Lithospheric Structure and Earthquakes beneath Kashmir Himalaya
Tuesday, 16 December 2014
Over the last two centuries, convergence between India and Tibet has outpaced the cumulative slip released through Himalayan earthquakes and have resulted in seismic gap across Kashmir Himalaya. Recent GPS geodetic data from Kashmir show that the ongoing convergence is accumulated as elastic strain within a ~200 km wide locked decollement and is sufficiently stressed to drive a magnitude 8 or greater event. Recently published focal mechanism of the mb 5.7 (2013) Kishtwar earthquake and hypocentral distribution of small-to-moderate seismicity for the past 60 years, showed that the down dip end of the locked decollement is currently active and could possibly be the site of initiation of a future great earthquake. In order to assess the seismic hazard in this Kashmir gap, we require a detailed knowledge of the lithospheric structure and use it to reliably locate active faults. A pilot seismological experiment, of nine broadband seismographs, have been deployed across the Kashmir Himalaya to achieve this goal. These stations are sited on the Siwalik Himalaya (AKNR, NGRT, SMVD, SUND and TAPN), the Lesser Himalaya (RAMN and UDHM) and the Higher Himalaya (BADR and PHAG), and straddle major Himalayan thrust zones. Most of these stations have recorded high quality broadband data for a year, which has been used to compute receiver functions and relocate local earthquakes. The Moho Ps is the strongest arrival on all the receiver functions, and highlights the base of the underthrusting Indian crust as a large impedance contrast boundary. Forward modeling of receiver functions show that the crustal thickness increases from ~40 km beneath the Siwalik Himalaya to ~48 km beneath the Lesser Himalaya and to ~52 km beneath the Higher Himalaya. The average crustal Vp/Vs points to a felsic Indian crust underthrusting the Kashmir Himalaya. Relocated local earthquakes cluster around the hypocenter of the Kishtwar earthquake and attests to the active downdip end of the locked decollement.