Automatic analysis of the Gorkha earthquake aftershock sequence: evidences of structurally segmented seismicity

We present the first 3 months of aftershock activity following the 2015 April 25 Gorkha earthquake M-w 7.8 recorded on the Nepalese Seismic network. We deployed an automatic procedure composed of three main stages: (1) coarse determination of the P and S onsets; (2) phase association to declare events and (3) iterative addition and refinement of onsets using the Kurtosis characteristic function. In total 9188 events could be located in the Kathmandu region with the majority having small location errors (<4.5, 9 and 10 km in the X-, Y- and Z-directions, respectively). Additionally, we propose a new attenuation law to estimate local magnitudes in the region. This new seismic catalogue reveals a detailed insight into the Gorkha aftershock sequence and its relation to the main shock rupture models and tectonic structures in the region. Most aftershocks fall within the Main Himalayan Thrust (MHT) shear zone or in its hangingwall. Significant temporal and lateral variations of aftershocks location are observed among them: (1) three distinct stages, highlighting subsequent jump-offs at the easternmost termination, (2) the existence of a seismic gap north of Kathmandu which matches with a low slip zone in the rupture area of the main shock, (3) the confinement of seismic activity in the trace of the May 12 M-w 7.3 earthquake within the MHT and its hangingwall through a 30 x 30 km(2) region and (4) a shallow westward-dipping structure east of the Kathmandu klippe. These new observations with the inferred tectonic structures at depth suggest a tectonic control of part of the aftershock activity by the lateral breaks along the MHT and by the geometry of the duplex above the thrust.