A probabilistic assessment of seismic hazard in Nepal Himalaya

Nepal Himalaya was formed from the result of a collision between the Indian and Eurasian Tectonic Plates and is the seismically active area which has experienced several huge earthquakes in its history with magnitude exceeding Mw 7. A probabilistic seismic hazard assessment (PSHA) has been performed for Nepal Himalaya to obtain the quantitative estimation of the probability of exceeding various ground-shaking levels. A composite earthquake catalog which combines historical documents and instrumental recordings was prepared. The catalog is composed of a total of 1228 events from 1255 to 2017. An artificial neural network method was employed for seismic zonation of the region based on the catalog. Then the Nepal Himalaya was divided into 16 seismogenic source zones. For the estimation of the seismic parameter, the study area was gridded with a size of 0.5°×0.5° and the seismicity within a 100 km buffer around each grid point was utilized. The Kijko's approach was used to compute the seismicity parameters of each zone. In addition, four sets of ground-motion prediction equations (GMPE) were selected for active shallow crustal and subduction interface earthquake regimes which shows the similar tectonic characteristics with the study area. The PSHA computations were performed using CRISIS 2015 software whose calculations are based on the well-known Cornell approach in which the basic input data are the geometry of seismic sources, seismicity parameters of each source, and ground motion relationships. A logic tree method was applied for considering epistemic uncertainties included in the input data, and then the final PSHA maps were prepared. This analysis produced a set of probabilistic seismic hazard maps. The hazard distribution pattern thus nearly followed the background seismicity distribution over the study area and the areas of highest potential hazard were likely distributed along the far western, central, and eastern parts. However, for 10% probability of exceedance in 50 years, the minimum and maximum peak ground acceleration (PGA) values were found to be 220-420 Gal, respectively. Similarly, for 5% and 2%, the PGA values were 280-580 Gal and 380-800 Gal, respectively.