Seismic Hazard Assessment of the NW Himalayan Fold-and-Thrust Belt, Pakistan, Using Probabilistic Approach

Abstract

The Seismic Hazard Assessment (SHA) based on probabilistic approach has been carried out for the entire seismically active NW Himalayan Fold and Thrust Belt in Pakistan. Additional information in the form of earthquake catalog, delineation of 41 active faults in a structural map, their relationship to the seismicity, and establishment of seismotectonic zones has also been undertaken. From the distribution of the 813 events within the study area, it appears that seismicity (≥4.0 Mw) is associated with both surface and blind faults. The clustering of events in specific parts along the surface faults shows that some fault segments, especially in the hinterland zone, are more active. In parts of the active deformational front, like the Salt Range, southern Potwar and Bannu, lesser seismic activity (≥4.0 Mw) could be due to damping effect of the thick Precambrian salt. A majority of the earthquakes (86%) range in magnitude from 4.0 to 4.9 Mw, followed by 107 events (13%) ranging from 5.0 to 5.9 Mw. The remaining 1% range from 6.0 to 6.7 Mw. There is a predominance of shallow seismicity (<50 km focal depth). Larger magnitudes events are more in the hinterland zone. In contrast, based on distribution of 683 (≥4.0 Mw) events, a deeper level of seismicity (50 to 200 km) prevails especially in the adjoining Hindukush Range of Afghanistan. Four seismotectonic zones have been established in the area. The b value is highest for the Peshawar-Hazara Seismic Zone (PHSZ) at 1.27, followed by 1.12 for the Surghar-Kurram Seismic Zone (SKSZ). The Swat-Astor Seismic Zone (SASZ) and Kohat-Potwar-Salt Range Seismic Zone (KPSZ) have b values of 0.99 and 1.03, respectively, thereby indicating the occurrence of more events of relatively higher magnitude as compared to the other two zones. The mean activity rate of the earthquakes (λ) ranges from 4.26 to 1.73. In decreasing order, the values are 4.26, 2.62, 2.07, and 1.73 for PHSZ, SASZ, KPSZ, and SKSZ, respectively. Using four regression relationships, the maximum potential magnitude (m1) has been determined for the 4 Quaternary faults. The highest value within each seismic zone represents its m1. Our calculations show that m1 is 7.8 in the hinterland (SASZ and PHSZ) and 7.4 in the foreland part (KPSZ and SKSZ). SHA, incorporating probabilistic approach, has been undertaken at 10 sites, along with disaggregation at the assigned amplitude of 0.2g. Pakistan does not have an attenuation equation of its own. The two equations of Ambraseys et al. (1996) and Boore et al. (1997) have been used and the results obtained using the equation of Boore et al. (1997) have been preferred. In the Probabilistic seismic hazard assessment (PSHA), the PGA values with 10% probability of exceedance in the 50 years, i.e., the return period of 475, have been determined using the EZ-FRISK (6.2 beta version) software. Best-estimated seismic hazard parameters (λ, m1, m0, and the β value) of the four seismic zones were used as the input. Results were generated in the form of total hazard curves. Values obtained range from 0.08g (for Bannu) to 0.21g (for Malakand and Kohat). For the other seven sites they are: Astore (0.082g), Kaghan (0.12g), Muzaffarabad (0.13g), Islamabad and Peshawar (0.15g), Talagang (0.16g), and Mangla (0.18g). High population density and more poorly constructed structures in Rawalpindi (twin city of Islamabad) and Peshawar make them more hazardous.