Seismotectonics and 3-D seismic velocity structure of the Arunachal Himalaya

Knowledge of seimotectonics of Arunachal Himalaya holds the key to comprehend the complex deformation scenario in the eastern Himalayan syntaxial (EHS) zone comprising India, Eurasia and Burma plates. Data from a network of 41 broadband seismic stations operated from 2009 December to 2016 March in a hitherto unmonitored zone, enabled us to investigate the distribution of local seismicity and 3-D crustal velocity structure. Seismicity trends reveal that the western Arunachal and the EHS zone are seismically active compared to central Arunachal. Seismicity in western Arunachal extends to the south of Mikir Hills in the Foreland, where it is associated with a WNW-ESE trending dextral fault in the Indian crust. While the entire crust is seismically active in the syntaxial zone, earthquakes in central Arunachal are confined to the upper 20 km. Further, low V-P (similar to 5.2-5.4 km/s) values in the sub-Himalaya indicate 5-6 km thick sediments. Low V-P/V-S (1.6-1.70) values in the mid-to-lower crustal depths may be associated with crustal anisotropy. Velocity images along with seismicity reveal a gently dipping Main Himalayan Thrust (MHT) without a clear signature of a mid crustal ramp in the region. The seismically less active flat portion of the MHT in central Arunachal region might be related to a partially creeping segment or less deformation at the Himalayan front.