Exploring the Seismogenic Structure of the 2016 Yanhu Earthquake Swarm Using Template-based Recognition Techniques

To improve our knowledge about earthquake swarm sequences in the Shanxi region, the template matched filtering technique was used to detect missed seismic events in the Yanhu earthquake swarm, resulting in 133 new locatable earthquakes being added, twice that of the original catalog. Next, using information on the 2016 Yanhu magnitude 4.4 earthquake swarm sequence, the precise locations of the aftershocks, focal mechanism of the mainshock, and seismotectonics were determined. Analysis of the epicenter distribution revealed that the tectonics of this earthquake differed from that of the previous swarm-type earthquakes generated in this region, as it had relatively simple tectonic units and occurred near the north-shore rupture of Yanhu. The results of the double-difference precise earthquake location analysis showed that the aftershocks were predominantly distributed in the northwest (NW) direction. Through fine localization, the focal depth was determined to be concentrated in the range of 1–20 km, with an average depth of 8.33 km, and the mainshock occurred at the end of the sequence. The deep spreading of the sequence was consistent with the focal mechanism solution obtained using the cut-and-paste method. The dislocation event of the mainshock in this sequence was a strike-slip-cum-thrust fault, and the strike and dip of the nodal plane I parameters were close to each other, indicating a high-angle slip fault. It was concluded that the sequence of the Yanhu earthquake swarm was dominantly distributed in the NW direction, and the precise localization results combined with the local mechanism solution indicate that the swarm was a NW-trending buried rupture earthquake.