Rupture history and seismotectonics of the 1991 Uttarkashi, Himalaya earthquake

Abstract

The 19 October 1991 Uttarkashi, India earthquake occurred in the main thrust zone of the Himalaya. With a moment magnitude of 6.8, this event is characteristic of the present-day motion on the thrust fault system. We examine this earthquake using different sets of data in order to understand better the faulting process of a major earthquake in the Himalayan region. Firstly, the modeling of the teleseismic records indicates that the mechanism is similar to the published CMT and indicates a shallow (between 10 and 15 km depth) low-angle thrust event. In the vicinity of the source, the earthquake was recorded by a network of accelerometers run by the University of Roorkee. Six three-component accelerometers were triggered within a radius of 60 km. Two of them were very close to the surface projection of the fault. Forward modeling of those records shows that the rupture propagated toward the west. This forward modeling gives us the possibility to confirm the epicenter location and to evaluate the timing of the accelerograms. The accelerogram records are inverted to obtain the distribution of slip on the fault 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. The slip source function obtained with near-field data is similar to the function obtained from teleseismic records and shows a low moment release at the beginning of the rupture and a maximum rate of moment release 4 seconds after. The relation between the slip distribution obtained by inversion, isoseimals, mapped faults and the aftershocks location is then discussed and we finally propose a seismotectonic interpretation 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. The Vaikrita thrust marks the line separating the very shallow-dipping detachment (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 in the Uttarkashi region.