A Seismic Hazard Map Based on Geology and Shear-wave Velocity in Rawalpindi-Islamabad,Pakistan

The 'twin cities' district of Rawalpindi-Islamabad is among the most endangered seismic regions in Pakistan, with the seismic hazard assessed (0.32 g) to intensity IX MMI for a 475-year return period. A seismic hazard map for Rawalpindi-Islamabad is presented herein, based on 85 shear-wave velocity (V-S) profiles obtained through geophysical H/V measurements and from the geological map of the region. Relationships between the average top thirty-meter shear-wave velocities (V-S30) and surficial geological units have been determined. The peak ground acceleration (PGA) maps for 150, 475 and 2475-year return periods were converted into a seismic intensity map. Intensity increments for different soils were used to compute PGA values for 150, 475 and 2475-year return periods. Sites located on softer ground experienced a higher degree of damage from moderate earthquakes. Due to the presence of soft clay or liquefiable soil and lateral spreading, a few locations may be classified as hybrid sites class C and D. This map is a critical step in facilitating code-based site classification and seismic design throughout Rawalpindi-Islamabad. Although the seismic hazard map based on seismic intensities is no longer used in engineering geology, it is still important in seismological analysis and for civil protection purposes.