Active Tectonics and Potential of seismic hazard in the Northwest Himalayan region, India

The Himalaya overrides the Indian plate along a decollement fault, referred as the Main Himalayan Thrust (MHT). The 2400 km long Himalayan mountain arc in the northern boundary of the Indian sub-continent is one of the most seismically active regions of the world. The Himalayan Frontal Thrust (HFT) is characterized by an abrupt physio-graphic and tectonic break between the Himalayan front and the Indo-Gangetic plain. The HFT represents the southern surface expression of the MHT on the Himalayan front. Slip rate 13-15 mm/yr has been estimated on the HFT (Wesnousky et al. 2006). Earthquakes recorded by WIHG shows a concentrated belt of seismic events located in the Main Central Thrust Zone and the physio-graphic transition zone between the Higher and Lesser Himalaya. However, there is quiescence in the Himalayan frontal zone where surface rupture and active faults are reported. GPS measurements indicate the segment between the southern extent of Microcosmic zone and the HFT is locked. The great earthquake originating in the locked segment rupture the plate boundary fault and propagate to the Himalaya front and are registered as surface rupture reactivating the fault in the HFFS. In the recent study the Seismic potential of the Himalayan segment (29-33°N; 75- 80°E) of the Northwest Himalayan, India has been estimated in order to evaluate the potential of seismic hazard in this region. The hypocentral parameters estimated incorporating the local crustal velocity model derived utilizing 25 seismic stations and relocated 462 earthquakes triggered in the NW Himalaya during 2004 and 2015 using 945 P-phase and 893 S-phase and also determined moment tensor solutions for 9 (Mw>4.5) earthquakes using waveform inversion. The geometry of the MHT plane has also been discussed in this study which varies along the strike of the Himalaya in flat and ramp segments. The earthquake potential prevailing in the western Himalayan seismic gap lying between the epicentral zone of the 1905 Kangra earthquake and the 1975 Kinnaur earthquake has also been estimated. The total amount of energy released since the last great event has been estimated which is significantly low in comparison of the estimated accumulated total energy. This indicates seismic hazard potential in this region is very high and can generate a major to great earthquake in future.