Characterizing Surface Geology, Liquefaction Potential, and Maximum Intensity in the Kachchh Seismic Zone, Western India, through Microtremor Analysis

Abstract This study presents results of microtremor analysis carried out over diverse geological formations and liquefaction sites in the Kachchh seismic zone of western India, which is host to the deadliest intraplate earthquake ( M w  7.7). Microtremors were recorded at 36 sites with single stations and 7 sites with an array. The microtremor array measurements were made using triangular arrays with sensors kept at 30–90 m radii. The Rayleigh‐wave velocities are estimated from the array data using high‐resolution frequency–wavenumber ( f ‐ k ) analysis, and the 1D shear‐wave velocity structure is determined by inversion. We found a good correlation between surface geology, resonant frequency and amplitude of the horizontal‐to‐vertical spectral ratio (HVSR) curve, and shear‐wave velocity. The ground vulnerability index ( K g ) values in the liquefied areas are found to be higher than those in the adjacent areas devoid of liquefaction. Broad or multiple peaks in the HVSR curves and large variations in the shear‐velocity models could be attributable to the structural and tectonic complexity in the maximum intensity area. Further, our results show that the average fundamental frequencies are around 0.32 Hz for the Quaternary, 0.80 Hz for Tertiary, 1.8 Hz for Cretaceous, 1.8 Hz for Jurassic, and 2.3 Hz for the Deccan trap formations. The inverted S ‐wave velocity varies from 190 to 1350  m/s down to a depth of 200 m for the Quaternary, 220 to 680  m/s down to 100 m for Tertiary, 236 to 1160  m/s for Cretaceous, 670 to 1780  m/s for Jurassic, and 260 to 2440  m/s for the Deccan trap sites. The results of microtremor analysis corroborate the observations from the available geophysical, geological, and borehole data.