Figure 5.(A) Study area (green solid rectangle) locates in the western part of the Gyeonggi Massif, middle part of the Korean peninsula. CFS: Chugaryung Fault System, YSF: Yangsan Fault, GY: Gyeongju earthquake, PO: Pohang earthquake. (B) The NNE-SSW directing Chugaryung Fault System (CFS in A) is composed of several fault lines numbered by 1 ~ 7. Gravity stations in the study area were about 5,000 points, which are indicated by green dots. (C) The Annual frequency of earthquake registered in the study area shows a clear difference before and after the Gyeongju earthquake (black rectangle). The annual earthquake frequency after the Gyeongju earthquake increased rapidly to more than 4 times.|@|~(^,^)~|@|The geological map of the study area shows that the Chugaryung fault system is mainly covered by Precambrian Gneiss, Jurassic Granites and Cretaceous volcanics. About 100 density measurements of igneous, metamorphic, and volcanic rocks are indicated by symbols that relate to the observed values (Park et al., 2009). The mean densities of the Granites, Volcanics and Gneiss are measured of 2.60, 2.56 and 2.67 g/cm3, respectively. ① –⑦ : Fault lines (see Fig. 2 for the identifications).|@|~(^,^)~|@|The complete Bouguer anomaly map (A) over the research area has a mean value of approximately –5.0 mGal and a range from –25.0 to 20.0 mGal. The higher Bouguer gravity anomalies relative to the mean value (–5.0 mGal) are generally observed to the west along the coast. The anomalies lower than the mean value (–5 mGal) reveal along the Chugaryoung fault system, where the Jurrasic Grantite are mainly exposed. ① – ⑦ : Fault lines (see Fig. 2 for the identifications). (B) The regional gravity anomalies are calculated using a wavelength of 20 km.|@|~(^,^)~|@|A residual anomaly map of the research area (A), which is calculated by subtracting the regional gravity field (Fig. 3B) from the Bouguer gravity field (Fig. 3A). The residual anomalies reflect the gravity effects of the crustal rocks much better than Bouguer anomalies (Fig. 3A). The dip curvature map (B) shows anticipated fault lines with a NE-SW direction (red dotted lines with aF2, aF5, aF8 and aF9) and a N-S direction (red dotted lines with aF1, aF3, aF4 and aF6). ① − ⑦ : Fault (see Fig. 2 for the identifications).|@|~(^,^)~|@|(A) Euler deconvolution technique are applied by using the residual gravity- (Fig. 4A) to obtain preliminary information on the 3D distribution of source points in the study area. The deep mass points greater than the mean depth (4,500 m), are mainly clustered in the western parts of the Dongducheon fault line marked with ①, while the eastern part of the Pocheon fault marked with ② is expected to be less than 4,000 m deep. Residual anomalies along the W-E profile (B) (refer to Fig. A for the location of the profile) ranges from –2.0 mGal to 6.0 mGal. The maximum source depth along the profile (D) are compared to the rock types (C).

© Econ. Environ. Geol.