Devajit Hazarika

The 2009 Bhutan and Assam felt earthquakes (Mw 6.3 and 5.1) at the Kopili fault in the northeast Himalaya region

Seismotectonics of the two recent earthquakes, one Mw 6.3 in the Bhutan Himalaya on 21 September 2009 and the other Mw 5.1 in the Assam valley on 19 August 2009, are examined here. The recent seismicity and fault plane solutions of these two felt earthquakes suggest that both the events occurred on the Kopili fault zone, a known active fault zone in the Assam valley, about 300 km long and 50 km wide. The fault zone is transverse to the east–west Himalayan trend, and its intense seismicity indicates that it transgresses into the Himalaya. The geologically mapped curvilinear structure of the Main Central Thrust (MCT) in the Himalaya, where the epicentre of the Bhutan earthquake is located, is possibly caused by the transverse Kopili fault beneath the MCT. This intensely active fault zone may be vulnerable to an impending larger earthquake (M > 7.0) in the region.

Crustal thickness and poisson’s ratio variations across the northwest Himalaya and eastern Ladakh

Crustal thickness and Poisson’s ratios are estimated across the northwest (NW) Himalaya and eastern Ladakh applying H-k stacking method on receiver functions of teleseismic earthquakes recorded at 16 broadband seismological stations. The results show significant lateral variation of crustal thickness from the Lesser and Higher Himalaya (∼50 km thick) to Ladakh (∼80 km thick) through the Indus Tsangpo Suture Zone (ITSZ). The Indian Moho is continuously traceable across the ITSZ which is consistent with the underthrusting of the Indian plate beyond the surface collision boundary. The estimated Poisson’s ratios in the Lesser and Higher Himalaya are low (0.249–0.253), suggesting felsic composition of the crust. The Poisson’s ratio is intermediate in the Tethyan Himalaya (0.269–0.273) and high beneath Ladakh (0.280–0.303), indicating the effect of aqueous fluid/partial melt present in the crust.

Earthquake precursory research in western Himalaya based on the multi-parametric geophysical observatory data

The opening of cracks and influx of fluids in the dilatancy zone of impending earthquake is expected to induce short-term changes in physical/chemical/hydrological properties during earthquake build-up cycle, which should be reflected in time-varying geophysical fields. With this rationale, eleven geophysical parameters are being recorded in continuous mode at the Multi-Parametric Geophysical Observatory (MPGO), in Ghuttu, Garhwal Himalaya, for earthquake precursory research. The critical analysis of various geophysical time series indicates anomalous behavior at few occasions; however, the data is also influenced by many external forces. These external influences are the major deterrent for the isolation of precursory signals. The recent work is focused on the data adoptive techniques to estimate and eliminate effects of solar-terrestrial and hydrological/environmental factors for delimiting the data to identify short-term precursors. Although any significant earthquake is not reported close to the observatory, some weak precursory signals and coseismic changes have been identified in few parameters related to the occurrence of moderate and strong earthquakes.