Evidence from earthquake fault plane solutions on upper crustal stresses in the Garhwal Himalaya

Abstract

The individual thrust-fault type fault plane solution of the 1991 Uttarkashi earthquake is considered along with a reverse fault type and a strike-slip type composite fault plane solutions for the Garhwal Himalaya. The two latter solutions are based on data from 49 small and micro-earthquakes. All 50 earthquakes occured in the vicinity of Main Central Thrust over a distance of about 150km from Yamuna to Alaknanda valleys. The data are consistent with the view that a thrust-fault type stress environment prevails throughout the upper crust in the region. The maximum effective principal stress (sigma 1') is horizontal to sub-horizontal with azimuth along N42 degrees everywhere in the upper crust except in the hypocental region of the Uttarkashi earthquake, where it is N26 degrees. Intermediate effective principal stress (sigma 2') is horizontal in the NW quadrant and the minimum effective principal stress (sigma 3')is vertical to sub-vertical correspondingly. Reactivation of the strike-slip faults occurs under the influence of sigma 1' and sigma 2'. This is within the theory of fault reactivation under general three dimensional stresses. Pore pressures approach lithostatic levels over most parts of upper crust in the region.