2 Seismicity and geology of the Korean
peninsula
The Korean peninsula can generally be characterized by three major
Paleoproterozoic massifs (lithotectonic domains), Nangrim, Gyeonggi, and
Yongnam, separated by the Imjingang and Ogcheon belts (Chough et al.,
2000). The new reflection seismic profile P3 was acquired on the
northwestern margin of the Gyeonggi massif crossing different
lithologies and the previously discussed faults.
The Gyeonggi massif in the central part of the peninsula is comprised of
granitic, gneissic and basaltic rocks. Geological units range from
Paleoproterozoic to Cretaceous except for the basalts and fluvial
deposits that are of Quaternary age. These rocks are crosscut by major
fault systems from Paleozoic to Quaternary (Choi et al., 2012; Bae and
Lee, 2016). The most represented lithologies are granite, gneiss and
schist, with also the occurrence of basalts, metasediments and dykes.
The area shows evidences of four main deformation phases. The first one
is related to N-S compression in Late Permian-Early Triassic, followed
by a N-S extensional phase in Middle-Late Triassic that resulted in
ductile shear and normal faulting. In Middle-Late Jurassic a NNW-SSE
compression caused NW-dipping thrust systems. The last phase is
characterized by N-S to NNE-SSW right strike-slip faults associated with
NNW-SSE folds and to the formation of different Cretaceous basins. The
three fault systems (Figure 2) that are the focus of this seismic survey
are related to the last deformation phase, which show evidence of
activity during the Quaternary and that are possibly still active
(KIGAM, 2008). Recent studies show the occurrence of seismic clusters
close to the main faults, especially to the Chugaryeong fault,
suggesting that they are in some way controlled by the current stress
field (Hong et al., 2021). During the last decades, several projects
have studied these faults, working on their ages, kinematics and
geometries, using trenches when possible (i.e., Han and Lee, 2019 and
references therein). Results of these studies suggest high dipping
angles for all the faults with the Chugaryeong fault considered to be
sub-vertical, Pocheon WNW dipping and Wangsukcheon ESE dipping (Han and
Lee, 2019). The main limitation for these studies is the poor exposure
of these faults on the surface. Therefore, most of the mapped faults are
reconstructed following lithological boundaries and morphological
valleys, with real information limited only to small areas. The entire
area is geothermally active, as can be seen by the occurrence of
different natural hot springs in the area (Lee et al., 2010).