Shear wave velocity structure over the eastern Indian subcontinent

Abstract

Fundamental-mode surface wave generated by twelve earthquakes, which occurred in northeastern India and nearby regions and were recorded at the Shillong (SHL) and Chiengmai (CHG) seismic stations are used to determine the shear wave velocity structure beneath these regions. The Frequency Time Analysis method is used to determine the group velocities for periods ranging from 4 to 50 s for fundamental-mode Rayleigh and Love waves. A high shear wave velocity (4.2–4.7 km s −1) is estimated in the lower 30–34 km of the crust. Crustal thickness is found to vary from 36 to 56 km with an overlying 2–4 km sedimentary thickness of shear wave velocity of 2.55–2.75 km s −1. The shear wave velocity in the upper 10 km of crust is 3.5–3.7 km s −1 below the sedimentary layer. The inferred high shear velocity for the lower crust beneath these regions suggests an oceanic affinity and they are different from the Indian shield structure. The lower crust must have an oceanic origin as derived from the reminiscent of a certain oceanic lithosphere. The available data do not permit us to estimate the upper mantle structure precisely; however, it shows a higher upper mantle shear velocity of 5.1–5.3 km s −1. This may be indicative of active tectonism beneath these regions in the uppermost mantle. The Moho boundary is not sharply defined here. The increase in crustal thickness may be due to the collision of the Indian and Eurasian plates in the north and subduction of the Indian plate beneath the Burmese Arc in the east.