An Integrated Approach for Site Characterization to Understand the Seismic Vulnerability of the Attock City, Pakistan

Pakistan has faced 12 major seismic events (Mw>6.0) during the last 30 years resulting in a heavy death toll and an economic loss of a few trillion dollars, which is immense for any underdeveloped country. Despite the generalized description of seismic hazards in various regions of Pakistan, densely populated cities still require a detailed and integrated vulnerability analysis to overcome the impact of a significant earthquake. This study integrates multiple techniques to understand the seismic hazard in the Attock city residing on the unlithified sediments of upper Pleistocene to Recent alluvial deposits. The study splits the region (about 200 km radius) into six seismic zones and uses area source parameters (including the number of events, the minimum and maximum magnitude, number of events above the minimum magnitude, activity rate per year, b-values etc.). The ground motion predictions equations compatible with the study area's seismotectonic environment are also used in this study. The resulting ground motions are obtained as peak horizontal ground acceleration and 5% damped spectral acceleration (SA). The site classification is carried within the Attock city, indicating the presence of SB (foundation condition with Vs30=760 m/sec), SC (foundation condition with Vs30=400 m/sec) and SD (foundation condition with Vs30=300 m/sec). The peak ground acceleration for SB, SC, SD is estimated as 0.23g, 0.28g and 0.30 g, respectively, for a return period of 475 years. Uniform hazard response spectra (for each site classification) is obtained by plotting spectral acceleration against time period for the return period of 475,975 and 2475 years. Furthermore, microtremor recordings are conducted at 20 sites within the Attock city to understand the fundamental frequency (f0) and H/V spectral amplification factor (A0). The values of f0 are found between 0.8 and 9 Hz, whereas the A0 is observed between 2.1 and 5. Despite the evidences of seismic vulnerability of the Attock city, the current building designs and infrastructure development is not synchronized with the uniform hazard response spectra and the site amplification, thus enhancing the vulnerability of the study area during a major earthquake. The study will be instrumental in pre-disaster mitigation strategies and can be helpful for urban planners and policymakers