Site Characterization and Shallow Shear Velocity through HVSR Measurements around the Hyderabad Metropolitan Region, India

Abstract

We investigate the site characterization and shallow shear velocity profiles from the analysis of the Horizontal to Vertical Spectral ratio (HVSR) around the Hyderabad metropolitan region (HMR), which falls under the southern Indian shield. This work uses both the ambient noise and microearthquake data to compute the HVSR, and additionally the Random Decrement technique to compute the HVSR of extracted Rayleigh waves. This study indicates comparable HVSR curves at each station with the three different datasets, from which we obtain the average dominant frequency (f0) and amplification value (A0). They are further used to calculate the seismic vulnerability index value (Kg). We observe that the value of f0 around the HMR is not fixed, but is varying in the range of 3.4 to 18 Hz, whereas the value of A0 is in the range of 1.7 to 12 approximately and Kg in the range of 0.16 to 1.68 approximately, with an exception of ∼ 33 at VKB (Vikarabad) station, which may be due to a local unconsolidated sub-surface structure. Based on the Diffused Field Assumption (DFA), we invert the average HVSR curves and average dispersion curves of Rayleigh waves, and provide the shallow shear velocity profiles up to 300 m, along with an approximate estimate of VS30 (in upper 30 m depth). The estimated VS30 values vary between 911 to 3143 m/s, falling under the classifications A and B (mostly Hard Rock and Rock type) of National Earthquake Hazards Reduction Program (NEHRP) (BSSC, 2001). However, our study shows some stations with shear velocity inversions at shallow depths within 300 m, indicating layers of low velocity, needing further study. In the absence of detailed near-surface findings, these findings are valuable inputs for geotechnical engineering studies and urban-city planning around the HMR, and emphasizes the effectiveness the HVSR method to determine sub-surface topography and/or unknown soil structures as an economical investigation viability.