Seismicity in far western Nepal reveals flats and ramps along the Main Himalayan Thrust

M Laporte,M Bhattarai,C Duverger,H Lyon-Caen,B P Koirala,M Riesner,L Bollinger,T Kandel,C Timsina,R Hoste-Colomer,J Letort,L B Adhikari

doi:10.1093/gji/ggab159

Abstract

SUMMARYUnravelling relations between lateral variations of mid-crustal seismicity and the geometry of the Main Himalayan Thrust (MHT) system at depth is a key issue in seismotectonic studies of the Himalayan range. These relations can reveal along strike changes in the behaviour of the fault at depth related to fluids or the local ramp-flat geometry and more generally of the stress build-up along the fault. Some of these variations may control the rupture extension of intermediate, large or great earthquakes, the last of which dates back from 1505 CE in far western Nepal. The region is also associated to lateral spatio-temporal variations of the mid-crustal seismicity monitored by the Regional Seismic Network of Surkhet–Birendranagar. This network was supplemented between 2014 and 2016 by 15 temporary stations deployed above the main seismic clusters giving new potential to regional studies. Both absolute and relative locations together with focal mechanisms are determined to gain insight on the fault behaviour at depth. We find more than 4000 earthquakes within 5 and 20 km-depth clustered in three belts parallel to the front of the Himalayan range. Finest locations reveal close relationships between seismic clusters and fault segments at depth among which mid-crustal ramps and reactivated tectonic slivers. Our results support a geometry of the MHT involving several fault patches at depth separated by ramps and tear faults. This geometry most probably affects the pattern of the coseismic ruptures breaking partially or totally the locked fault zone as well as eventual along strike variations of seismic coupling during interseismic period.

Highlights

Most of the continental or oceanic subductions megathrust systems produce large and great earthquakes resulting from the release of decades to centuries of stress build-up (e.g Burgmann et al 2005; Avouac 2015; Herman et al 2018)
We document the hypocentral location of 4500 earthquakes recorded by the Himalayan-Karnali Network (HiKNet), a temporary seismological network deployed during 2 yr (2014–2016) in the trace of 1505 earthquake in far western Nepal
The main seismic clusters we monitor develop along the Main Himalayan Thrust (MHT) fault, at the toe of active mid crustal ramps, while some secondary clusters develop within the contact between tectonic slivers, passive thrusts or out-of-sequence active faults in the hanging-wall of the Main Thrust system

Summary

Introduction

Most of the continental or oceanic subductions megathrust systems produce large and great earthquakes resulting from the release of decades to centuries of stress build-up (e.g Burgmann et al 2005; Avouac 2015; Herman et al 2018). These active fault systems frequently produce small and moderate size earthquakes, which generally occur along the downdip edge of the interseismic locked zone. In Nepal, the Main Himalayan Thrust (MHT), the plate boundary fault between India and the Himalayas, produces infrequent large (M7) and great (M8+) devastating earthquakes rupturing the locked zone up to the surface. The density of these local earthquakes recorded at the Nepalese seismological network during the 20 yr that preceded the 2015 Mw 7.9 Gorkha earthquake, depicts some significant

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Discussion

Conclusion