Analyzing Earthquake Focal Depth Distribution in the Southern Korean Peninsula

The distribution of focal depths can enhance the understanding of seismogenic characteristics in a region. However, determining accurate focal depths is highly challenging, and the reliability of such determination is often limited, requiring careful consideration during analysis and interpretation. This study investigates the depth distribution of earthquakes in the southern Korean Peninsula to better understand regional seismicity. To minimize focal depth errors, the criteria for reliable focal depth determination were examined. For the local crustal velocity model of Kim et al. (2011), P- and S-wave travel times were computed under conditions of arbitrary seismic networks with a 150 km aperture, consisting of 1,680 seismic stations and originating from focal depths of 5 to 25 km. From these conditions, 10 seismic stations were randomly selected to determine the hypocenter of an event, which was then compared with the original focal depth. This process was repeated 100,000 times for each focal depth without noise and an additional 100,000 times with random noise ranging from -2 to 2 seconds added to the travel times. Consequently, a total of 4.2 million sets of arrival times were generated. To account for epistemic uncertainty in the crustal structure, three local velocity models, including that of Kim et al. (2011), were used for earthquake location. Various metrics were evaluated to develop selection criteria for ground truth events with well-located hypocenters. The Gradient Boosting method identified the minimum distance to a station as the most important metric. In this study, a new metric was introduced by gridding the epicentral distance within 100 km and the azimuth while considering the distribution of seismic stations within each grid. Using this metric, along with considerations of epicentral distance and azimuthal gaps, criteria were established to ensure the reliability of input data for accurately determining focal depths. Seismic events from 2018 to 2022 were located after meticulously inspecting seismic phases and selected based on the criteria proposed in this study to analyze the focal depth distribution in the southern Korean Peninsula.