Matched-filter Detection of the Missing Foreshocks and Aftershocks of the 2015 Gorkha earthquake

Abstract:

The 25 April 2015 Mw 7.8 Gorkha earthquake occurred at the bottom edge of the locking portion of the Main Himalayan Thrust (MHT), where the Indian plate under-thrusts the Himalayan wedge. The earthquake is followed by a number of large aftershocks but is not preceded by any foreshocks within ~3 weeks according to the NEIC, ISC and NSC catalog. However, a large portion of aftershocks could be missed due to either the contamination of the mainshock coda or small signal to noise ratio. It is also unclear whether there are foreshocks preceding the mainshock, the underlying physical processes of which are crucial for imminent seismic hazard assessment. Here, we employ the matched filter technique to recover the missing events from 22 April to 30 April. We collect 3-component broadband seismic waveforms recorded by one station in Nepal operated by Ev-K2-CNR, OGS Italy and eleven stations in Tibet operated by the China Earthquake Networks Center. We bandpass the seismograms to 1-6 Hz to retain high frequency energies. The template waveforms with high signal-to-noise ratios (> 5) are obtained at several closest stations. To detect and locate the events that occur around the templates, correlograms are shifted at each station with differential travel time as a function of source location based on the CRUST1.0 model. We find ~14 times more events than those listed in the ISC catalog. Some of the detected events are confirmed by visual inspections of the waveforms at the closest stations. The preliminary results show a streak of seismicity occurred around 2.5 days before the mainshock to the southeast of the mainshock hypocenter. The seismicity rate is elevated above the background level during this period of time and decayed subsequently following the Omori’s law. The foreshocks appear to migrate towards the hypocenter with logarithmic time ahead of the mainshock, which indicates possible triggering of the mainshock by the propagating afterslip of the foreshocks. Immediately after the mainshock the seismicity rate increased to the peak level during the period of observation. A large portion of aftershocks are located at ~100-150 km to the southeast of the hypocenter, where the expansion of aftershocks with logarithmic time is more prominent.