Probabilistic Seismic Hazard Assessment of Nepal

Author:

Stevens, V. L.  Shrestha, S. N.  Maharjan, D. K.  

Journal:

BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA

Issue Date:

2018

Abstract(summary):

We carry out a new probabilistic seismic hazard analysis (PSHA) for Nepal. The 2015 M-w 7.8 Gorkha, Nepal, earthquake (hereafter the Gorkha earthquake) highlights the seismic risk in Nepal, allows better characterization of the geometry of the Main Himalayan thrust (MHT), and enables comparison of recorded ground motions with predicted ground motions. These new data, together with recent paleoseismic studies and geodetic-based coupling models, allow for good parameterization of the MHT's characteristics. Other faults in Nepal remain less well studied. Unlike previous PSHA studies in Nepal that are exclusively area-based, we use a mix of fault and area source models to describe six seismic sources in Nepal. For each source, the Gutenberg-Richter (GR) a - and b-values are found, and the maximum magnitude earthquake (M-max) estimated, using a combination of earthquake catalogs, moment conservation principles, and similarities to other tectonic regions. We use OpenQuake to carry out the analysis, and estimate peak ground acceleration (PGA) at 2% and 10% probability of exceedance in 50 yrs, along with hazard curves at various locations. We find that PGA reaches 0.6g at 10% probability of exceedance in 50 yrs and is high over most of Nepal. In contrast to previous seismic hazard models, our hazard is high in southern Nepal and fairly evenly distributed across the westnorthwest-east-northeast direction. The MHT is the principal seismic hazard in Nepal so we study the effects of changing several parameters associated with this fault and find that uncertainties in a and b-values have a much more significant effect on estimated PGA than the geometry or M-max. We compare the results from a modeled Gkha earthquake scenario using different ground-motion prediction equations (GMPEs) with observations, and find that none of the trialled GMPEs fully account for all the features observed. Developing a region-specific GMPE would be a next step for future seismic-hazard analysis in Nepal.

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