Figure 9.(A) The study area (black solid rectangle) locates in the northern part of the Yangsan fault line (white dotted line). 2016M5.8_GY: Gyeongju earthquake epicenter, 2017M5.4_PO: Pohang earthquake epicenter. (B) Geologic map indicates that the Yangsan fault line in the study area is the boundary between the Cretaceous biotite Granite and the Cretaceous grey Sandstone. (C) presents measured gravity stations (dots) and electrical resistivity survey line (DD-08) in the Naengsuri. 2D electrical resistivity structure presented in (D) will be used as one of the important constraints to model density structure.|@|~(^,^)~|@|Simplified schema of Euler deconvolution method (A), Curvature analysis (B), and 3D modeling algorithm to understand theoretical background of gravity inverse interpretations and forward modelling (C). SI: Structure Index, RHO: Density|@|~(^,^)~|@|(A) Gravity measurement line parallel to the electrical survey line (ER_DD-08). (B) The complete Bouguer anomaly along the WE profile with a mean value of approximately 28.5 mGal and a range from 27.7 to 29.5 mGal. The lower Bouguer gravity anomalies are generally observed in the western part of the Yangsan fault line, while the area to the east is characterized by higher anomalies. A prominent anomaly change from 27.5 to 29.0 mGal is well coincided with the low resistivity area (LRZ in A).|@|~(^,^)~|@|(A) Bouguer anomalies across the Yangsan fault line are shown (refer to Fig. 1C and Fig. 3A for the location of the profile). (B) Application of the Euler deconvolution technique by using the complete Bouguer anomaly field (A) to calculated x and z-derivative. (C) Distribution of source points. The Yangsan fault core is characterized by the prominent disturbances of xand z-derivatives, source depths are about 120 meters, definitely deeper than the mean depth (50 meters).|@|~(^,^)~|@|A shows that the Bouguer anomaly map over the study area (Sinkwangmyen, Pohang) has a mean value of approximately 30 mGal with a range from 20 to 40 mGal. The higher Bouguer gravity anomalies in the NE area from Sinkwangmyen, while the SW is characterized by lower anomalies. (B) The mapped Dip-curvature shows a prominent density contrast along the Yangsan fault line. Several density discontinuities along the Yangsan fault line (marked with S1-S5) are anticipated to be the fault segmentations.|@|~(^,^)~|@|(A) Results of the application of the Euler deconvolution technique to the Bouguer anomalies in Fig. 5A get information on the 3D distribution of source points. (B) The Bouguer anomalies along the WE profile varies from about 27.0 mGal to about 29.2 mGal. (D) The distribution of the mean source depth showing the fault core extends to a depth of 2 km. Surface details in (C).|@|~(^,^)~|@|(A) Index map is the same as Fig. 6(A). (B) the Bouguer anomalies along the SSW-NNE profile ranging from about 28.5 mGal to about 29.2 mGal. (C) The mean source depth (black dotted line) shows that the depth of the Yangsan fault core is ranging from 2000 to 800 meters, which can be explained by different depths of fault segments.|@|~(^,^)~|@|The final gravity model along the E-W profile in Naengsuri. The subsurface structure (C) is modeled with iterative modifications by using all available constraints from the interpretation of the gravity field (A), results of electrical resistivity survey (B), and the source depths analysis (B).|@|~(^,^)~|@|(A) Geological map with locations of E-W and SSW-NNE profiles. Final gravity models (C and E) along the E-W and SSW-NNE profiles (locations refer to Fig. A) are shown, which are constrained by the interpretation of the gravity field (B and D) and calculated source depths (Yellow rectangles in Fig. C and E).

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