SHOULD ALL OF NEPAL BE TREATED AS HAVING THE SAME EARTHQUAKE HAZARD?

Current earthquake hazard maps for Nepal predict substantial variations in hazard within the nation, with noticeable differences between maps. An alternative hypothesis is that, given current knowledge, all of Nepal may be regarded as equally hazardous and perhaps vulnerable to much larger earthquakes than those currently known because of their long recurrence times. This hypothesis is based on the limitations of the historical earthquake record, the recognized deficit in seismic moment release, and GPS data showing a similar level of coupling along the arc.

We explore the two possibilities via Probabilistic Seismic Hazard Assessment (PSHA), which attempts to forecast the fraction of sites on a map where ground shaking will exceed the mapped value within some time period. Studies evaluating the performance of PSHA models have compared past shaking data to the models in a hindcast (i.e. Stein et al., 2015) for Italy, Japan, the Central and Eastern U.S., and California. By and large these studies conclude that hazard is overestimated. Why that is and whether it is systematic or a result of biased data remains unclear. We investigate these issues for Nepal, using the Global Earthquake Model, whose model for Nepal is that of Nath and Thingbaijam (2012). In this, higher hazard is concentrated in two regions along strike of the Himalayan Front. Lower hazard is estimated between these two, in the region known as the Central or Western seismic gap. To evaluate the performance of the model, we compare it to historical shaking datasets spanning 1636 – 2015. We use two performance metrics; one based on the total number of sites where modeled ground motion is exceeded (M0), and one based on the magnitude of the difference between the expected and observed ground motion at a site (M1). Relative to the PSHA evaluations for other areas mentioned above, the GEM model for Nepal has the highest (by a small amount) M0 score out of all 475-yr return period models. It is on par with U.S. models for M1, which is middle-of-the-pack. Because previous work has suggested that Nepal might be better treated as having uniform hazard, we use the performance metrics to explore this and other hazard issues for Nepal.