is continuing at the rate of 3-4 cm per year

and stresses generated by this convergence movement have

given rise to extensive southward directed thrust system which is

successively migrating to the south and being accommodated by

the major shear zones including those of Kashmir and Hazara.

The active tectonics created by the plate movements or deep

crustal disturbances induce body waves which are seismic and

travel through the earths interior, spreading outward from the

epicenters in all directions. These waves are capable of

triggering the dormant faults and could transform an Aseismic

zone into seismic. Seeber et al (1983) based on seismic data

delineated two major deformational anomalies at crustal level

called Detachment Fault and Basement Fault. The Detachment is

nearly a horizontal fault and separates the under thrusting Indo-

Pak shield from the overlying metamorphic and sedimentary

rock formations. The Basement marks the line separating the

shallow dipping Detachment Fault from the steeper dipping

Basement. Thus under this context the fault in the Kashmir-

Hazara terrain are categorized either being the offshoots of

Basement or Detachment. In case of Detachment a rupture

occurs in vast area in a single event and it generates great

earthquakes. The earthquakes are categorized as shallow at

depth range up to 70 km beneath the surface, intermediate

between 70 -350 km and deep between 350 670 km.

Figure 1 Regional Seismotectonic Setup

3. REGIONAL GEOLOGY

The BB fault dips northeast. Near Muzaffarabad, the fault

separates Precambrian limestone and shale on the northeast from

Miocene Murree Formation on the southwest; farther southeast,

the fault is entirely within the Murree Formation or forms the

contact between the Murree and Kamlial formations (Fig.2).

The Kohistan arc can be divided from the Asian plate by the

Northern or Shyok Suture and from the Indian plate by the Main

Mantle Thrust (MMT). The internal zone is separated from the

external zone un-metamorphosed to low-grade metamorphic

Precambrian sediments and dominantly Mesozoic to Eocene

Tethyan shelf sediments by the Punjal Thrust (PT). The Main

Frontal Thrust (MFT) delineates the southernmost extent of the

foreland basin fold and thrust belt.

Figure 2. Geology of Bagh-Balakot Fault & Location/FMS of Main KH Earthquake

The Balakot Formation red beds lie in thrust contact with the

Paleocene aged shallow marine Patala Formation and Lockhart

Limestone below, and are tectonically intercalated with an

underlying dark gray marl formation (Mahdi 2007). Jhelum

Fault is a NE dipping strike-slip fault following the western

margin of HKS bend. Rocks belonging to Miocene, Cambrian

and Pre-Cambrian periods exposed along its trace are highly

deformed due to recurring shear zones. Individual blocks of the

Punjal Volcanic and Triassic limestones have been found

dragged for several kilometers southward. An accumulative left-

lateral offset of about 31 km is indicated on the western limb of

the Syntaxis. It apparently dislocates from the Main Boundary

Thrust and terminates at the eastward continuation of some of

the geological structures of North West Himalayan Fold and

Thrust Belts. These tectonic relationships indicate Jhelum fault

as the youngest major tectonic feature in the syntaxial zone

(Mehdi et. al. 2006).

4. SEISMOTECTONIC SETTING

Earthquakes in the northern part of Pakistan are the result of

ongoing northward subduction of the Indian plate beneath the

Eurasian plate at a rate of around 40 mm/year. The N & NE

directed compression has produced major thrust faults like the

Main Karakoram Thrust (MKT), Main Mantle Thrust (MMT),

and the Main Boundary Thrust (MBT) shown in Figure-1, as

well as many active faults of variable length. Transpressional

features in the area include strike slip faults named as Jhelum,

Thakot, Puran and Raikot Faults. In addition to these, existence

of shallow to deep crustal faults, like the NW trending Indus

Kohistan Seismic Zone (IKSZ) of Armbruster et al. (1978) and

Bagh Balakot Fault (BBF) of Khan and Ali (1994), has also been

proposed. The Himalayan region has been experiencing major

earthquakes, like the 1905 Kangra earthquake, 1934 Bihar

Nepal, and 1950 Assam earthquake. All these earthquakes have

approached or exceeded Mw = 8.0, but none was as destructive

as the October 8, 2005 Kashmir-Hazara earthquake of Mw = 7.7.

These great earthquakes occurred on the detachment under the

outer and Lesser Himalayas rocks from the under thrusting

Indian shield rocks. Gahalaut (2006) indicated the presence of

three seismic gaps in some segments of the detachment and

named one of them as the Kashmir Seismic Gap, which lies in

the southeast of the NW Himalayan Syntaxis or Hazara Kashmir

Syntaxis (HKS). The seismically active nature of Pakistan and

its adjacent region is well known because of the occurrence of

some of the biggest earthquakes of the world. Some events that

caused loss of life and destruction in northern Pakistan during

the recent past are the 1974 Pattan earthquake of Mw = 6.2,

Rawalpindi earthquake of 1977 having Mw= 5.5, two Bunji

earthquakes of Mw= 5.3 and Mw = 6.3 that occurred in 2002,

two Batgram earthquakes of Mw 5.5 and Mw = 5.8 that took

place in 2004, and the October 8, 2005 Kashmir Hazara

earthquake with Mw = 7.7. The fault that ruptured during the

October 08, 2005, Mw 7.7 earthquake is a thrust has been

identified characteristically along the Jhelum valley from

Muzaffarabad to Garhi and farther south.

5. INDUS KOHISTAN SEISMIC ZONE

On the basis of a micro-earthquake survey in this region during

19731974, through WAPDA MSMS data a wedge-shaped NW

trending structure was recognized by Armbruster et al. (1978)

who named it as IKSZ. Later workers confirmed the presence of

this 100 km long feature between the HKS and the MMT. This

50-km-wide zone of seismicity has a nearly horizontal upper

surface and a NE dipping lower surface. Seeber, et. al. (1983)

has used microseismic data from the Pakistans Tarbela and

Chasma microseismic networks to develop a seismotectonic

model for the Hazara Kashmir Syntaxis (HKS) region in

Pakistan. According to them there exist a sub-horizontal

decollement as an interface between the sedimentary and meta-

sedimentary wedge and the basement. However, they have also

discerned two parallel clusters of epicentres associated with the

basement faults, extending towards NW from the KHS, which

they interpreted as the deeper level NW extensions of the

structural trends in the Kashmir Himalaya, east of Syntaxis. Out

of the two zones/clusters the NE one, which they preferred to

call Indus Kohistan Seismic Zone (IKSZ), is currently more

active and indicates predominantly thrust type movement. The

IKSZ strikes parallel to the north-western Himalaya, but extends

beyond the HKS. This seismicity extends north-westwards the

belt of seismic activity that follows the front of the entire

Himalaya. Ni et al. (1991), on the basis of relocated hypocenters,

have identified two seismic zones within the IKSZ: a shallow

zone extending from the surface to a depth of 8 km and a more

pronounced midcrustal zone lying at depths of 12 to 25 km. The

upper boundary at a depth of about 12 km is considered to

represent a decollement surface that decouples the sediments and

metasediments from the basement. The IKSZ is seismically the

most active structure in the region, as it is capable of generating

large events. Prior to 2005, the most destructive earthquake,

associated with the IKSZ, was the 28 December, 1974 Pattan

earthquake with mb 6 magnitude. The question arises whether

the IKSZ or the detachment is responsible for the Muzaffarabad

earthquake.

6. DATA ANALYSIS AND FOCAL MECHANISM SOLUTIONS

More than 15,000 aftershocks were recorded by the WAPDA

Micro Seismic Monitoring System (MSMS) between the

October 8, 2005 Kashmir Hazara (KH) earthquake and

December 2010. The WAPDA MSMS uses its own P-wave

velocity model, whereas for S-wave velocities, it is assumed that

the Vp/VS ratio is 1.73. About 173 aftershocks were documented

in the first day after the initial shock, one of which had a

magnitude of Mw = 6.4). A total of 25 earthquakes (Mw 4)

have been selected for the FMS, out of which ten are pre KH

earthquake (19642004) and fifteen are post KH earthquake,

including the main shock (Table 1). All the 25 pre and post KH

earthquake FMS are plotted on a map (Fig.3) and are discussed

below in relation to the structures present in the area. FMS of

pre KH earthquakes are named as P1, P2. . . P10 and the KH

earthquake plus aftershocks FMS as 1, 2, 3. . . 15 (Fig 3.). All of

pre KH earthquakes except P8 are situated near or along the

MMT that forms the northern boundary of the study area. The

event P8 is located on the right lateral Thakot Fault, which is

one of the important active faults in the area. For FMS 6, 7, FMS

and 10, the focal depth is 10 km, confirming the activation of

IKSZ. Focal depths for the events 2, 4, 5, and 8 the depth ranges

from 49 to 55 km (Table 1.) There is general agreement that

thrusting is the major deformational process operating at these

levels. However, in the case of FMS 9, 12, and 13 (aftershocks),

the focal depth is 10 km, that is again confirming the activation

of the IKSZ.

Table 1. Source parameters of the 25 earthquakes used in FMS determination FMS Nos. Date (D/M/Y) Time (H:M:S) Latitude (N) Longitude (E) Depth (km) Magnitude (Mw)

Pre-2005 FMS P1 8/11/65 21:23:09 34.6 73.3 65 5.1

P2 6/4/66 1:51:53 34.91 73.06 54 5.6

P3 28/4/71 15:12:42 34.44 73.6 43 5.3

P4 27/12/71 20:59:39 34.98 73.02 55 5.7

P5 27/09/72 20:24:56 35.07 72.91 49 5.3

P6 28/12/74 12:11:46 35.06 72.91 15 6.4

P7 28/12/74 22:38:53 34.99 73.1 21 5.3

P8 7/4/75 6:41:02 34.91 72.97 53 5.5

P9 28/12/84 16:28:01 34.61 73.61 47 5

P10 17/7/2000 5:26:00 34.59 72.89 10 5.5

KH Earthquake FMS

1 08/10/05 03:50 34.42 73.52 13 7.6

2 08/10/05 10:46 34.76 73.28 8 6.4

3 08/10/05 12:08 34.56 73.20 10 5.7

4 08/10/05 12:25 34.76 73.40 10 5.7

5 08/10/05 21:13 34.77 73.45 10 6.0

6 08/10/05 21:45 34.65 73.36 10 5.7

7 09/10/05 08:30 34.56 73.51 10 5.8

8 09/10/05 19:20 34.38 73.79 10 5.5

9 12/10/05 20:23 34.90 73.46 10 5.8

10 13/10/05 20:49 34.74 73.19 10 5.7

11 19/10/05 02:33 34.66 73.53 05 5.8

12 19/10/05 03:16 34.90 73.38 10 5.5

13 23/10/05 15:04 34.88 73.19 10 5.9

14 28/10/05 21:34 34.74 73.13 10 5.5

15 06/11/05 02:11 34.47 73.54 10 5.7

Figure 3. Focal Mechanism Solutions of Earthquakes.

Focal mechanism solutions of these events support this

contention, as all solutions illustrate thrusting. All include a

plane that strikes in the NWSE direction and dips NE, except

P2 and P4 that have NE and N striking planes, respectively. P-

axis orientations of these events are NESW (Table 2).

Following Armbruster et al. (1978), it is believed that the

IKSZ is responsible for the shallow seismic activity, whereas

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, the

deeper earthquakes are considered to be intraplate events

within the lower crust (lower part of the Himalayan crust).

Table 2. FMS parameters obtained for the 25 earthquakes FMS Nos. Nature of FMS Fault plane (FP) Auxiliary plane (AP) P-Axis T-Axis

Strike Dip Strike Dip Strike Plunge Strike Plunge

Pre-2005 FMS P1 THRUST 142o 33

oNE 289 o 62 o 31 o 15 o 164 o 69 o

P2 THRUST 308 o 7 oNE 40 o 90 oSE 136 o 44 o 303 o 45 o

P3 THRUST 339 o 26 oNE 166 o 64 oSW -106 o 19 o 82 o 70 o

P4 THRUST 331 o 12 oNE 87 o 85 oS 186 o 39 o 345 o 49 o

P5 THRUST 320 o 13 oNE 154 o 78 oSW 242 o 33 o 68 o 57 o

P6 REVERSE 348 o 52 oNE 105 o 60 oSW 225 o 5 o 321 o 53 o

P7 THRUST 329 o 21 oE 135 o 70 oS -131 o 24 o 37 o 65 o

P8 THRUST 7 o 11 oNE 175 o 79 oNW -93 o 34 o 82 o 56 o

P9 THRUST 284 o 34 oNE 162 o 70 o -129 o 21 o 108 o 56 o

P10 RLSS 312 o 85 oNE 42 o 88 oNW 177 o 2 o -267 o 4 o

KH earthquake FMS

1. THRUST 342 o 57 o NE 101 o 53 o NW 42 o 2 o 310 o 53 o

2. THRUST 335 o 54 o NE 107 o 48 o NW 42 o 4 o 305 o 63 o 3. THRUST 340 o 57 o NE 102 o 51 o NW 42 o 4 o 306 o 56 o 4. THRUST 339 o 56 o NE 97 o 54 o NW 38 o 1 o 307 o 53 o 5. THRUST 357 o 62 o NE 145 o 32 o NW 75 o 15 o 300 o 69 o 6. THRUST 339 o 51 o NE 102 o 56 o NW 219 o 3 o 314 o 57 7. THRUST 353 o 69 o NE 101 o 52 o NW 50 o 10 o 310 o 44 o 8. THRUST 321 o 64 o NE 179 o 31 o NW 65 o 17 o 198 o 66 o 9. THRUST 313 o 49 o NE 95 o 48 o NW 24 o 0 o 293 o 70 o 10. THRUST 1 o 77 o NE 113 o 31 o NW 69 o 27 o 302 o 50 o 11. THRUST 320 o 43 o NE 124 o 48 o NW 138 o 3 o 329 o 82 o 12. THRUST 334 o 60 o NE 105 o 41 o NW 43 o 10 o 294 o 62 o 13. THRUST 309 o 48 o NE 101 o 46 o NW 25 o 1 o 291 o 75 o 14. THRUST 341 o 57 o NE 99 o 54 o NW 41 o 2 o 309 o 53 o 15. THRUST 157 o 81 o NE 52 o 33 o NW 88 o 28 o 35 o 45 o

FMS P6 and P7 like P2 and P4 are also located near the MMT.

P6, with a magnitude of Mw = 6.4, was the second largest

earthquake to have occurred in the study area. Focal depths for

these events are 15 and 21 km, respectively (Table 1). From the

earlier description, it is known that the IKSZ occurs at depths

ranging from 12 to 25 km. According to Ni et al. (1991), 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 under thrusting of the

Indian plate beneath the IKSZ. Various workers named P6 as

the Pattan earthquake of 1974 and determined its FMS. The

epicenter is located approximately 10 km north of MMT in a

terrain of garnet granulites overlying ultramafic rocks.

Solutions obtained in the present study are of reverse faulting,

with a component of right-lateral strike slip in the case of the P6.

FMS 2, 4, 5, 10, and 14, are situated midway between the MMT

and the Syntaxis region faults. The Indian Plate to the south of

MMT may comprise a stack of nappes. The IKSZ underlies this

area. In this domal structure, basement uplift is an ongoing

process and may be the reason for generating right-lateral strike

slip faults in this part of the study area. The earthquake with

FMS P10 is located close to the nearly NS trending strike slip

Thakot Fault, which is considered to be the surface expression of

the Tarbela seismic zone. This zone, lying between depths of 8

and 18 km, overlies the HLSZ and comprises of thrusts and

strike slip faults with strikes in both the NW and NE directions.

Considering the strike (NWSE), dip (NE), sense of thrust, and

depth (10 km) of FMS 3 and 6, it is anticipated that the IKSZ

was activated. The events P3, 1, 7, and 15 are situated within the

HKS and P9and P8 at its eastern limb. FMS 3, 6, 7 and 15 are

shallow earthquakes with focal depths of 510 km, whereas P3

and P9 are of 54 and 43-km depth, respectively. The latter two

provide further confirmation of the attachment of the crystalline

basement of a crust thickened to about 58 km. Thrust solutions

with NWSE striking rupture plane have been obtained for all

the events including FMS 1, which is for the October 8, 2005

KH earthquake. Considering the trend and dip direction of the

Bagh Basement Fault, the plane dipping NE at 26 is inferred to

be the rupture plane for P3 and P9. This suggests the existence

of more than one basement fault in the core of the Syntaxis.

IKSZ is about 95 KM NE off Tarbela Dam Project (3470 MW)

and about 60 km NW off under construction Neelum Jhelum

Hydropower Project (1000 MW) (Fig 4). The Figure 4 indicates

that both Projects are surrounded by very active tectonic

features. From the safety monitoring view deployment of

seismic instruments is necessary. Tarbela Dam Project is well

surrounded by MSMS instruments, however, the Eastern side

Neelum Jhelum Project lacks such setup.

7. CONCLUSIONS

The October 8, 2005 KH earthquake is the deadliest of all the

Himalayan earthquakes. Historical and instrumental data

propose that this earthquake occurred in the region where

seismic activity is very high. From the thorough study of the 25

FMS in relation to the structure and tectonics of the area, three

significant conclusions can be drawn: (1) Shallow events (depth

Figure 4. Seismotectonic of Northern Pakistan & WAPDA MSMS

10 km) with prominent strike slip solutions are associated with

the surface strike slip faults (e.g., BBF) and/or due to the

uplifting of the Besham domal structure; (2) moderate depth

events (depth 1025 km) with thrust/reverse solutions but having

minor right-lateral strike slip component are associated with

IKSZ; and (3) deeper earthquakes (depth below IKSZ) with pure

thrust/reverse solutions may be related with the under-thrusting

of the Indian plate beneath the IKSZ, which represents a major

thrust zone. The FMS of 15 aftershocks and the KH earthquake

are strongly indicative of a NWSE trending, NE dipping thrust

fault, about 90 km in length. Some 35 km of this proposed fault

follows the NWSE trending BalakotBagh Fault. It is therefore

propose that the IKSZ is seismically active and was the source

of the KH earthquake. The FMS of this earthquake is also a

NWSE striking thrust with minor right-lateral strike slip

component. The present study shows that the IKSZ extends

further southeast into the center of Syntaxis. Under construction

Neelum Jhelum & Kohala Hydro Project needs installation of

MSMS instruments from the safety monitoring point of view.

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