RUPTURE HISTORY AND SEISMOTECTONICS OF THE 1991 UTTARKASHI, HIMALAYA EARTHQUAKE

The 19 October 1991 Uttarkashi, India earthquake occurred in the main thrust zone of the Himalaya. With a moment magnitude of 6.8, this even t is characteristic of the present-day motion on the thrust fault syst em. We examine this earthquake using different sets of data in order t o understand better the faulting process of a major earthquake in the Himalayan region, Firstly, the modeling of the teleseismic records ind icates that the mechanism is similar to the published CMT and indicate s a shallow (between 10 and 15 km depth) low-angle thrust event, In th e vicinity of the source, the earthquake was recorded by a network of accelerometers run by the University of Roorkee. Six three-component a ccelerometers were triggered within a radius of 60 km. Two of them wer e very close to the surface projection of the fault. Forward modeling of those records shows that the rupture propagated toward the west. Th is forward modeling gives us the possibility to confirm the epicenter location and to evaluate the timing of the accelerograms. The accelero gram records are inverted to obtain the distribution of slip on the fa ult plane. The results show a complex rupture process. The slip maxima (1.5 m) occurred 10 km west and 15 km southwest of the hypocenter, Th e slip source function obtained with near-field data is similar to the function obtained from teleseismic records and shows a low moment rel ease at the beginning of the rupture and a maximum rate of moment rele ase 4 seconds after. The: relation between the slip distribution obtai ned by inversion, isoseimals, mapped faults and the aftershocks locati on is then discussed and we finally propose a seismotectonic interpret ation of this earthquake. The Uttarkashi earthquake probably occurred along the detachment surface which coincides with the upper surface of the subducting Indian lithosphere. This detachment surface is gently dipping under the Lesser Himalaya and south of the Vaikrita thrust, Th e Vaikrita thrust marks the line separating the very shallow-dipping d etachment (along which big earthquakes like the Uttarkashi earthquake could occur) from the steeper-dipping, aseismic basement thrust. This observation is important for correctly estimating the seismic hazard i n the Uttarkashi region.