Spatiotemporal patterns and nucleation of the 2020 Haenam earthquake sequence in the southwestern Korean Peninsula

In 2020, an earthquake sequence occurred in the Haenam area of the southwestern Korean Peninsula, where 74 cataloged earthquakes were recorded within two weeks (26 April–8 May 2020). The activity was concentrated at ~20 km depth, where seismicity is relatively uncommon on the Korean Peninsula, and it comprised an unusually persistent sequence with dozens to thousands of events occurring over the two-week period. In addition, the largest event (M_W 3.2) occurred about one week after the first cataloged event rather than at the onset of the sequence. Previous studies reported that hundreds of additional microearthquakes occurred during the same period, and some interpreted the activity as a swarm, citing the lack of a clear mainshock–aftershock decay, distributed seismicity, and hypocentral migration consistent with fluid diffusion. Here, we focus on a spatiotemporal analysis of the Haenam sequence. To ensure catalog completeness, we applied an improved template matching technique, resulting in an enhanced dataset of 1,345 events. We performed precise relocation and magnitude calibration for a subset of well-recorded events. To place the catalog on a consistent magnitude scale, we estimated magnitudes for the remaining events from S-wave peak amplitude ratios using relative-magnitude scaling. Relocated hypocenters define an E–W striking plane (strike ~92°, dip ~62°) consistent with strike-slip faulting. Spatiotemporal clustering identifies five distinct clusters. Within each cluster, seismicity is largely confined to the estimated rupture radius of the largest event in each cluster, consistent with aftershock-like behavior. Successive clusters preferentially initiate near the edge of the preceding rupture area, suggesting cascade-like triggering. We further observed a brief deepening of seismicity to 21.3–21.5 km only immediately after the mainshock (M_W 3.2), implying a transient downward extension of the effective lower cutoff of seismicity in the lower crust. Using magnitudes on a consistent M_W scale, the enhanced catalog yields a b-value of 1.05 ± 0.03. We also estimated seismic moments from M_W using the moment–magnitude relation and found that the largest earthquake accounts for ~39.4% of the total seismic moment released, indicating a mainshock-dominated sequence. Our results demonstrate how high-resolution spatiotemporal analyses and magnitude calibration can clarify the geometry, clustering, and nucleation of small intraplate sequences on the Korean Peninsula.