Seismic Hazard analysis of Nepal

Abstract

Nepal is situated in the central part of the Himalaya, which is one of the most seismically active zones in the world. Nepal has witnessed several mega-quakes, with magnitude above 8, and thousands of smaller earthquakes. The 2015 Mw 7.8 Gorkha earthquake is the most recent significant event in the country. Large and moderately large earthquakes can have a tragic impact on people and society as a whole. The preparation of seismic hazard map and evaluation of site specific ground motion due to earthquake are recognized as the fundamental steps towards the earthquake risk reduction. An attempt is made to develop seismic hazard maps of Nepal using available data of earthquakes, recent knowledge of seismotectonics and refined geological features. A comprehensive and magnitude-homogenized earthquake catalogue is prepared from the processing of earthquake catalogues collected from different sources. Thus prepared catalogue contains earthquake data since 1100 AD to 2017 AD. Nine sets of potential earthquake source models are delineated after the analysis of the seismicity distribution, geological structures, results of different geophysical and geodetic surveys and the experience of 2015 Gorkha Earthquake. Probabilistic technique of seismic hazard analysis is applied to evaluate seismic hazard in Nepal. The result shows high hazard in an east-west elongated belt that runs parallel to the front of the Higher Himalaya. The maximum PGA reaches about0.45 g in farwest Nepal and about 0.4 g in Kathmandu Valley for 10 % probability of exceedance in

50 years, at engineering rock site.

The effect of the sediments of the Kathmandu sedimentary basin is investigated using strong motion data of earthquakes recorded in the Kathmandu Valley. During the Mw 7.8 (Gorkha) earthquake, the largest PGA (0.25 g) was recorded at Kirtipur (KTP), which is a rock site. The record shows a single and unusual spike, which is attributed to the permanent displacement of the Main Himalayan Thrust. The second largest PGA (0.23 g) was recorded at the Central Department of Geology (Tribhuvan University (TVU)), which is a soil site. During the Mw 7.8 earthquake, the PGAs were vii

comparatively smaller at all soil sites, whereas the peak ground velocity (PGV) was small at KTP (rock site) and were larger at all other soil sites. The strong motion records of the earthquakes depict the predominant period being between 3 and 5 seconds at the soil sites. The high frequencies (>2.5 Hz), were strongly damped and the low frequencies (<2.5 Hz) were amplified at the soil sites, therefore tall buildings suffered more and the low rise buildings suffered less when compared. The amplification factor was small during the stronger earthquakes and was large during smaller earthquakes; thus demonstrating non-linear response of soil. The analysis shows that the azimuthal effect of earthquake sources was not observed in the Kathmandu Valley. Earlier reports on the 1934 Bihar-Nepal Earthquake (Mw 8.2) and 1833 North Kathmandu Earthquake (Mw ~7.6) report localized massive destruction in the Kathmandu valley and attribute the massive destruction to local geology of the basin (Pandey and Molnar, 1988). Contrary to these claims, amplification of ground acceleration was not observed in the Kathmandu valley during the 2015 Gorkha Earthquake but the low frequencies were amplified. The seismic hazard map and ground response results are mandatorily considered in policy making, planning, formulation and revision of building codes, design of new infrastructures and retrofitting of existing infrastructures. It facilitates the seismic risk assessment, insurance policies and many other related matters.