Crustal Structure and Earthquakes Beneath Sikkim Himalaya

Abstract:

We compute P-wave receiver functions from teleseismic data recorded by our local network of broadband stations in Sikkim Himalaya operated between 2006 and 2013. Our network spreads across the Lesser Himalaya (stations NAMC and RABN), Higher Himalaya (stations GTOK, MANG, CHTG and LCHG) and Tethyan Himalaya (station YUMT). Using Common Conversion Point (CCP) stack, we observe a Low Velocity Zone (LVZ) at depth of 9-12 km beneath the Lesser Himalaya which deepens to 14-16 km beneath the Higher Himalaya. This LVZ is the basal decollement (Main Himalayan Thrust) and marks the top of the downgoing Indian plate. The shallow segment of this fault is frictionally locked and the 2006 (M_w 5.7) Sikkim earthquake occurred at the downdip end of this locked zone. On the CCP stacks, the Moho is highlighted as a strong impedance contrast interface beneath all the stations and to the first order dips gently (~9°) to the north. However, we observe second order dip variations on the Moho beneath the Lesser and Higher Himalaya, with segments dipping to the south. To model the detailed structure within the crust and the second order Moho dip variations we jointly invert P-wave Receiver Functions with Rayleigh wave group velocity dispersion data. The higher frequency (5 to 15 s) dispersion data is acquired by analysis of ambient noise between pairs of simultaneously operating stations in Sikkim Himalaya, while the longer period dispersion data (> 15 s) is taken from our previous study of group velocity tomography of India. Our inversion results show that the Moho beneath Lesser Himalaya is at depth between 43 and 48 km; beneath Higher Himalaya at depths between 53 and 55 km; and beneath Tethyan Himalaya at a depth of 60 km. Using this velocity structure, we relocate all earthquakes (> m_b3.5) which originated within Sikkim Himalaya between 2006 ans 2013 and found that they occur both within the downgoing Indian plate and the Himalayan wedge.

Keywords: Sikkim Himalaya, P-wave Receiver Functions, Crustal structure, Earthquakes.