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.