Abstract:
Seismic Hazard Assessment (SHA) of the entire seismically active NW Himalayan Fold and Thrust Belt Pakistan that incorporates probabilistic approach was carried out. Additional information in the form of earthquake catalogue, delineation of 40 active faults in a structural map, their relationship to the seismicity, establishment of seismotectonic zones was also undertaken and is represented in the form of a map. Distribution of 813 events within study area indicates that seismicity (³ 4.0 Mw) appears to be associated with both the surface and blind faults. At the same time, 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 Salt Range, southern Potwar and Bannu, lesser seismic activity (³ 4.0 Mw) could be due to the damping effect of the thick Precambrian salt. Considering a number of geological and seismological factors, four seismotectonic zones were established. The b value for the Peshawar-Hazara Seismic Zone (PHSZ) is 1.16 followed by 1.12 for the Surghar-Kurram Seismic Zone (SKSZ). The other two, Swat-Astor Seismic Zone (SASZ) and Kohat-Potwar-Salt Range (KPSZ) have identical values of 0.95, thereby indicating occurrence of more events of relatively higher magnitude as compared to the other two seismic zones. Mean activity rate of earthquakes l 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 4 regression relationships, the maximum potential magnitude (m1) has been determined for the 40 Quaternary faults. In each seismic zone, the highest value within the seismic zone represents its m1. The results 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 was undertaken at 10 sites (Astore, Bannu, Kaghan, Kohat,.Mangla, Malakand, Muzaffarabad, Peshawar, Talagang and Islamabad). In the Probabilistic Seismic Hazard Assessment (PSHA), the peak ground acceleration (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 (l, m1 m0 and the b value) of the four seismic zones were used as the input parameters. The 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 sites these 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 even more hazardous. In addition, disaggregation at the assigned amplitude of 0.2g was also carried out for the ten sites.
Page(s):
287-295
DOI:
DOI not available
Published:
Journal: Proceedings of Pakistan Academy of Sciences, Volume: 42, Issue: 4, Year: 2005