One-Dimensional Velocity Model of Sikkim Himalayan Region

A preliminary one-dimensional (1D) velocity model for Sikkim region has been developed using P- and S- wave travel-time data. The work has been performed in Seisan by taking 276 local earthquake events. Out of 276 events 76 best events has been selected for inversion. Most of the earthquake events are concentrated in depth range 10 to 40 km. The 1D velocity model obtained for the study region has six uniform layers with interfaces at depths of 0, 10, 20, 30, 40, and 50 km with P wave velocity of 5.23, 5.35, 5.85, 6.59, 7.49, and 8.03 km/sec and S-wave velocity of 3.03, 3.08, 3.38, 3.37, 4.19, 4.61 km/sec, respectively. Mainly the events are more clustered in the area lying between latitude 27.2°N to 27.8°N and 88°E to 88.6°E, which shows high seismotectonic activity in the area due to the strain accumulation caused by dipping of Indian plate under the Eurasian plate. From velocity model it can be observed that largest velocity occurs at a depth of 40 km which shows the major lithological variation at this depth. The approximate thickness of the upper crust (granitic layer) is around 30km which can be noticed from the velocity data. The analysis depicts that the layers with thickness range 10-20 km and P wave velocity 5.35 km/s and thickness range 30-40 km with P wave velocity 6.59 km/s contains 19 hypocenter within them. This study will play a vital role in the assessment of regional tectonics, earthquake hazards and will provide evidence of the evolutionary model of the Sikkim Himalayan region. Keywords: Velocity model, Sikkim Himalayan region, Regional tectonic, Earthquake hazards, Evolutionary mode