Earthquake-generating Process in Pakistan-Kashmir Himalayan Front: A Tectonic Overview

摘要

The Harvard Centroid Moment Tensor (CMT) focal mechanism solutions of the 8 October 2005 Muzaffarabad earthquake (MUZ 7.6) mainshock (moment magnitude 7.6), the 28 December 1974 Pattan earthquake (PAT 5.9) mainshock (body wave magnitude 5.9) and their associated aftershocks are examined to study the tectonic deformation and seismicity behaviour at Indus Kohistan Seismic Zone (IKSZ), in northwestern Pakistan - Kashmir Himalayan front. The CMT solutions for both the earthquake sequences depict similar nodal planes and indicated positive slip with predominance of shallow northeast-dipping thrust faults. The northeastern elevated hanging wall of the IKSZ is found to be slipped up by about 1.5 -1.9 m above the southwestern footwall during the MUZ 7.6 earthquake. The deviatoric compression (P) axes are horizontal and mainly from SW direction, which is the direction of the convergence of Indian plate. The depth-section of the seismic events indicated that all the epicentres are confined in a zone, between plastic decollement and ductile basement, having a different rheological behaviour in rock type. These earthquakes occurred due to the release of accumulated stress resulted from the thrust produced by the southwestward leading-front of overthrusted high-hydrothermal fluid-rich mid-crustal rocks (Basement Thrust Front), differential sediment loads on the decollement due to major river incisions and high crustal exhumation rate along anomalous topographic rise of Hazara-Kashmir syntaxis, IKSZ and Indus syntaxis. Our observation revealed that the pre-existing IKSZ is the principal source zone for the PAT 5.9 and MUZ 7.6 earthquake sequences. The MUZ 7.6 earthquake may have occurred on the unaffected southern segment by re-rupturing the IKSZ after a gap of 31 years. The rupture process around the seismogenic IKSZ may have facilitated by fluids in the partial melt matrix rather regional tectonic stress alone.