Crustal properties in the epicentral tract of the great 1897 Assam earthquake, northeastern India

Abstract

The velocity structure for the upper and middle crust in the epicentral tract of the Great 1897 Assam Earthquake (western half of the Shillong massif) was estimated using locally recorded microearthquake data. The relatively homogeneous upper crustal layer has P and S wave velocities of 5.9 ± 0.2 and 3.4 ± 0.1 km/s, respectively, with a thickness of 11 to 12 km. The average P and S wave velocities in the middle part of the crust down to a depth of about 26 km were estimated to be 6.3 ± 0.6 and 3.5 ± 0.2 km/s, respectively. The larger scatter in P velocity estimate of the mid-crustal layer was investigated using synthetic examples. Our preference is for the model in which this crustal region is assumed to comprise of a number of thin layers with alternate low and high seismic velocities. From theoretical considerations we confirm that it is possible to estimate the velocity structure for a horizontally layered model using the velocity estimation procedure (a combination of the Wadati, Riznichenko and Bune method) we have used. We also carried out detailed model analysis to check how effective this velocity estimation procedure is for the type of array data we have. Combining the estimated velocity structure with our earlier work on seismicity of this area we propose that the upper homogeneous crustal layer in the Shillong massif may be moving relatively southward across an intracrustal thrust zone. We suggest that the Great 1897 Assam Earthquake may represent the most recent episode of this relative southward movement of the massif.