1 Introduction

The Korean peninsula is an important tectonic link between eastern China and the Japanese islands (Figure 1). Comprised mainly of deformed basement rocks (granitic intrusions and volcanic rocks, and high-grade gneiss and schist, ranging in age from 1.1 to 2.7 Ga), the Korean crust has experienced a long deformation history. The peninsula has until recently been in a stable intraplate seismic state, although major earthquakes are known from historical recordings (hard copies and drawings), occurring both on the southern part and close to metropolitan Seoul (Lee and Yang, 2006; Park et al., 2020). After the 2011 Tohoku-Oki mega-thrust Mw 9.0 earthquake in Japan, several plus Mw 5.0 earthquakes have been recorded in the peninsula. The 2011 mega-thrust perturbed the Korean crust resulting in coseismic displacements of ~2-4 cm around the east and west coasts of the peninsula and is likely responsible for the subsequent increased seismic activity in the country (Hong et al., 2017). In 2016, the largest event, the Gyeongju Mw 5.4 earthquake (ML 5.8), recorded in the recent history of modern seismic monitoring in South Korea occurred.
Metropolitan Seoul is densely populated and is home to over 20 million inhabitants; a strong earthquake in the city can be devastating. For preparedness and public safety purposes, it is essential to investigate potentially active fault systems, their structures, geometries and triggering mechanisms. This can help to estimate potential damages, magnitudes and places where reinforcements are necessary for infrastructure and housing (Singh et al.,1980; Rosenblueth et al.,1990; Wells et al., 1994). The need to investigate subsurface structures below major cities is a common problem since many mega-cities are in high geo-hazard risk regions. However, these environments are typically noisy (both electric/electromagnetic and ambient seismic noise) and with logistical challenges, making many geophysical investigations difficult to impossible. Nonetheless, different methods have been attempted, including drilling as one example (Zoback et al., 2013). Geophysical methods, especially seismic ones, can be a method of choice although ambient vibrational noise and ground-receiver coupling are major challenges in mega-cities (Sato et al., 2009; Malehmir et al., 2011; Ishiyama et al., 2016). Despite these problems, different studies show that adopting solutions specifically developed for high-resolution imaging in urban settings can bring important results via high-quality imaging of the subsurface structures, also helping to delineate fault systems at depth that may be important for understanding geo-hazards and for preparedness purposes (Sato et al., 2009; Brodic et al., 2015; Ishiyama et al., 2016; Malehmir et al., 2015, 2016, 2017, 2022).
To evaluate the feasibility, and develop specific strategies, for high-resolution seismic imaging of geo-hazards in mega-cities, two novel active-source reflection seismic profiles (P1 and P2) were acquired in November 2020 in the central and wider metropolitan area of Seoul in South Korea. The survey aimed at showing a spatial relationship between the Chugaryeong fault system (one of the crustal-scale fault systems crossing the entire peninsula and active until the Quaternary) and the recorded seismicity in the last 10 years (Hong et al., 2018 and 2021). Recent studies suggest that the Chugaryeong fault is near-vertical and still active showing a dominant strike-slip mechanism from focal mechanism solutions (Hong et al., 2018, 2021). The Pocheon and Wangsukcheon faults, mapped east of the Chugaryeong fault, appear to form splay faults as they approach each other in the city (Figure 1). Other smaller faults are also expected between these three major crustal fault systems although with poor or no surface exposures. P1 was located on the northern side of metropolitan Seoul while P2 was positioned in the central part of the city where the Chugaryeong fault intersects a cluster of seismicity. Given the encouraging results from the 2020 survey, especially along P1 where reflections down to 8-9 km depth were imaged and showed both spatial and temporal correlation with the clustered seismicity (Malehmir et al., 2022), a new survey was justified to shed light on the overall subsurface geometry of the three major fault systems. In July 2021, a new seismic profile (P3), approximately 40 km, was acquired (Figure 1). It was positioned between P1 and P2 on the northern outskirts of metropolitan Seoul where it was logistically possible to acquire the new profile. P3 intersects all the three major fault systems, Chugaryeong, Pocheon and Wangsukcheon, and marks the longest single profile ever recorded in South Korea. The main goals of this study are (1) establish a depth relationship of the three major faults for the first time, (2) reconstruct fault geometries and intersections and (3) investigate correlation between reflections and the recorded seismicity in the region. We demonstrate that the new profile allows 3D positioning of these fault systems and speculate on a fault-bend fold structure suggesting a reverse movement component along it. Some of the recorded seismicity may also occur at the intersection of the Chugaryeong fault with other major faults.