Seismicity in the Kingdom of Bhutan (1937–2003): Evidence for crustal transcurrent deformation

Abstract

We investigate moderate‐sized historic to recent earthquakes in the Bhutan Himalaya spanning the years from 1937 to 2003. We find that few moderate‐sized earthquakes occurred in the region during this time period. In order to better characterize the seismicity, we relocate all earthquakes and estimate focal mechanisms for events for which we have adequate data. We use first motion data for older events and waveform model digital seismograms for three earthquakes that occurred in 1980 (Mw = 6.4), 1995 (Mw = 5.4), and 2003 (Mw = 5.4). For the modern events, we utilize three techniques for focal mechanism and depth determination: regional full waveform time domain modeling of the two moderate (Mw = 5.4) events, and surface wave spectral method and body wave depth modeling approach for the Mw = 6.4 event. We find that the first motion and digital data focal mechanisms are mostly strike‐slip, with midcrustal to deep crustal depths. Although no recent great event has occurred along the main Himalayan thrust, the Indian plate is undergoing significant, midcrustal to deep crustal transcurrent deformation, likely due to oblique convergence of the Indian‐Asian collision in this region. Unlike oceanic‐continent oblique subduction, strike‐slip and thrust partitioning occurs throughout the crust, not in distinct zones. The geological and geophysical nature of the Bhutan Himalaya appears significantly different than other Himalayan regions due to the strike‐slip nature of many of the events, an out of sequence thrust fault, and the implications of deformation in the Shillong Plateau.