Lateral variations of the midcrustal seismicity in western Nepal: Seismotectonic implications
Résumé
The midcrustal seismicity along the Himalayas depicts some lateral variations. This seismicity is often related to stress build up, at depth, along the downdip extent of the fault segments producing large and great earthquakes. To better characterize these variations, we deployed the first temporary seismological network in West Nepal (HiKNet) as a complement to the Nepalese national network. We used automatic picks, manually validated, to perform absolute and relative location of the earthquakes. The resulting catalog contains 2190 local events, shallow to midcrustal (8-16 km). The comparison between the seismicity and the geologic balanced cross-sections shows that most earthquakes occur within the hanging wall of the Main Himalayan Thrust fault nearby ramps or suspected contacts between Lesser Himalayan slivers. Some of the events associated with these geological structures are clustered within weeks-long earthquake swarms with seismic envelopes migrating at 35-50 m/day. These transients are possibly related to fluid migrations or geodetically unresolved slow slip events. The lateral variations of some of the structures associated to this seismicity are likely partially controlling the extent of the coseismic ruptures during intermediate earthquakes that break partly the locked fault zone, in a similar way to what was reported after the Mw7.8 2015 Gorkha-Nepal earthquake. A better characterization of the faults segmentation, including a better location and extent of the seismogenic segments at depth, is an important input for seismic hazard studies.
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