Seismic Imaging of Two-layer Deformation Lithosphere in the Indian Ocean

We obtain a very deep seismic image from the offshore northern Sumatra that shows faults down to 45 km depth by boosting low frequency signal during data processing, which is much better than the traditional processed result. The amplitude of these deep reflectors in the mantle decreases linearly with depth down to 25 km and then remains constant down to 45 km. We also find that the number of faults, the number of earthquakes and the cumulative moment released as a function of depth show a similar pattern, suggesting that the lithospheric mantle deformation in this region can be divided into two layers: a highly fractured serpentinized upper layer and a pristine brittle lithospheric mantle where great earthquakes initiate and large stress drop occurs. It is also consistent with the depth of second layer of double Benioff zone in most of the subduction zones.