Monitoring a short-lived earthquake swarm during April-May 2020 in Haenam, Korea, and its preliminary results

Abstract

On 3 May 2020, an M-L 3.1 earthquake occurred in Haenam, southwestern Korea, in an area devoid of recorded seismicity since instrumental observations began in 1978. Careful examination of the temporal occurrence of seismicity, and the magnitude distribution of the sequence before and after the M-L 3.1 earthquake, indicates typical swarm-like behavior. The earthquake swarm started with an M-L 0.6 event on 26 April 2020, intensified up to 3 May 2020, and abruptly terminated with an M-L 1.0 event on 9 May 2020. The Pusan National University Geophysics Laboratory (PNUGL) deployed a temporary seismic array with eight three-component short-period instruments to monitor the short-lived bursts of seismicity. During the monitoring campaign, we detected > 700 microearthquakes by applying a matched-filter technique to the combined dataset produced by PNUGL, the Korea Meteorological Administration, and the Korea Institute of Ocean Science and Technology. We determined earthquake parameters for 299 earthquakes that were detected at four or more seismic stations. We also determined the focal mechanism solutions of the 10 largest earthquakes in the swarm using first-motion polarities with S/P ratios. The focal mechanism, hypocentral depth, and stress orientation of the largest earthquake in the sequence were also determined using waveform inversions. The distribution of earthquake hypocenters, together with focal mechanism solutions, indicates that the earthquake swarm activated deeply-buried faults (similar to 20 km) oriented either NNE-SSW or WNW-ESE. We also report details of the temporary seismic monitoring network, including the instrumentation, detection of microearthquakes, and variations in event-detection threshold influenced by anthropogenic and natural noise fluctuations. We also discuss the limitations associated with lowering the detection threshold of microearthquakes by increasing the number of seismic stations or by adopting advanced event-detection techniques.