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).