Relation of shear wave velocity variations with depth for different lithologies: A contribution towards mitigating the region's seismic risk

Abstract

The state of Uttarakhand in India lies in a highly seismically active zone, therefore earthquake re- sistant design criteria have been followed for major construction practice in this region. One of the major inputs for earthquake-resistant design is average shear wave velocity (VS ) at 30 m depth. In the present work, at twenty-six different sites, the Horizontal to vertical spectral ratio (HVSR) technique and multichannel analysis of surface wave (MASW) is used for joint analysis to obtain a one-dimensional velocity model that supports both the HV spectrum and dispersion curve obtained from HVSR and MASW methods, respectively. Shear wave profiles obtained at various sites are compared with the bore log data obtained from drilling and it clearly shows that the (VS ) for the same formation have a strong dependence on the depth at which it occurred. Data from twenty- four shear wave profiles have been used to prepare a linear regression relation of (VS ) for different lithological formations with respect to their depth of occurrence. Root mean square error obtained from the developed relationship for various lithologies clearly shows that the results are fairly well within the range of acceptance. The developed relations have been further validated by calculating Vs profile from bore log data obtained at two new location sites that are not included in the data set used for the preparation of regression relations. A comparison of two velocity sections clearly shows that the velocity profile computed from regression relation matches closely with that obtained from the seismic survey and thereby establishing the efficacy of developed regression relationship for its practical implementation. This developed relation allows soil type classification for zoning purposes.