Seismicity and decollement geometry beneath the Sikkim-Darjeeling Himalaya using local earthquake tomography: Implications for seismotectonic and Seismogenesis

In this study, we attempted to establish the plausible depth of disposition of the basal de ' collement through 3-D seismic imaging using local earthquake tomography to understand the linkage between seismogenesis and collisional tectonics of the Sikkim-Darjeeling Himalayan region. We analysed 2105 events recorded by 33 broadband seismograph stations during 2005-2011. Detailed 3-D seismic imaging unravelled heterogeneous velocity structures (Vp and Vs) with distinct variations in Poisson's ratio (sic) at different depth ranges. A well -resolved low-velocity zone is found to persist consistently at a depth of about 20 km corresponding to the gentle north dipping de ' collement plane, exhibiting significant velocity perturbations across this interface. The geometry and depth of the de ' collement surface vary laterally, and its deepest disposition is observed at a depth of-30 km. The shallower low-velocity zone up to 10 km with higher-(sic) suggests the presence of the quaternary piedmont sediments with high fluid saturations. In contrast, the higher velocity perturbation with low (sic) at the mid-crustal layer is indicative of the competent parts of the crust, where seismogenesis is related to the occur-rence of earthquakes of varying strengths beneath the Sikkim -Darjeeling Himalayan region. Our seismic im-aging corroborates with the past moderate earthquakes that were confined to the vicinity of high and low velocities (Vp and Vs) and (sic) zones, whilst the strong size earthquakes (M > 6.0) occurred mostly below the de ' collement plane where high-velocity basement thrust exists along with Indian subducting plate. This study clearly expresses the well-defined disposition of the de ' collement zone that controls the nature and extent of seismogenesis.