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 structures and hypocenters of 420 events beneath the western 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 underthrusted 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 crust. From the Salt Range to the HTZ, the Indian plate dips at a shallow angle, about 2°–3° to the northeast. North of the HTZ the underthrusting Indian plate dips gently to the northeast with an increased slope of 5° to 8° 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 midcrustal wedge directed to the southwest. The larger events in the IKSZ are interpreted as occurring on a major thrust 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 microearthquakes occurring at 12–14 km. Seismicity along the HTZ suggests that the Panjal, and Murree thrusts are active.