Geotechnical, Geological and Geophysical Investigations for Seismic Microzonation and Site-Specific Earthquake Hazard Analysis in Gujarat

Geotechnical, geological and geophysical investigations for seismic microzonation and site-specific earthquake hazard analysis adopted in Gujarat, western India, are explained. Geology of the area is studied to understand basic earthquake hazards. Seismicity and tectonics are studied up to 50 km distance in detail and 300 km distance in general. To know the nature of soil layers and drainage patterns, a geomorphological map is prepared by remote sensing and ground check. Depth and seismic shear velocity of near-surface soil/rock layers are estimated at 2 km grid by 30–90 m borehole drilling and geophysical methods like MSWA, analysis of seismograms, microearthquake recording and PS-logging. The 2D and 3D soil models are prepared. Deep layers and fault details are estimated by gravity, seismic reflection and magnetotelluric geophysical surveys. The Peak Spectral Acceleration (PSA) is estimated on the grid pattern with a spacing of 0.5 km. The methodology is divided into three parts: (i) Establishment of Engineering Bed Layer (EBL) (a layer above which the soil effect is to be estimated) from borehole data, SPT N-Values and seismic shear velocity of soil layers, (ii) Estimation of strong ground motion at EBL using strong motion simulation technique and (iii) Estimation of surface strong-motion parameters and soil amplification by passing the EBL ground motion through ground model prepared for each borehole. The 1D ground response analysis is conducted through SHAKE program and strong-motion parameters are estimated at surface. The PGA maps and spectral acceleration (Sa) maps for 0.1–1.25 s are prepared. Liquefaction potential is estimated.