SEISMIC HAZARD ASSESSMENT OF NW HIMALAYAN FOLD AND THRUST BELT, PAKISTAN

Mona, Lisa (2005) SEISMIC HAZARD ASSESSMENT OF NW HIMALAYAN FOLD AND THRUST BELT, PAKISTAN. Doctoral thesis, Quaid-i-Azam University, Islamabad.

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Abstract

Seismic Hazard Assessment (SHA) of the entire seismically active NW Himalayan Fold and Thrust Belt that incorporates both probabilistic and deterministic approaches has been carried out for the first time. Additional information in the form of earthquake catalogue, delineation of 40 active faults in a structural map, their relationship to the seismicity, focal mechanism studies of 45 events, establishment of seismotectonic zones has also been undertaken. Distribution of 813 events within study area indicates that seismicity (≥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 (≥4.0 Mw) could be due to the damping effect of the thick Precambrian salt. Majority of the earthquakes (86%) range in magnitude from 4.0 to 4.9 Mw followed by 107 events (13%) having magnitude 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). Even within this depth range, about 81 % of the events have focal depths of <25km. Larger magnitudes events are more in the hinterland zone. In contrast, based on distribution of 683 (≥4.0 Mw) events from the adjoining areas, a deeper level of seismicity (50 to 200 km) prevails especially in the Hindukush Range of Afghanistan. Focal Mechanism Solutions (FMS) of 45 earthquakes (Mw≥4), including 21 from the Hinterland zone and 24 from the Foreland zone show dominance of strike slip faulting (27 being strike slip solutions). 11 indicate thrusting, 6 reverse faulting and 1 normal faulting solution. Tectonic complexity seems to be due to interplay of a variety of factors. Thrust and reverse solutions near the northern collisional boundary have mostly NE-SW directed P-axis orientations. Imbricate thrusting, breaking and thickening of the crust is believed to be occurring due to the steep bending of the underthrusting plate at the collisional boundary. Another compressional E-W direction in the southwestern portion suggests the presence of a restraining bend in the Chaman Fault (the western plate boundary). A similar direction of compression is obtained for a basement fault (one thrust solution in HKS) in the eastern portion of the study area. Out of 27 strike slip solutions, a total of 15 FMS from the Foreland and 12 from the Hinterland zone have been obtained. The relatively deeper events (10-52km) are from the hinterland zone and shallower ones (mostly to a depth of less than 17km) from the foreland zone. In the hinterland zone, strike slip solutions are related to the ongoing uplifting in the Nanga Parbat-Haramosh massif and the Besham dome. Two major, nearly N-S trending sinistral faults, the Chaman Fault and the Jhelum Fault at the western and eastern margins respectively form a regional shear couple developed in this study in order to explain the strike slip deformation in the Foreland zone. Deformation within the Hazara Kashmir Syntaxis on the eastern side is also having an influence. Most of the FMS are interpreted as Synthetic shears (Riedel shears) with a few being Secondary synthetic shear (P shears). The kinematic change from convergence to transpressional especially in the Foreland zone is more pronounced in the upper portion of crust as indicated by shallow focus events. The basement seems to be involved in the deformation process indicating that the thin skin models implying the presence of a decollement may not be valid. Four seismotectonic zones i.e. the Swat-Astore (SASZ), Peshawar-Hazara (PHSZ), Kohat-Potwar-Salt Range (KPSZ) and the Surghar-Kurram (SKSZ) Seismic Zones have been identified. The b value for the seismic zones shows that PHSZ has the highest value of 1.16 followed by the SKSZ with a value of 1.12. The other two seismic zones (SASZ and KPSZ) have identical values of 0.95 and these lower values indicate that the occurrence of more events of relatively higher magnitude as compared to the other two seismic zones. Mean activity rate of earthquakes (A.) 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. Results show that the m1 is greater in the hinterland zone (SASl and PHSl) i.e. 7.8 as compared to the foreland part (KPSl and SKSl) i.e. 7.4. SHA incorporating both deterministic and probabilistic approaches has been undertaken at 10 sites (Astore, Bannu, Kaghan, Kohat, Mangla, Malakand, Muzaffarabad, Peshawar, Talagang and Islamabad) representing the different parts of the investigated area. 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. Firstly applying Deterministic Seismic Hazard Assessment (DSHA), the PGA values have been estimated considering the maximum potential magnitudes of the line sources and their shortest distance to a particular site within a radius of 100km. Results for the 84-percentiles show higher PGA's, for Muzaffarabad (0.79g), Islamabad (0.75g), Kohat (0.75g) and Peshawar (0.64g) as compared to the other sites. Earthquake activity along the Main Boundary Thrust (MBT) can potentially be hazardous for Muzaffarabad, Islamabad and Kohat where as the Khairabad Fault is identified as the main danger to Peshawar. Lowest value obtained for Bannu (0.27g) indicates that the affect of the nearby active Karak Fault is negligible. For the sites of Malakand (0.5g), Astore (0.47g) and Kaghan (0.46g) future seismic activity on MMT can be hazardous. In the case of Talagang (0.47g) and Mangla (O.4g), the DiI Jabba Fault and Kalar Kahar Fault respectively are identified as the critical tectonic features. 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 El-FRISK (6.2 beta version) software. Best-estimated seismic hazard parameters (A, m1, mo and the p value) of the four seismic zones were used as the input parameters. Results of PSHA were in the form of total hazard curves for the same ten sites used for DSHA. PGA range from 0.08g (for Bannu) to 0.21g (for Malakand and Kohat). For other sites they are: Astore (0.082g), Kaghan (0.12g), Muzaffarabad (0.13g), Islamabad and Peshawar (0.15g), Talagang (0.16g) and Mangla (0.18g). Due to high population density the sites of Islamabad and Peshawar have been considered more hazardous. Peshawar and Rawalpindi (twin city of Islamabad) with more poorly constructed structures were regarded even more hazardous. Disaggregation at the assigned amplitude of 0.2g has also been carried out for the ten sites. Finally the seismic design parameters i.e. Operational Basis Earthquake (OBE) and Maximum Credible Earthquake (MCE) accelerations have been determined.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: seismic hazard assessment, nw himalayan fold, seismotectonic zones, foreland zone, hinterland zone, deterministic seismic hazard assessment, probabillstic seismic hazard assessment, operating basis earthquake, maximum credible earthquake
Subjects: Q Science > Q Science (General)
Depositing User: Muhammad Khan Khan
Date Deposited: 01 Sep 2016 08:39
Last Modified: 01 Sep 2016 08:39
URI: http://eprints.hec.gov.pk/id/eprint/734

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