Seismic evidences of faulting beneath the Panvel flexure

Microseismic studies in the Konkan plains encompassing the Panvel flexure, adjoining Mumbai, reveal low seismicity associated with the 150 km long flexure. About 158 earthquakes (Mc 1.0-3.6), including 41 events of magnitude (Mc >= 2.5) occurred along the flexure and its flanks during the period 1998-2005. Historical seismicity indicates the occurrence of about 24 moderate earthquakes of intensity >= IV in this region. Hypocentres estimated for 20 events reveal that the earthquakes occur at three depth levels - near surface (<2 km), shallow (2-15 km) and deep (>15 km). The near-surface earthquakes occur within the Deccan traps possibly due to failure of the faults flanking the flexure, while the shallow and deep crustal earthquakes suggest reactivation of the basement faults. The 2 km contrast in crustal thickness across the flexure all along its axis coupled with geological evidences of block tectonics suggest isostatic adjustments leading to normal faulting to be the preferred mechanism for the observed seismicity. This study brings out the existence of an active fault system beneath the Panvel flexure, possibly linked to west-coast tectonics.