Delineation of subsurface profiles beneath the Kopili fault zone in northeast India utilizing coda portion

Abstract

The Kopili fault (KF) zone, is witness to many seismic activities including the 1869 earthquake (7.8 MS), the 1943 earthquake (7.3 MS) and recent 2016 earthquake (6.7 MW) which occurred at certain time intervals. Considerable portion of KF zone and its neighbouring is characterized by alluvial soils which have higher potential of trapping seismic waves. This leads to intense amplification of ground shaking, necessitating a detailed study of subsurface profile beneath KF. The H/V Spectral Ratio of strong-motion recordings is an effective tool to evaluate such nonlinear site responses, and to develop a subsurface velocity profile is an important step toward hazard mitigation. In this work, we used 14 seismic stations, nearby this fault to construct the subsurface profile. This study suggests that H/V ratios obtained from coda portion of seismogram are almost independent of the coda window length. We applied diffuse field concept to produce an inverted S-wave as well as P-wave velocity profile. The inverted profiles show significant correlation with the underlying site class as well as exhibit an excellent match with already reported attenuation quality factors for this region. For few selected sites, inferred velocity estimates become 500 m/s at a depth of 25 m or shallower. Interestingly, it is observed that two nearby receiving sites are characterized by different shear wave velocity profiles at shallower depths. However, in deeper depths, they are found to converge. All results are comprehensively analyzed and their implications are accordingly outlined.