A seismotectonic study of the 21 May 2014 Bay of Bengal intraplate earthquake: evidence of onshore-offshore tectonic linkage and fracture zone reactivation in the northern Bay of Bengal

Abstract

The earthquake of 21 May 2014 (Mw 6.0) in the northern Bay of Bengal (BOB) highlights the importance of studies on intraplate earthquakes in the oceanic regime for understanding the stress state of the oceanic lithosphere. The epicenter of the earthquake is located at a water depth of 2.5 km where the sediment thickness is nearly 12 km, and it occurs at a depth of ~50 km within the upper mantle. Its location on the seismotectonic map of the region shows that the epicenter is far from the seismically active zone of the Burmese Arc in the east and low-to-moderately active seismic region of the east coast of India in the west. The fault plane solution of this earthquake indicates that it was a strike-slip event with a right-lateral sense of motion on a NW-oriented nodal plane, and it occurred on one of the NW-SE-trending fracture zones previously mapped in the BOB. Based on a compilation of long-term (1900–2011) intraplate earthquakes along with available focal mechanisms in the BOB and the adjoining east coast of India, we conclude the following: (1) the Precambrian structural trends, basin-scale faults and minor lineaments on the east coast of India are favorably reactivated in their offshore extensions up to the shelf-slope areas of the margin; (2) earthquake occurrences in the BOB region can be correlated with the fracture zone trends in the central BOB and along the Ninetyeast ridge or at the intersections of fracture zones with the subsurface trace of the 85°E ridge. The 21 May 2014 earthquake is the result of reactivation of such a NW-SE-trending fracture zone lying in the lithosphere of >100 Ma in age. Further evaluation of this event in light of the global occurrence of oceanic intraplate earthquakes in the older lithosphere (>80 Ma) suggests that such reactivation is possible in the high ambient stress state.