Abstract:
Earthquakes can have intense consequences on any societies and their surrounding structures. Furthermore, ground motions amplified by surficial materials can intensify the situation, often making the difference between minor and major damage. Classification of sites by shear-wave velocity has become more common; the shear wave velocity of soil and rock is one of the key components in establishing the design response spectra, and therefore the seismic design forces, for a building, bridge, or other structure. The method has been proposed for evaluating global seismic site conditions, or the average shear velocity to 30 m depth (V𝑠30), from the Shuttle Radar Topography Mission (SRTM) 30 arc-sec digital elevation models (DEMs). The basic principle of the method is that the topographic slope can be used as a reliable proxy for V𝑠30 in the absence of geologically and geotechnical based site-condition maps through correlations between V𝑠30 measurements and topographic gradient. However, to validate the data based on available geological map of Pakistan, the results extracted from the proxy-based method are then compare with the classification of area based on strata/soil present in the area and the measured V𝑠30 values. This study collected high quality geotechnical investigation reports with standard penetration test (SPT) N-values. While the proxy-based V𝑠30 map is developed using a correlation between V𝑠30 and topographic slopes for active tectonic regions. From slope derived V𝑠30map of Pakistan, the major area of both Punjab and Sind provinces has stiff soil, areas of Sind alongside with the sea have soft loam, and the different parts of both Khyber Pakhtunkhwa and Baluchistan are rocky, very dense, soft, and stiff soil. Northern areas of Pakistan are mountainous; therefore, it has higher values of V𝑠30. The highest V𝑠30 values in the Northern areas are estimated from hard rocks, including porphyritic granite-granodiorite, augen flaser gneisses, tourmaline granite, and pegmatite in the Mansehra orthogenesis unit. Some areas within the region of Rawalpindi-Islamabad have mix site classified as C and D, dependent on the presence of limited soil settings. (e.g., presence of soft clay or liquefiable soil) that cannot be determined solely with VS profiling. The coastal areas of Sind and some areas of Punjab including Muzaffargarh, Layyah, Attock etc. are susceptible to soil liquefaction as Site Class in these areas are mostly D1 and have ability to liquify during earthquake occurrence.
