Seismicity acceleration and clustering before the 2015 Mw 7.9 Gorkha earthquake, Nepal

In the last decade, several observations of peculiar seismic and geodetic phases preceding large earthquakes have been documented. Despite being a posteriori, these observations provide a better understanding of the processes involved in the nucleation of earthquakes. In this study, we investigate the foreshocks and the pre-seismic phase of the M$_w$7.9 25 April 2015, Gorkha earthquake in Nepal by applying a matched-filter technique to its nucleation zone. We use the seismic signals of 1851 local earthquakes and the continuous signal recorded at the nearest station for the 6 years preceding the mainshock. The pre-seismic phase depicts a long-term increase of seismicity rate and several seismic swarms less than 20 km away from the mainshock epicenter. The longest swarm occurs one month before the Gorkha earthquake, lasts two weeks and consists of 38 repetitive earthquakes located at the northwestern edge of the rupture zone. Another increase in seismicity rate starts six days before the mainshock and includes small foreshocks that develop less than 10 kilometers from the future earthquake hypocenter. These observations suggest that the Gorkha earthquake was preceded by a pre-seismic phase related to a possible initiation of a slow slip with fluids involved at the northwestern boundary of the rupture zone.