Lateral structure variations and transient swarm revealed by seismicity along the Main Himalayan Thrust north of Kathmandu

The midcrustal seismicity along the Main Himalayan Thrust in Nepal presents lateral variations along the rupture of the 2015 Gorkha earthquake. In order to resolve these variations, we relocate the seismicity north of Kathmandu, during a period well covered by the Nepal National Seismological Network, using a double-difference algorithm. The 550 relocated events highlight a complex pattern of clustered seismicity within the unstable-stable transition zone. Part of the seismicity is densely clustered on a southward dipping plane which ruptured on January 31st 1997 (ML = 5.8), activating a backthrust with a geometry consistent with the centroid moment tensor of this event calculated in this study. At its eastern end, the midcrustal cluster is offset by 20 km to the south suggesting the presence of a tear fault. The analysis of the time sequence allows constraining a scenario involving stress transfer between these local midcrustal structures, beginning more than one month before the 1997 main shock. The temporal evolution of the seismicity is strikingly similar for two other transient seismic swarm episodes which developed hundreds of kilometers apart along the Main Himalayan Thrust at the same time. The local stress field appears responsible for the higher sensitivity of these regions to subtle strain transients developing along the Main Himalayan Thrust.