Persistence of seismicity at fault bends along the Main Himalayan Thrust

Abstract:

At subduction zones and continental collisional margins, the relative motion between tectonic plates is accommodated by shortening along megathrusts characterized by extensive seismogenic zones that produce some of Earth's largest earthquakes. Elucidating the mechanics of down-dip segmentation of the seismogenic zone is key to better determine seismic hazards in the region. Most of the relative convergence between the Indian and Eurasian plates is currently accommodated along the Himalayan megathrust, which follows the surface-emergent Main Frontal Thrust (MFT) that soles into a gently dipping décollement called the Main Himalayan Thrust (MHT). However, the geometry of the MHT and its impact on seismicity remains controversial. Also, the impact of fault bends on down-dip segmentation of the MHT is lesser known, due in part because the detailed structural layout of the MHT is debated. Here, we develop seismic cycle simulations tuned to the seismo-geodetic data of the 2015 Mw 7.8 Gorkha, Nepal earthquake to better constrain the megathrust geometry and its role on the demarcation of partial ruptures. We show that partial ruptures of the seismogenic zone similar to the 2015 Gorkha earthquake can be obtained either on a wide frictionally unstable décollement or on one with additional fault bends, as long as the characteristic size for earthquake nucleation is much smaller that the seismogenic zone width. However, a sequence with a mainshock followed by shallow dipping aftershocks consistent with observations can only be obtained with a ramp in the middle of the seismogenic zone. Structural complexity may play an important role during the seismic cycle, fault bends possibly obstructing rupture propagation and attracting seismicity.