Did Structural Segmentation of the Main Himalayan Thrust Control the 2015 Mw 7.8 Gorkha Earthquake Rupture in Nepal?

Abstract:

The collision of India with Asia continues to uplift the highest mountain range in the world. The April 25^th 2015 M_w7.8 Gorkha earthquake, largest recent thrust event in the range, ruptured the Main Himalayan Thrust (MHT), which dips gently northwards beneath central Nepal and forms the boundary between the Indian and Eurasian plates. Although the geology of the range has been studied for decades, fundamental aspects of its deep structure remain disputed. Here we develop a 3D, geologically informed, model of the MHT that is consistent with seismic observations from the Gorkha earthquake. The model is based on the existence of two deep ramps on the MHT that link segments of décollement, one to produce the Gorkha-Pokhara anticlinorium and the other to explain the deeper rocks exposed by the Main Boundary Thrust compared to the Main Frontal Thrust. Comparing our model to a joint slip inversion based on GPS, InSAR and seismic data from the earthquake shows that the slip patch matches closely with an oval-shaped, gently dipping fault surface bounded on all sides by steeper ramps. The Gorkha earthquake rupture seems to have been limited by the geometry of that fault segment. This is a significant step forward in understanding the deep geometry of the MHT, and suggests that subsurface geometry may play a key role in limiting earthquake ruptures. Such an approach can be extended along strike to identify other slip patches consistent with historical earthquake magnitudes, and could be used in conjunction with dynamic modeling to assess the likelihood of single or multiple-segment events. Our result emphasizes the importance of adequately constraining subsurface fault geometry in mega-thrust or subduction zones, in order to better assess the sizes and locations of future earthquakes.