Geodetic constraints and tectonic implications of the Mw=7.6, 8 October 2005, Kashmir earthquake Abstract

abstract

  • Four points of the sparse westernmost Himalaya GPS network installed in 2001 in Pakistan were within one rupture dimension of the 2005 Kashmir Earthquake. The closest points in the footwall were displaced NW by 8 and 56 cm respectively: two others provide null constraints indistinguishable from interseismic deformation. The data are consistent with mean reverse slip of more than 3 m on a main fault with average strike of 330º ± 15º. They provide a numerical calibration of the 2D image of coseismic surface displacements observed with InSAR amplitude analysis. The InSAR observations are modeled as a rupture plane of 80-90 km in length, 39º dip, minimum depth of ~1 km (near Balakot), and maximum depth of 25 km. Coulomb stress calculations indicate that aftershocks to the NW occur in a region of enhanced failure. Coulomb stress also increased on strike-slip faults SW of the epicenter bringing the Jhellum and Tarbela faults closer to failure. However, few large aftershocks occur in a similar region of increased Coulomb stress to the SE, although this region has not failed in a large event since 1555. Regional tectonic velocities relative to Eurasia are oblique to the cosiesmic slip direction, suggesting that the tectonic stress field must play a significant role in modulating Coulomb stress. The 2001-2005 velocity for Peshawar indicates north-south directed convergence of 21±2 mm/year across the plate boundary between the Pamir and Peshawar, and an additional 8 mm/yr between Peshawar and the Indian plate at the southern edge of the Potwar plateau. Thus if no earthquakes are missing from the seismic record, the Kashmir earthquake has released less than half of the 8.5 m slip deficit accumulated since 1555. Moreover, its along-strike dimension (80-90 km) falls at least 200 km short of removing the slip deficit from the apparently mature seismic gap south and west of Kashmir. We conclude that an Mw=8.2 earthquake is possible in the Pakistan/Indian Himalaya, contiguous with the recent earthquake extending SE to the Kangra 1905 Mw=7.8 rupture. In contrast to the inferred loading in this SE region, the 2005 rupture has effectively reduced compressive stresses in a NE/SW direction that might otherwise have driven a detachment-type earthquake near Islamabad. However, vergence of the thin-skinned tectonics here is orthogonal to this stress regime and it is unclear whether, or by how long, the Islamabad region has been reprieved from a future shallow faulting event of the sort that possibly destroyed Taxila in the first century.

authors

publication date

  • 2006