Three‐dimensional velocity structure and hypocenters of earthquakes beneath the Hazara Arc, Pakistan: Geometry of the underthrusting Indian Plate

Abstract

The three-dimensional P and S wave velocity stmctures and hypocenters of 420 events beneath the westem Hazara Arc are obtained simultaneously by inverting travel time data observed at fifteen Tarbela seismic stations. In general, the P and S wave velocity distribution of the top layer (0-6 km depth) correlates well with surface geology. Within this layer we find a low-velocity region beneath the Hazara Thrust Zone (HTZ) corresponding to the underthmsted Murree Formation, and there are high-velocity regions south of the Main Mantle Thrust (MMT) which are associated with the exposed Cambrian, late Paleozoic, and Tertiary granites. A low-velocity zone immediately to the west of the Hazara-Kashmir Syntaxis (HKS) indicates the existence of a Miocene foreland basin which is covered by late stage southeasterly directed thrusts along the Hazara Arc and is consistent with the idea that the HKS is detached from the lower cmst. From the Salt Range to the HTZ, the Indian plate dips at a shallow angle, about 20-3 ø to the noaheast. North of the HTZ the underthrusting Indian plate dips gently to the northeast with an increased slope of 5 ø to 8 ø until it reaches the Indus-Kohistan Seismic Zone (IKSZ). Along the NW trending IKSZ the Indian plate bends more steeply to the northeast beneath a seismically active midcmstal wedge directed to the southwest. The larger events in the IKSZ are interpreted as occurring on a major thmst zone that can be followed to a depth of 24 km. The IKSZ appears to consist of an upper seismic zone (from the surface to about 8 km) and a lower seismic zone (12 km to 24 km) separated by an aseismic region about 4 km thick. The lower IKSZ may represent the leading edge of a southwestward directed slab which has not yet ruptured the surface. Hypocenters of relocated earthquakes indicate that the HTZ is about 30 km wide with most of the larger microemthquakes occurring at 12-14 km. Seismicity along the HTZ suggests that the Panjal, and Murree thrusts are active.

Recent seismic activity in the NW Himalayan Fold and Thrust Belt, Pakistan: focal mechanism solution and its tectonic implications

“…The 28 December 1974, Pattan earthquake (m b 6), another destructive earthquake that occurred before the 2005 Muzaffarabad earthquake was also due to the IKSZ. This zone was first identified by Armbruster et al (1978) using micro-earthquake survey data (1973)(1974) and is further confirmed by Seeber & Armbruster (1979) and Ni et al (1991). The IKSZ is believed to be divided into two zones, i.e.…”

Crustal Structure Along the Lawrencepur-Astor Profile in the Northwest Himalayas

“…The top of this layer is at approximately 13 km depth and its maximum thickness is about 5 km. This mid-crustal layer, to the northeast of Hazara Syntaxis, may represent the leading edge of a slab detached from the basement of the Indian plate as suggested by NI et al (1991). The base of the low velocity layer has a velocity of 6.2 km s )1 similar to that towards the southeast.…”

“…The base of the low velocity layer has a velocity of 6.2 km s )1 similar to that towards the southeast. The velocity model of NI et al (1991) shows a continuous increase of velocity from 5.9 km s )1 (5-8 km depth) to 6.6 km s )1 (maximum 30 km depth) to the southwest of Hazara Syntaxis. They do not agree with the presence of a mid-crustal low velocity zone (LVZ), between Lawrencepur and Hazara Syntaxis shown by GUERRA et al (1983).…”

“…163, 2006 Crustal Structure Along the Lawrencepur-Astor Profile (TAPPONIER et al, 1981;SEBER et al, 1981;COWARD and BUTLER 1986;BOSSART et al, 1988) have been given for its peculiar geometry. NI et al (1991) are of the opinion that its bend is formed by a stack of thrust sheets made up of various metasediments which have overthrusted the early Tertiary formation.…”

“…Due to this, several international projects (BELOUSSOV et al, 1980;KAILA et al, 1983;CHAMBERLAIN et al, 2002;MELTZER et al, 2001;ZEITLER et al, 2001) have been launched to study the Pamir, Nanga Parbat and Hindukush regions. Ni et al (1991) computed a 3-D velocity structure from the earthquake data, in the region of Hazara Syntaxis and Main Mantle Thrust. JOHNSON and VINCENT (2002) have developed a gross 3-D velocity model, based on earthquake tomographic studies for a large part of India and Pakistan region.…”