Investigating 2D shear waves velocity (\({{\varvec{V}}}_{{\varvec{s}}}\)) structure beneath Garhwal Himalaya, Uttarakhand, India, using microtremor H/V spectral ratios and surface wave dispersion measurements

Abstract

The shallow seismic response to earthquakes is important for ground motion prediction and is controlled by the major structural heterogeneities including topography. In this study, constrained inversion of microtremor horizontal-to-vertical spectral ratios (HVSRs) and Rayleigh phase velocity dispersion curves, along with well-constrained single station HVSRs inversion, produce the first 2D near-surface shear wave velocity (\({V}_{s}\)) model of Garhwal Himalaya, India. We analyse passive source seismic data from 158 sites, along with two-sided active seismic array records from 96 locations, to evaluate the 1D \({V}_{s}\) structure for different geo-tectonic units in the studied area. To explain the observed HVSR response, we calculate the HVSR curves using both theoretical (modified Haskell matrix) and numerical (modal summation) approaches. The simulated HVSR curves agree well with the observations in ~ 1.0–25.0 Hz. We examine the feasibility of the obtained 1D \({V}_{s}\) profiles through extensive synthetics. The resultant 1D \({V}_{s}\) profiles were then compiled to create the 2D near-surface \({V}_{s}\) models for various litho-tectonic units. High \({V}_{s}\) anomalies correlate well with the major tectonic features, such as the Kaliyasaur fault, North Almora thrust, and the anticline structures, while the syncline structures, Singtali thrust, and the depression fault zones exhibit low \({V}_{s}\) anomalies. The 1D \({V}_{s}\) profiles of ten known stratigraphic sections clearly delineate the interface boundaries between various rock strata. For the upper 30 m depth, the \({V}_{s}\) 30 value ranges from 280 m/s to 600 m/s. Our velocity model demonstrates intense rock folding and faulting beneath the region, which can be used to evaluate the local site response for improved seismic hazard assessment of the region.