Spatiotemporal patterns of the 2020 Haenam earthquake sequence, South Korea: lineament and migration implying fluid-driven earthquake swarm

Abstract

From late April 2020, 71 consecutive earthquakes with low magnitudes (the largest, Mw 3.2) were catalogued in the county of Haenam, South Korea. In this region, moderate to strong earthquakes were not reported by instrumental, historical, and geological records. We identified 155 uncatalogued events in the source region that occurred between April 25, 2020 and May 8, 2020 and determined the hypocenters of the 226 catalogued and detected events with relative meter-scale location errors based on the cross-correlation approach with data from permanent seismic networks. The clear lineament of the hypocenter distribution defines the fault plane as being 0.3 km by 0.3 km at a depth of ∼20 km, trending WNW-ESE with a dip of ∼70° in the SSW direction. The retrieved moment tensor of the largest event presents a strike of 98°, a dip of 65°, and a rake of 7°, which is close to the inferred fault geometry from relocation results. The hypocenters of the earthquake sequence progressed toward the upper east along the fault-strike direction until the largest event occurred on May 3, 2020 with left-lateral strike-slip motion at the shallower part of the fault plane where the migration is headed. We discuss potential mechanisms that cause non-volcanic earthquake swarms and suggest that the spreading of fluid flow induced the migratory behavior of the Haenam earthquake sequence. This conception of a fluid-driven earthquake swarm is supported by the presence of a seismic migration front that is comparable to a hydraulic diffusivity of 0.012 m2/s in conjunction with inherited WNW-ESE structural sets showing a fault-fracture mesh geometry that can channel the fluid flow. The final part of the sequence forming boundary of the hypocenter distribution could be interpreted as the possible fault-valve behavior at the western end and the permeability barrier at the eastern end, respectively. Our spatiotemporal patterns of the 2020 Haenam earthquake sequence are valuable for understanding earthquake swarms in intraplate stress fields.