Seismic site characterization and nonlinear ground response analysis of Islamabad, Pakistan

Abstract

This study evaluates nonlinear site amplification in Islamabad, Pakistan, a region located near the Himalayan tectonic front and characterized by high seismic hazard. Local soil conditions play a critical role in modifying ground-motion characteristics and influencing seismic demand. A comprehensive dataset of 180 boreholes, including 30 newly drilled and laboratory-tested sites, was used to construct representative subsurface profiles reflecting the heterogeneous stratigraphy of the study area. Shear-wave velocity (Vs) profiles were estimated using a region-specific SPT-V_s correlation and extended to 30 m depth to determine V_S30-based site classification. Ten representative soil profiles, corresponding to BCP (2021) Site Classes C and D, were analyzed using DEEPSOIL with a pressure-dependent hyperbolic GQ/H model calibrated against Darendeli (2001) modulus-reduction and damping relationships. A suite of seven recorded ground motions was used to evaluate nonlinear site response. Results indicate that shallow stiff-soil sites, Class C, exhibit pronounced short-period amplification at T ≈ 0.15 to 0.30 s, with elevated surface accelerations. In contrast, deeper soft-soil sites, Class D, show broader amplification extending toward intermediate periods, associated with thicker and lower-velocity deposits. The derived average site coefficients, Fa ≈ 1.59 and Fv ≈ 1.16 for Class C, and Fa ≈ 1.78 and Fv ≈ 1.47 for Class D, differ from current BCP (2021) values, indicating potential underestimation of short-period demand and overestimation of long-period response in some cases. These results highlight the importance of site-specific analysis for improving seismic design and microzonation in Islamabad. However, given the limited number of representative profiles and associated uncertainties, the results should be interpreted as site-specific and indicative rather than broadly generalizable.