7 Conclusions

We have acquired an approximately 40-km long reflection seismic profile intersecting three major fault systems (Chugaryeong, Pocheon and Wangsukcheon faults) in the broader metropolitan area of Seoul in South Korea. The data are consistent with the Chugaryeong fault being a sub-vertical structure below the location where it is geologically mapped. From a seismic viewpoint, surface-waves show strong back scattering from it and it shows no coherent reflectivity. A 30-degree NW dipping zone of reflectivity underlying a dome-shaped reflective package is imaged in the central part of the migrated stacked section and it is interpreted as a fault. A dyke or sill system intersected by the propagating fault has resulted in the fault-bend fold structures (dome-shaped reflectivity). This fault if projected downwards would intersect a series of seismic events at the intersection with the Chugaryeong fault. This further suggests, and confirms a recent speculation, a relation between the two faults concerning the intersection and the seismicity, with the implication that both faults may be active and form splays. If this fault is projected to the surface it will intersect Pocheon fault surface expression and related strong surface wave back scattering, unfortunately there is no imaging of the near surface geometry of Pocheon fault to confirm or reject this possibility. The Wangsukcheon fault, on the other hand, is found to have a N20E/60E geometry, implying an opposite dip angle from the other two faults. There is no related seismicity associated with the Wangsukcheon fault in the area, which may justify why it has completely different geometry with respect to the two other faults.
Data availability . Original data underlying the material presented are available by contacting the corresponding author. However, as the dataset is the subject of other PhD studies, there is a period of 3years embargo on their availability.
Acknowledgments. We thank the contributions of many students and post-docs from Yonsei University and Uppsala University, Geopartner and C&H Company. Chiara Colombero and Politecnico di Torino provide access to the surface waves back scattering energy computation code.
Author contributions . TKH initiated the project, organized the acquisition and was involved in the discussions of the results and their interpretations. SZ is responsible for the data preparation, for most of the data processing, for the traveltime tomography, for the crossdip analysis, for the interpretations and for the writing of the article. AM led the data acquisition, processed most of the landstreamer data, helped with the overall processing and especially with the writing of the article and the interpretational aspects and discussion. CJ worked in the first part of the data processing highlighting the Wangsukcheon fault reflection and modeling its reflection traveltime. JL helped on the acquisition setting and organization and prepared the basis for the maps. MP worked on the surface-wave back scatterings analysis. BB contributed in the data acquisition and planning. TKH, BB, CJ, JL, MP, SP, DC, BK and JeL contributed to the final preparation of the paper.
Financial support . This work was supported by the Korean Meteorological Administration (KMA) Research and Development Program under grant KMI2022-00710, and partly by the Basic Science Research Program of National Research Foundation of Korea (NRF-2017R1A6A1A07015374).