Three-dimensional Crustal S-wave Velocity Structure Beneath the Southwestern Korean Peninsula from Helmholtz Tomography; [헬름홀츠 토모그래피를 이용한 한반도 남서부지역 3차원 S파 지각 속도 구조 연구]

초록

The ML 4.8 earthquake that occurred on June 12, 2024 approximately 4 km south-southwest of Haengan-myeon, Buan-gun, Jeonbuk Province, highlights the need for an improved understanding of the crustal structure and seismogenic mechanisms of the southwestern Korean Peninsula. In this study, we investigate the three-dimensional S-wave velocity structure of the southwestern Korean Peninsula to evaluate its relationship with major fault systems and seismic activity. A total of 331 earthquake waveforms, recorded between January 1, 2021 and April 30, 2025, were collected from 62 seismic stations deployed across the study area. Phase and amplitude measurements were used to perform Helmholtz tomography. Rayleigh-wave phase-velocity maps for periods of 10–40 s were first derived, and a three-dimensional S-wave velocity model at depths of 5–40 km was subsequently constructed through iterative least-squares inversion. The resulting model reveals northeast-southwest-trending linear low-velocity anomalies in the upper crust beneath the Jeonbuk and southern Chungnam provinces. These low-velocity anomalies spatially coincide with the geological distribution of granitoids formed during extensive Mesozoic orogenic activity. Furthermore, seismicity is observed to cluster along these linear anomalies, exhibiting a clear spatial correlation with major fault systems in the study area. By integrating the distribution of low-velocity anomalies with microearthquake patterns, we propose the possible existence of a previously unidentified subsurface fault near the source region of the Buan earthquake. ©2026 The KSEEG. Printed by Hanrimwon Publishing Company. All rights reserved.

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