Shear wave velocity and crustal structure along Satluj valley, Northwest Himalaya

Abstract

The continent-continent collision between India and Asia has resulted in the spectacular Himalayan mountain range, development of large fold and thrust sequences and greater than 1500 km shortening of the crust (Molnar & Tapponnier, 1975; Yin, 2006). The Indian plate is being underthrusted beneath the Eurasian plate along a north-dipping detachment surface named as Main Himalayan Thrust (MHT), separating down-going Indian plate from overriding Himalayan wedge (Seeber et al., 1981). The over-thrusted wedge was formed due to uplift along south-verging thrust planes located from north to south. The Himalayan wedge adjoins with the lithotectonic units: Tethyan Himalaya, Higher Himalaya, Lesser Himalaya and Sub-Himalaya. These lithotectonic units are separated by faults viz. South Tibetan Detachment System (STDS), Main Central Thrust (MCT) and Main Boundary Thrust (MBT), respectively. The Himalayan wedge is confined between the Himalayan Frontal Thrust (HFT) at the south separating the Sub-Himalaya from the Indo- Gangetic Plain (IGP) and Indus-Tsangpo Suture zone (ITSZ) towards the north, which is considered as the surface collision boundary of India-Asia collision. The major thrust faults are assumed to sole into the detachment- MHT (Seeber & Armbruster, 1981; Ni & Barazangi, 1984; Zhao et al., 1993; Yin, 2006; Avouac, 2007). The MHT accumulates the strain energy originated due to collision and releases accumulated energy in the form of large/great earthquakes e.g. the 1905 Kangra earthquake of magnitude Mw 8.0 and the 1934 Bihar-Nepal earthquake of magnitude Mw 8.0 (Middlemiss, 1910; Ni & Barazangi, 1984; Kayal et al., 2001). Seismological studies reported a ramp structure on the MHT that causes earthquakes (Pandey et al., 1995; Caldwell et al., 2013; Duputel et al., 2016). Significant variation of seismicity along the strike of the northwest (NW) Himalaya has been reported by recent studies (Gahalaut & Kalpna, 2001; Arora et al., 2012). Seismicity in the NW Himalaya mostly concentrates in the Himalayan Seismic Belt (HSB), a narrow belt of 30–50 km width spanning the northern Lesser Himalaya and the southern Higher Himalaya (Arora et al., 2012).