Abstract
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 vicinityof 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 uppercrust in the region. The maximum effective principal stress (s1) is horizontal to sub-horizontal with azimuth along N42° everywhere in the upper crust except in the hypocental region of the Utrarkashi earthquake, where it is N26°. lntermediate effective principal stress (s2)is horizontal in the NW quadrant and the minimum effective principal stress (s3) is vertical to sub-vertical correspondingly. Reactivation of the strike-slip faults occurs under the influence of s1 and s2. This is within the theory of fault reactivation under general three dimensional stresses. Pore pressures could be relatively high and may approach lithostatic levels over most parts of upper crust in the region.
Published Version
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