Abstract
The Himalayan megathrust accommodates most of the relative convergence between the Indian and Eurasian plates, producing cycles of blind and surface-breaking ruptures. Elucidating the mechanics of down-dip segmentation of the seismogenic zone is key to better determine seismic hazards in the region. However, the geometry of the Himalayan megathrust and its impact on seismicity remains controversial. 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 a ramp in the middle of the seismogenic zone is required to explain the termination of the coseismic rupture and the source mechanism of up-dip aftershocks consistently. Alternative models with a wide décollement can only explain the mainshock. Fault structural complexities likely play an important role in modulating the seismic cycle, in particular, the distribution of rupture sizes. Fault bends are capable of both obstructing rupture propagation as well as behave as a source of seismicity and rupture initiation.
Highlights
The Himalayan megathrust accommodates most of the relative convergence between the Indian and Eurasian plates, producing cycles of blind and surface-breaking ruptures
Since structural complexity may play a significant role in megathrust behaviour, it is paramount that we reduce uncertainties in fault geometry
We consider endmember scenarios that allow us to study the impact of fault bends on the seismic cycle
Summary
The Himalayan megathrust accommodates most of the relative convergence between the Indian and Eurasian plates, producing cycles of blind and surface-breaking ruptures. The seismic cycle at the MHT embodies both along-strike and down-dip segmentation[12], with surface-breaking ruptures, like the 1934 Nepal-Bihar event[13,14], that unzip the entire seismogenic width, and blind ruptures, like the 1833 Kathmandu event[15,16], that break only a small section of the megathrust. We will 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 than the seismogenic zone width
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