Table 1Location and geology of study area. (A) Study area, located in the north-western part of the Gyeonggi-Massif, the central part of Korean peninsula. (B) Geological map, which shows that the western and eastern part of the research area are mainly covered by Paleozoic Misan Fm. and Cretaceous volcanics, respectively. (C) Location of earthquake epicenters and a seismic profile by Malehmir et al. (2022), which across the Dongducheon fault (DF). CFS represents Chugaryung Fault System and ① Dongducheon fault.|@|~(^,^)~|@|Rock densities. (A) Densities of igneous, sedimentary, and volcanic rocks from Park et al. (2009) (see also Table 1). (B) Vp-ρ (black line) and Vs-ρ (grey line) relationship that was used for calculating densities from the seismic velocities (Hermann, 2005, Cho et al., 2007).|@|~(^,^)~|@|Construction of subsurface density structure model along the seismic profile. (A) Seismic reflection profile (Malehmir et al., 2022; see Fig. 1C for its location), which shows prominent reflectivities at depths of about 1, 3, 5, 7 and 8 km (R1 – R5). Comparing these reflectivities and the geologic map (Fig. 1B), we speculated rock types at each depth. (B) P-wave velocity distribution, which shows the core of the Dongducheon fault (DF) corresponds to the low velocity zone with a width of about 200 m. (C) Density distribution, which was calculated from the P-wave velocity (Fig. 2B). (D) the shear wave velocity model (Kil et al., 2021) along the seismic profile. The western region from the DF is characterized by high shear wave velocity to a depth of 3 km, while the eastern region shows relatively low velocity. Densities for each 1 km thick layer are calculated from the shear-wave velocity (Fig. 2B). (E) Comparison of all available geophysical information, which will be used as constraints for the gravity forward modeling.|@|~(^,^)~|@|The complete Bouguer anomaly map of the study area. Higher gravity anomalies are observed in the west region of the Dongducheon fault (DF), while lower gravity anomalies are observed on the east side. Microseismicities are concentrated mainly in the low gravity region between high gravity anomalies (HG1 and HG2).|@|~(^,^)~|@|Location of gravity measurements along seismic profile. The existence of high gravity anomaly region (HG1) is reliable because the two gravity data sets obtained at different times agree well with each other.|@|~(^,^)~|@|Reconstructed fault system inferred from the dip-curvature analysis of gravity. (A) Schematic illustration of dip-curvature analysis. (B) Dip-curvature map, which shows anticipated fault lines with a NW-SE direction (EE1 ~ EE6 and WSF) across the Dongducheon fault (DF) and a N-S direction (SF1 ~ SF3). (C) Reconstructed fault system, where the NW-SE directing- and N-S extending fault lines are assumed to be the En echelon lines and strike-slip duplex structure, which are formed by dextral strike slip movement of the DF.|@|~(^,^)~|@|Comparison of our fault system model with the classical model of the strike-slip duplex structure model. (A) Distribution of microseismicities and anticipated fault lines. (B) A classical tectonic model of the strike-slip duplex structure (Twiss and Moores, 1992). (C) Application of the classical model to our study area. It is observed that the microseismicities are distributed along the fault lines inferred by gravity analysis. Distribution of microsesmicities and consistency with the classical strike-slip duplex model support that our model of En echelon lines and strike-slip duplex structure across and along the Dongducheon Fault (DF) are reliable.|@|~(^,^)~|@|Gravity forward model along the seismic profile. A 2D gravity forward model is suggested by using all available constraints from gravity field (A, B), geologic map (A), rock type with densities (C), and spatial distribution of seismic reflectivities (Malehmir et al., 2022). The final model indicates that the prominent gravity lows (DF, SF1~SF3 in B) are mainly affected by the fault cores, which are characterized by low densities about 15% less than surroundings.|@|~(^,^)~|@|Gravity model along the Dongducheon Fault line. A 2D gravity forward model is suggested by using all available constraints from gravity field (A, B) and geologic map (A), and rock type with densities (C). The final model indicates that the prominent gravity highs (HG1 and HG2 in A and B) are mainly caused by the high density material (HD1 and HD2 in C), which may be uplifted from the basement of the Gyeonggi Massiv (GM). The vertical stress distribution (D) at depth of 4.5 km shows a prominent vertical stress change from 3 MPa to –3 MPa in the area between EE1 and EE2 that can be explained by the density difference between En echelon lines.

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