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

Abstract

In this study, we attempted to establish the plausible depth of disposition of the basal décollement 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 (ϭ) 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 décollement plane, exhibiting significant velocity perturbations across this interface. The geometry and depth of the décollement 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-ϭ suggests the presence of the quaternary piedmont sediments with high fluid saturations. In contrast, the higher velocity perturbation with low ϭ at the mid-crustal layer is indicative of the competent parts of the crust, where seismogenesis is related to the occurrence of earthquakes of varying strengths beneath the Sikkim - Darjeeling Himalayan region. Our seismic imaging corroborates with the past moderate earthquakes that were confined to the vicinity of high and low velocities (Vp and Vs) and ϭ zones, whilst the strong size earthquakes (M > 6.0) occurred mostly below the décollement plane where high-velocity basement thrust exists along with Indian subducting plate. This study clearly expresses the well-defined disposition of the décollement zone that controls the nature and extent of seismogenesis.