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.