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Stress pattern in two seismogenic sources in Nepal-Himalaya and its vicinity

Paudyal H., Singh H. N., Shanker D., Singh V. P.

Acta Geophysica

2008

Vol. 56, no. 2

313-323

The composite stereographic projection of orientations of the compression and tension axes using thirty-nine fault-plane solutions of earthquakes from two active seismogenic sources of Nepal and adjoining areas were examined and the nature of stress pattern and their influence on tectonics in the region have been studied. The seismogenic source located in Eastern Nepal region, which has been the site of 1934 Bihar-Nepal great earthquake of M 8.4, is presently experiencing N-S to NE-SW directed compressive stresses. The inferred pattern of compression axes in Western Nepal region suggests a shallow compressive stress, dipping N-S to NE-SW. Approximately similar nature of the stress regime is observed in Western and Eastern regions of Nepal, separated by nearly 700 km; it shows N-S to NNE-SSW direction of compression and underthrusting of the Indian Plate beneath the Himalaya at a shallow angle. Present study indicates that the stress is being released along the strikes of some of the transverse faults present in the region since the compressive stress exerted by the northward movement of the Indian Plate is approximately perpendicular to the Himalayan collision belt. Unilateral stress pattern generated by the northward movement of the Indian Plate in the central part of the Himalaya reveals that the present day collision occurs roughly perpendicular to the local strike of the Himalaya.

Application of the time-predictable model in Peninsular India for future seismic hazard assessment

Shanker D., Singh H. N.

Acta Geophysica

2007

Vol. 55, no. 3

302-312

It has been the belief among Earth scientists that the Peninsular Shield is aseismic, as the region attained stability long ago. However, the earthquake at Koyna (10 December 1967), Bhadrachalam (13 April 1969), Broach (23 March 1970), Hyderabad (30 June 1983), Khillari (30 September 1993), Jabalpur (22 May 1997), Gujarat (26 January 2001), and additional ones of smaller magnitudes, altered this concept. This area has experienced many widely distributed shallow earthquakes, some of them having large magnitudes. It is now widely accepted that seismic activity still continues with moderate events. Therefore, a need has arisen to take into consideration recent seismological data to assess the future seismic status of Peninsular India. Earthquake generation model has been studied to develop the statistical relations with surface wave magnitude (MS ≥ 4.5). Five seismogenic sources showing clustering of earthquakes and including at least three main shocks of magnitude 4.5 ≤ MS ≤6.5 giving two repeat times, have been identified. It is mainly based on the so-called "regional time-predictable model". For the consid-ered region it is observed that the time interval between two consecutive main shocks depends on the preceding main shock magnitude (Mp) not on the following main shocks magnitude Mf suggesting the validity of time predictable model in the region.