Estimation of earthquake local site effects using microtremor observations for the Garhwal–Kumaun Himalaya, India

Abstract

The Garhwal–Kumaun region of the Himalaya encompassing the state of Uttarakhand, India, has experienced several earthquakes in the past. Damage due to earthquakes is controlled by local site conditions, primarily resonance frequency and wave amplification from the ground. We present local site parameters with their site geology for 37 sites using ambient noise data. Horizontal to vertical spectral ratio technique is used to estimate the spectral ratio curves. Based on the type of curve, sites are classified into four classes, viz. clear peak, broad peak, double and multi‐peak, and flat H/V curve. Sites seen with clear or broad peaks are located on either soil or weathered rocks, thus indicating large impedance contrast and sharp discontinuity with large velocity contrast. Multiple peaks are observed in either soil or boulder bed and reveal large impedance contrast, probably representing shallow and thick strata. Sites with flat curves are found on weathered/phyllite/granite gneiss/granite schist rock types within highly dissected hilly areas. Fourteen sites have a peak frequency >6 Hz with a dominance of broad and clear peaks in the Lesser and Higher Himalayan regions. On the contrary, foothills and part of Siwalik sites exhibited a peak frequency between 1.14 and 4.94 Hz. The results demonstrate that sites with thick soil cover and boulder bed areas, that is, Doon valley and foothills, show low‐frequency peaks and hard rock or shallow bedrock sites, that is, Lesser and Higher Himalaya exhibit a higher frequency range. The estimated H/V amplitude and peak frequency values have shown a good correlation with site geology and geomorphology.

Ground vulnerability derived from the horizontal‐to‐vertical spectral ratio: Comparison with the damage distribution caused by the 2017 ML 5.4 Pohang earthquake, Korea

“…Earthquake damage is primarily controlled by seismic vibrations, which reflect the combined effects of the earthquake source, attenuation of seismic energy along ray paths, and amplification due to soil (or ground) conditions specific to the site. Among these factors, site-specific effects prevail in both the far and near fields (Badrane et al, 2006;Abd El-Aal, 2010;Singh et al, 2017;Sunaryo, 2017;Sairam et al, 2018;Naik et al, 2019;Gaytan et al, 2020;Pudi et al, 2020). Devastating examples of seismic amplification within unconsolidated shallow sediments include, but are not limited to, the M W 7.6 Niigata earthquake in 1964 (Ishihara and Koga, 1981), the M W 8.0 Mexico City earthquake in 1985 (Anderson et al, 1986), the M W 6.9 Kobe earthquake in 1995 (Holzer, 1995) and the M W 7.7 Chi-Chi earthquake in 1999 (Kao and Chen, 2000;Kim et al, 2010).…”