Three-Dimensional Velocity Structure and Hypocenters of Earthquakes Beneath the Hazara Arc, Pakistan: Geometry of the Underthrusting Indian Plate (original) (raw)
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Journal of Seismology, 2009
This paper deals with the data obtained from local networks in northern Pakistan for 251 earthquakes of magnitude ≥4.0 for October 8, 2005 to December 31, 2006 period. The study presents focal mechanism solutions (FMS) of 12 pre- (1904–2005) and 17 post- (October 8, 2005–December, 2005) Muzaffarabad Earthquake, their detailed tectonic interpretation, and correlation with surface evidence of co-seismic rupture with published synthetic aperture radar data. Distribution of landslides obtained from National Engineering Services of Pakistan and the earthquake damages are also discussed. Aftershock distribution, which is more prominent in the crystalline zone (northwest of Muzaffarabad), defines a 50-km-wide NW–SE trending zone that extends for 200 km from the main mantle thrust to the center of the Hazara–Kashmir Syntaxis. The FMS of the main shock and 16 aftershocks having magnitude ≥4.0 indicate thrusting to be the dominant mechanism with rupture planes having NW–SE trend and NE dip. In addition, 12 FMS of pre-Muzaffarabad Earthquake (1904–2004) from the same area have been determined and results are compared. This leads to the conclusion that the wedge-shaped NW–SE trending blind zone, referred to by earlier workers as the Indus Kohistan Seismic Zone (IKSZ), has been activated during the Muzaffarabad earthquake. The right-lateral component in all FMS, supported by the surface evidences, suggests the involvement of Balakot–Bagh Fault (BBF). We propose that the IKSZ is the source of the October 8, 2005 Muzaffarabad earthquake that reactivated the BBF. Furthermore, the IKSZ does not end at the nose of the syntaxis but extends further southeast of it. Tectonic complexity seems to be due to a variety of factors. Also, thrust and reverse solutions near the northern collisional boundary (main mantle thrust) have mostly NE/SW-directed P-axis orientations. From the detailed FMS analysis, three conclusions have been drawn: (1) Shallow events (depth ≤10 km) with prominent strike slip solutions (earlier earthquakes) are associated with the surface strike slip faults (e.g., Muzaffarabad Fault) and/or the Besham domal structure; (2) moderate depth events (depth 10–25 km) with thrust/reverse solutions but having minor right-lateral strike slip component (all Muzaffarabad earthquakes and two earlier) are associated with the IKSZ; (3) deeper earthquakes (depth below IKSZ) with pure thrust/reverse solutions may be related to the under-thrusting of the Indian plate beneath the IKSZ, which represents a major thrust zone. Imbricate thrusting and breaking and thickening of the crust are considered to be caused by steep bending of the under-thrusting plate at the collisional boundary.
Journal of Geophysical Research, 1980
Relocations of earthquakes, recorded by a local network of stations in Afghanistan and Tadjikistan in 1966 and 1967, indicate a narrow seismic zone (width • 30 kin) dipping steeply into the mantle to a depth of 300 km beneath the Pamir and Hindu Kush ranges. Very low seismicity was observed at depths less than about 70 kin, the approximate depth of the Moho. Clear gaps in activity exist also within the zone of intermediate depth seismicity. One gap, about 50 km wide near 37øN and at depths greater than lO0 kin, separates a steeply northward dipping zone to the southwest from a steeply southeastward dipping zone to the northeast. This gap probably marks either a tear in the downgoing slab or a gap between two oppositely dipping slabs. Fault plane solutions, determined by Soboleva for events between 1960 and 1967, generally show steeply plunging T axes approximately within the planar seismic zone. They therefore are grossly similar to those at island arcs where no deep earthquakes occur and presumably result from gravitational body forces acting on a relatively dense slab of lithosphere. At the same time there is a very large variation in the fault plane solutions, much larger than is common at island arcs. Introduction Although it does not have an island arc structure, the Pamir-Hindu Kush region is the source of very high intermediate depth seismicity. This region is one of the most active sources of earthquakes felt within the USSR, even though most of it lies outside of the USSR, in Afghanistan. Accordingly, Soviet seismologists have devoted considerable attention to its study. An extensive network of stations has been operated in Tadjikistan for 20 years by the Tadjik Institute of Seismo-Resistant Construction and Seismology (TISSS) of the Academy of Sciences of the Tadjik SSR and by the Institute of Physics of the Earth (IFZ) of the Academy of Sciences of the USSR (in Moscow). Moreover, in 1966 and 1967 a special network was installed in Afghanistan and along the Soviet-Afghan boundary by the IFZ to study the seis-1966 and 1967 allowed the most precise determinations of hypocenter that were possible at that time [Lukk and Nersesov, 1970]. These hypocenters defined an approximately planar zone that dips steeply into upper mantle and extends in an east-west direction for nearly 700 km. With careful analytical and graphical techniques, but without the aid of high-speed computers, Lukk and Nersesov [1970] simultaneously determined a velocity structure for the crust and upper mantle and located the earthquakes. In the present paper we extend their study and present relocations of these same events using a computer. The data obtained with this network were also used to infer a high-velocity zone surrounding Massachusetts 02139.
An enhanced image of the Pamir-Hindu Kush seismic zone from relocated earthquake hypocentres
Geophysical Journal International, 1998
We determine the shape of the seismic zone in the Pamir^Hindu Kush region de¢ned by [30^42 0 N, 68^78 0 E] by obtaining improved hypocentral locations with 90 per cent con¢dence limits of less than 30 km (the depth error bar for most of the earthquakes) of about 6000 shallow and intermediate-depth earthquakes. Available S and depth-phase arrival times are also used together with the P-wave arrival times in the joint hypocentre determination technique. To obtain the best possible hypocentral locations, the study region is divided into three depth ranges, 0^60, 60^160 and b160 km. The 0^60 km depth zone is then subdivided laterally into 19 blocks, with the deeper regions divided into two blocks each. The improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate-depth seismicity in the PamirĤ indu Kush region is most simply explained by a single S-shaped seismic zone, 700 km long and no more than 30 km wide and with most activity concentrated at 100^300 km depth. The main features observed are: (1) the eastward steepening of the north-dipping Hindu Kush seismic zone through to its overturning at its eastern end beneath the Pamirs, where it dips to the southeast; (2) the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the Hindu Kush seismic zone; (3) the very abrupt cut-o¡ in intermediate-depth seismicity at 90^110 km depth with no extension to shallower depths under the Pamirs, and with a persistent gap between the intermediate and shallow seismicity in the northern Pamirs; and (4) the unusual horizontal Taxes for intermediate-depth earthquakes of the Pamir seismic zone, which align with its curvature. This study shows that the seismic zone under the Hindu Kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity, whereas the Pamir region has horizontal Taxes that follow the trend of the contorted seismic zone. This suggests that the Pamir seismic zone is a slab deformed due to £ow in the upper mantle.
Geometry of the Pamir-Hindu Kush intermediate-depth earthquake zone from local seismic data
Journal of Geophysical Research B: Solid Earth, 2013
We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. A total of 9532 earthquakes were detected, located, and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algorithms, waveform cross-correlation, and multi-event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir-Hindu Kush intermediate-depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes: one beneath the Pamir and the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (approximately 10 km width), curviplanar arc that strikes east-west and dips south at its eastern end and then progressively turns by 90°to reach a north-south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate-depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20-25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east-west, yet bends northeast, toward the Pamir, at its eastern end. It may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth. In the upper part, events form a plane that is 15-25 km thick in cross section and dips sub-vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub-parallel planes. The observed geometry could be reconciled either with a model of two-sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitates a contortion and oversteepening of the latter.
Geological setting of the 8 October 2005 Kashmir earthquake
Journal of Seismology, 2009
The source of the 8 October 2005 earthquake of M 7.6 was the northwest-striking Balakot–Bagh (B–B) fault, which had been mapped by the Geological Survey of Pakistan prior to the earthquake but had not been recognized as active except for a 16-km section near Muzaffarabad. The fault follows the Indus–Kohistan Seismic Zone (IKSZ); both cut across and locally offset the Hazara–Kashmir Syntaxis defined by the Main Boundary and Panjal thrusts. The fault has no expression in facies of the Miocene–Pleistocene Siwalik Group but does offset late Pleistocene terrace surfaces in Pakistan-administered Jammu-Kashmir. Two en-échelon anticlines near Muzaffarabad and Balakot expose Precambrian Muzaffarabad Limestone and are cut by the B–B fault on their southwest sides, suggesting that folding and exposure of Precambrian rocks by erosion accompanied Quaternary displacement along the fault. The B–B fault has reverse separation, northeast side up; uplift of the northeast side accompanied displacement, producing higher topography and steeper stream gradients northeast of the fault. No surface expression of the B–B fault has been found northwest of the syntaxis, although the IKSZ and steeper stream gradients continue at least as far as the Indus River, the site of the Pattan earthquake of M 6.2 in 1974. To the southeast, northwest-striking faults were mapped by the Geological Survey of Pakistan. One of these faults, the Riasi thrust, cuts across the southwest flank of an anticline exposing Precambrian limestone. Farther southeast, in Indian-administered territory, Holocene activity on the Riasi thrust has been described. In the Kangra reentrant still farther southeast, active faulting may follow the Soan thrust, along which Holocene and Pleistocene offsets have been described. The Soan thrust, rather than the south flank of the Janauri anticline, may represent the surface projection of the 1905 Kangra earthquake of M 7.8.
Geophysical Journal International, 2012
We provide a new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake region based on magnetotelluric (MT) investigations carried out close to the epicentre zone. 2-D inversion of broad-band MT data of two profiles of lengths 32 km (AA) and 52 km (BB) revealed a thick (∼3 km) highly conductive (1-4-m) surface layer of fluviomarine Mesozoic-Cenozoic sediments. The models delineate the hypocentre zone located at ∼20-25 km depth that manifests the high resistivity-conductivity transition zone. The accumulation of compressive stresses post-rifting along this weak zone has resulted in the reverse slip of Bhuj 2001 earthquake. The reverse fault (F 1) associated with the earthquake is believed to be an ancient normal fault formed during the rifting phase. Contrary to earlier suggested theories, we suggest that F 1 got initiated along the high resistivity-conductivity transition zone causing the Bhuj 2001 event. The geoelectric models revealed a laterally extending partially resistive zone at 20-30 km depth range showing a tendency to extend further deep. Model calculations using synthetic data also support this observation. Therefore, we hypothesize the presence of a basal detachment, marking the transition zone between the continental crust and the lithospheric upper mantle at ∼40 km depth, intersected by the F 1. The geoelectric models suggest that the crustal thinning caused the asthenospheric upwelling and/or serpentinization leading to the ascent of volatiles and melts. The subsurface geometry in Kachchh basin suggests the thickskinned deformation.
A 1D velocity model of the Tehri region in the Garhwal Himalaya is estimated from the travel-time inversion of 145 well-located local events having 1177 P and 1090 S arrivals. The velocity model consists of six layers up to 24 km depth, with P- and S-wave velocities ranging from 4.42 to 6:78 km=s and 2.41 to 3.71 km=s, respectively. The depth of the Moho, estimated using travel-time curves of crustal phases, is about 46 km. A low-velocity layer deciphered between 12 and 14 km depths is ascribed to fractured basement thrust representing the upper surface of the Indian plate. Using the proposed velocity model, 1457 events are relocated. About 70% of the locatable events occur in the Inner Lesser Himalaya between the Main Central thrust (MCT) and the Srinagar thrust. The postulated depth of the basement thrust in the vicinity of the MCT is about 10–12 km. The depth distribution of events delineates the geometry of the seismically active Main Himalayan thrust (MHT) below a 300-km-long segment of the MCT. The MHT is composed of two shallow-dipping fracture zones that seem to represent seismically active thrust zones dipping in opposite directions. Two seismicity zones, at 10 and 15 km depths with a 5 km vertical separation, define a flat-ramp-flat type structure of the MHT in the vicinity of the MCT. The postulated front of the underthrusting Indian plate is at a depth of about 15– 18 km. The lower-flat seismicity zone bifurcates into two, indicating further slicing of the lower-flat zone. The postulated thickness of the brittle part of the underthrusting Indian crust is about 20 km in the vicinity of the MCT.
2010
During the wee hours of October 08, 2005, a devastating shallow focused (16.2 km) earthquake with moment magnitude of Mw 7.7 occurred in the Pakistan's Kashmir Hazara Region. Its tremors were felt in a radius of over 1000 km with damages taking place in an area of 36000 sq km. More than 0.1 million people died and the rehabilitation of infrastructures damages are estimated to cost around five billion dollars. The Kashmir Hazara terrain is located on the NW margin of lesser Himalaya. The KHS is one of the bold tectonic scars which physically isolate this terrain from rest of the Himalaya. Other major tectonic features sculpturing this terrain in the shape of folds and faults are: Main Mantle Thrust (MMT), Main Boundary Thrust (MBT), Panjal Thrust (PT), Hazara Thrust (HT) and the Indus Valley Faults. All these mega structures are the abode of variable seismicity and generate earthquakes of low to high (damaging) magnitude. The seismic zones of the mega-crustal deformations in the Kashmir Hazara terrain, from where the earthquakes emanate generally lies between 10-60 km surface depths. The earthquakes generated at this depth are categorized as shallow and are usually more hazardous. The earthquake resulted from the subduction of Indo-Pakistan plate beneath the Eurasian plate and it ruptured the southwest Jhelum Thrust (JT) fault. The fault was previously inferred to be as active in a region where the river incises directly into the Murree sandstones on the west side of the valley (footwall of JT), while it has abandoned large inset terraces along the east side (hanging wall of JT). The occurrence of Kashmir-Hazara earthquake confirms that the active Jhelum Thrust (JT) and Jhelum Fault (JF), in a region located well north of the Main Himalayan Frontal Thrust, accommodate roughly EWoriented, present day shortening related to "zipper tectonics" within the part of the Kashmir Hazara Syntaxis (KHS). Maximum Modified Mercalli Intensity was X at Balakot, situated on the hanging wall side of the causative fault and the maximum ground motions in the same area were inferred to be 0.90 'g' from overturned vehicles in the direction parallel to the axis of valley.
Tectonophysics, 1983
Long period body waves are examined to show that the Hamran (1972.9.3), Dare1 (1981.9.12) and Patan (1974.1228) earthquakes in Kobistan had focal depths of about 8-10 km. All involved high angle reverse faulting (th~sting) and had seismic moments of about 2.2 to 2.7. lO25 dyne cm. These shallow depths contrast with the deeper h~~ntr~ found in the Hindu Kush and northeast Karakoram to the north and in Hazara to the south. The Hamran and Patan shocks were assigned depths of 45 km by the ISC, indicating that even well-recorded events in this region may have focal depths in error by 30 km