Crustal velocity structure and seismotectonics of the Kinnaur region of northwest Himalaya: New constraints based on recent micro-earthquake data

Abstract

• New ‘minimum’ 1-D crustal velocity model of Kinnaur Himalaya through VELEST algorithm. • High micro-earthquake activity to the north of HHSB in the Kinnaur Himalaya. • Crustal seismicity is concentrated between 5 and 25 km depth above the Main Himalayan Thrust. • Two distinct zones with thrust fault mechanism in the HHSB and normal fault mechanism to the northern part. • Major improvement in the crustal structure and the seismotectonics of the region. The occurrence of micro-earthquakes is monitored in the Kinnaur region of NW Himalaya using a local seismograph network of 12 broadband stations. About 800 micro-earthquakes recorded from 2008 to 2012 within the magnitude range of 1.0 to 4.5, are relocated using the newly developed 1-D ‘minimum’ crustal velocity model. The velocity model developed using the VELEST program based on P- and S-phase data divides the upper 35 km thick crust into 8 layers in which Vp/Vs appears to be quite variable. The seismogenic zones in this part are mainly located within a depth range between 5 and 25 km. On the basis of integration of different studies, this region around the epicentre zone of the M S 6.8 earthquake of 1975 can be divided into several seismogenic zones. The southern zone falls in the Higher Himalayan Seismic Belt, where the occurrence of M L > 5.0 earthquakes is rare. However, in comparison with other regions of the NW Himalaya, the northern zone in the South Tibetan Detachment shows a unique feature of high seismicity. Seismicity is bounded to the east by the Kaurik-Chango fault zone. The north–south oriented Kaurik Chango normal fault is transverse to the strike of major tectonic features of the Himalaya and follows the trend of present-day seismicity along with the fault plane solution of M S 6.8 Kinnaur earthquake. Results from microseismicity and fault plane solutions provide valuable information to understand the seismotectonics of the region. The stress inversion shows variable stress patterns from north to south.