Imaging of high frequency seismic radiation during the 2015 Mw 7.8 Gorkha (Nepal) earthquake using multiple global seismic arrays

Abstract:

The Gorkha earthquake provides a unique opportunity to study the dynamics of the Himalayan faults in Nepal, specifically the Main Himalayan Thrust, accommodating north-south compression due to convergence between Indian and Eurasian plate. We use four independent global seismic arrays to backproject the high frequency seismic radiation to its source, and produce high-resolution images of rupture propagation during this earthquake (Figure 1). The rupture propagates unilaterally from west to east for 120 km for at an average velocity of 2 km/s. The duration of rupture is estimated to be 60 s. Along the dip direction, rupture propagates downward from the epicenter for the first 50 s, and shows complexity after that. The details of the rupture revealed by the arrays indicate strong control of stress and/or geometrical heterogeneity over the rupture pattern. Interestingly, high frequency seismic radiation is spatially separate from the low-frequency patches derived from inversion of low frequency seismic data [NEIC, USGS]. High frequency sources are located downdip of peak co-seismic slip. As a result of this separation, sources of high-frequency radiations are located farther away from Kathmandu, the most densely populated city in Nepal, compared to the large slip patches. This earthquake broke the downdip part of the locked patch of the Main Himalayan Thrust. The updip part, with strong seismic coupling [Ader et al., 2012], still remains unbroken and can potentially produce large damaging earthquakes. Mapping of high frequency radiations using multiple arrays are providing robust details of rupture characteristics allowing us to assess the relationship between fault coupling, frictional heterogeneity and future earthquake hazard in the Nepal Himalayas.