Pre-stack Reverse-Time Migration Method for Imaging Subsurface Structures of the Himalaya-Tibet Collision Zone

Abstract:

We use pre-stack reverse-time migration (RTM) of converted waves to image crust and mantle structures of the Himalaya-Tibet collision zone. Multi-component seismic data are back propagated using FDTD method of elastic wave. P and SV components are separated through the divergence and curl of the reconstructed wave-field. Cross-correlation imaging condition is applied between the back propagated P and SV wave-fields to reconstruct the location of the points where conversion (e.g., P-to-S or S-to-P) occurred. Unlike traditional CCP receiver function stacking, this method does not rely on 1-D assumptions about the geometry of structures, so that it is able to cope with dipping interface, steep faults. Then we applied this method on data of the southern segment of Hi-CLIMB. The chosen seismic array consists of 76 stations and it is 260-kilometer-long extending northward from the Ganges Basin and across the Himalayas. By pre-stack RTM, we have constructed an image of the crust and upper mantle beneath the Himalayas. The image reveals that the Moho dip from a normal depth of 45 km at distance of 120 km ~ 190 km to a much deeper depth of 65 km at 200 km ~ 270 km under southern Tibet. There are also some weak signal in the crust with the same trend of Moho at this region. This may also be related to the underthrusting of Indian plate beneath Tibet. This result of pre-stack RTM consists with previous results obtained by receiver function, but has a steeper change of the depth of Moho.