A hybrid probabilistic seismic hazard model for Northeast India and Bhutan combining distributed seismicity and finite faults

In Probabilistic Seismic Hazard Analysis (PSHA), a widely used approach to model earthquake sources consists of using homogenous source zones. This approach suffers the limitation of assuming that the observed seismicity can occur anywhere with same probability over a specific area, which might lead to the potential undervaluation of the predicted ground motion level due to an effect of smearing of seismic potential. To compensate that, a hybrid model is used, accounting both for distributed seismicity and localized seismogenic structures. In this study, we perform PSHA for Northeast India and Bhutan, that are the most seismically hazardous regions on the planet. The region was partitioned into seismogenic source zones of supposedly homogeneous seismic potential and seismotectonic characteristics. Earthquake recurrence parameters for each zone were obtained from direct magnitude-frequency analysis on a precompiled global catalogue. Seismogenic faults are added to the model by converting slip rates from GPS velocity data to seismic activity. Complementary information was derived from the analysis of moment tensor solutions of large events and from a detailed literature review. Using this hybrid model, seismic hazard was calculated for a region bounded by lat/long 24.0°-28.8°N/88.0°-94.5°E. Calculations were performed for Peak Ground Acceleration (PGA) and several Spectral Acceleration (SA) periods for a Probability of Exceedance (POE) of 10% in 50 years, corresponding to 475 years return period, and for a reference rock condition with Vs = 800 m/s. The results highlight significant acceleration levels (about 0.77g at PGA) in the Arunachal Pradesh region (Northeast India), due to the presence of the Himalayan Frontal Thrust (HFT).