Seismicity and the state of stress from investigations of local earthquakes in the Kumaon Himalaya

Abstract

Results of investigations of local earthquakes in the region of the Main Central Thrust (MCT) in the Kumaon Himalaya, between and adjacent to the valleys of the Bhagirathi and Yamuna rivers, are presented. Records of over 250 earthquakes were analysed and the following facts emerged: (1) Earthquakes in the Himalayas occur in specific areas and belts. One such belt has been identified in the region under investigation, hypocentral estimates being more reliable for earthquakes occurring in the middle segment approximately 70 km long of this belt, crossing the Yamuna river between the villages of Barkot and Syanachatti. (2) All but a few epicentres in this middle segment, lie to the southwest of the surface trace of the MCT in a zone with a width of 10–30 km. (3) Most of the earthquakes in this segment occur at depths of less than 10 km below the ground surface, the maximum estimated depth being 32 km. Using observations of first motion for a composite focal mechanism solution, the nodal planes were observed to be near vertical and the compression axis near horizontal and normal to the local strike of the seismic belt and of the MCT. We conclude that although the Main Central Thrust itself is not seismically active in this region, there is considerable activity immediately to the southwest of it. Furthermore, the mode of faulting as inferred from the records of these earthquakes, is strike slip.