Matched-filter detection of the missing pre-mainshock events and aftershocks in the 2015 Gorkha, Nepal earthquake sequence

The 25 April 2015 Mw 7.8 Gorkha, Nepal earthquake occurred at the bottom edge of the locked portion of the Main Himalayan Thrust, where the Indian plate underthrusts the Himalayan wedge. The earthquake was followed by a number of large aftershocks, but not preceded by any foreshocks within ~ 3 weeks according to the NEIC or ISC catalog. However, due to the limited station coverage of the local seismic network, a large portion of events may not be reported in routine catalogs. Here, we employ the matched filter technique to recover the undocumented earthquakes for the period beginning 80 days before through 30 days after the 2015 Gorkha earthquake. We detect twice as many aftershocks as those listed in the ISC catalog. We observe an along-strike aftershock expansion after the mainshock and before the largest Mw 7.3 aftershock that occurred on 12 May 2015. Repeating earthquakes are found to the ESE of the mainshock rupture area, suggesting that the expansion may be partially driven by afterslip. In addition, we observe a significant increase in seismicity rate ~ 3-4 days prior to the mainshock, initiating ~ 6 h after a M 5.2 earthquake, located ~ 240 km to the NW of the mainshock. The increase of seismic activities occurs in a wide region around the Gorkha principal slip zone, indicating the effect of delayed dynamic triggering may contribute to the large-scale unloading process prior to the 2015 Gorkha mainshock.