Source parameters of earthquakes and intraplate deformation beneath the Shillong Plateau and the Northern Indoburman Ranges

Abstract

We determined the fault plane solutions and focal depths of 17 earthquakes beneath the Shillong Plateau and the northern Indoburman ranges by combining results from the inversion of long‐period P and SH waveforms and amplitudes, from polarities of first motions, and from the identification of pP and sP phases on short‐period seismograms. Fault plane solutions of 15 earthquakes show mixtures of thrust and strike‐slip faulting, but the P axes for these events are nearly horizontal and consistently oriented north‐northeast–south‐southwest. All of these earthquakes occurred at depths greater than 29 km. Beneath the Shillong Plateau, one event occurred at a depth of 52 km. The relatively large depths for earthquakes in an intraplate setting suggest that the Indian lithosphere in this area is especially cold. Earthquakes beneath the northern Indoburman ranges define a gently east‐southeast dipping zone from 30 to 45 km beneath the Bengal basin to 40 to 90 km beneath the ranges. This zone seems to steepen and connect with the zone of intermediate depth seismicity that dips eastward beneath Burma. These earthquakes cannot have occurred along the interface between a subducting Indian plate and the overriding Indoburman lithosphere, because the P axes, not the nodal planes, are parallel to the north‐south trending seismic zone. Although a couple of the earthquakes might have occurred within the Indoburman lithosphere, most of this seismicity seems to have occurred within the Indian plate, recently and currently being subducted eastward beneath the Indoburman ranges. The consistent north‐northeast trend of the P axes implies that the orientation of maximum compressional strain in the Indian plate throughout its northeastern pan is nearly perpendicular to that responsible for roughly north‐south trending folds of the Indoburman ranges. Thus, either recently in geologic time (since 1 Ma) the orientation of maximum compression changed dramatically, or, more likely, the deformation in the Indoburman ranges is decoupled from that in the underlying Indian plate. Meanwhile, the seemingly identical northward displacement of India and the Indoburman ranges with respect to south China must be accommodated farther east, along the Sagaing and other faults.