Investigation of shallow structures using ambient seismic noise data recorded at permanent broadband seismic stations in the Eastern Indian Shield and adjoining regions

Abstract

The present study provides a characterization of the soil structure underneath nine permanent Broadband Seismograph (BBS) stations spread across the Eastern Indian Shield region. The region is tectonically quite stable and less vulnerable to earthquake activities as compared to the other active regions in India. The objective of this study is to use the properties of the ambient seismic noise to study the site response for hazard analysis and to understand subsurface structure at each recording site in the Eastern Indian Shield (EIS) and adjoining regions. We analyzed continuous ambient seismic noise for horizontal-to-vertical spectral ratio (HVSR) at the seismic stations, which indicates that the predominant frequency is independent of the seasonal effects, however, significant variation in amplification is observed. The peak amplification of the spectral ratio from the seismic noise data is often found smaller than that estimated from the earthquake waveforms; however, they found similar at a few stations. Rayleigh-wave ellipticity inversion at all the BBS stations shows that the upper most layer (depth ≤ 20 m) has invariably lower shear wave velocity (Vs, 150–300 m/s), and correlated with soft to stiff soil. The second layer (20–95 m depth) with Vs ranging between 300 and 600 m/s is associated with very dense soil/soft rock. At six stations, the third layer (>100 m) is likely associated with rock, with Vs varying between 600 and 1200 m/s. At Dhanbad (DHA) site, however, the second (Vs ~ 1900 m/s) and third (Vs ~ 3000 m/s) layers correspond to weathered and hard granite gneisses, respectively. The results from the present study would be helpful in constraining the hazard of the region, which eventually may contribute to the disaster mitigation efforts in the region.