Constraining the Earthquake Focal Depth Distribution in the Southern Korean Peninsula

ABSTRACT The accurate determination of earthquake focal depth is critical for understanding regional seismic processes, characterizing seismogenic behavior, and assessing seismic hazard. However, precise focal depth determination remains a significant challenge owing to theoretical limitations, sparse station geometry, uncertainties in velocity models, and errors in arrival-time picking. This study evaluated the uncertainty of earthquake focal depths in the southern Korean Peninsula using seismic station geometry and investigated regional seismogenic characteristics. We selected 304 earthquakes with magnitudes between 2.0 and 4.9 recorded from 2018 to 2022, for which P- and S-wave arrivals were manually picked from dense seismic stations. To quantify uncertainty, we conducted Monte Carlo simulations using synthetic datasets generated with multiple velocity models, random sampling of station locations, and varied initial focal depths. Gradient boosting analysis identified the minimum distance to a station and the number of near-epicentral P and S arrivals as the dominant factors reducing focal depth errors. We propose geometry-based criteria that allow approximately 95% of local crustal events to be located with focal depth errors within 5 km and epicentral errors within 2 km: (1) at least seven stations within 100 km, including two within 50 km and one within 10 km of the epicenter; (2) at least one S wave within 50 km; and (3) primary and secondary azimuthal gaps less than 160° and 220°, respectively. These criteria are region-specific, and further validation is required for application elsewhere. Applying these constraints revealed a bimodal focal depth distribution in the southern Korean Peninsula, with primary concentrations at 5–12 and 12–22 km. Shallow earthquakes occur widely across the southern Korean Peninsula, whereas deeper events are preferentially located near the boundaries of the Okcheon fold belt and in the Gyeongsang basin. Our findings also highlight the need for caution when interpreting offshore focal depths, which may be biased by insufficient station geometry.