Probabilistic Seismic Hazard Analysis of Manshera District

Abstract

Structures are designed to withstand a specific intensity of ground shaking. Intensity is generally expressed as Peak Ground Acceleration (PGA), since this is directly usable by a structural engineer in computing inertia forces for seismic design of structures. PGA for any specified site can be determined by performing Seismic Hazard Assessment (SHA). The quantification of seismic risk at a site involves assessing the probability of exceedance of any given value of Ground Acceleration as a result of combined effects of frequent moderate earthquakes and infrequent large earthquakes. In this study, Probabilistic Seismic Hazard Analysis (PSHA) has been carried out to determine the probability of exceedance of different values of Ground Acceleration for the city of Manshera and its surrounding areas. Uniform hazard spectra were then developed to obtain the PGA values for seismic design of structures. Since Manshera has both rock and soil geology, both have been included in the analysis. Disaggregation analysis was also performed to determine the distance and magnitude of earthquake contributing maximum to the hazard curve for the study site. A detailed statistically analysis was also performed to obtain SHA parameters such as annual activity rate, magnitude of maximum probable earthquake at each fault, b value required to determine the exceedance rate of earthquake of particular magnitude at any fault. In addition, seismicity pattern of the area has also been studied using both historic and instrumentally recorded earthquakes. Seismicity map of the study area was prepared using ArcGIS software. From the distribution of the 1287 earthquake events (recorded from 1928-2010) within the study area and seismicity pattern, it appears that seismicity ≥ 4.0 Mw (Mw is the moment magnitude of an earthquake) 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 north - eastern side of the Manshera city, are more active. A majority of the earthquakes (94.8%) range in magnitude from 4.0 to 4.9 Mw, followed by 62 events (4.8%) ranging from 5.0 to 5.9 Mw, and the remaining 0.4% are more than 6.0 Mw. There is a predominance of shallow seismicity (<20 km focal depth) which indicates that future earthquake, if follow the same pattern, can be quite destructive. From statistical analysis, the mean activity rate, λ (number of events per year) of earthquake having magnitude greater than 4.0 is found to be 7.2 which indicate that it is highly earthquake prone area. Magnitude – frequency relationship of earthquakes occurred in the past in the study area can be best described by an exponential relationship. An earthquake