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[SCG52-P15] Gravity anomaly around the Lake Ohara fault in the Chugoku region
Introduction
The Lake Ohara fault is a right-lateral active fault that extends northeast-southwest from southwestern Shimane Prefecture to central Yamaguchi Prefecture (e.g., Mizuno et al., 2003), and is about 42 km long (HERP, 2016). Furthermore, the Ogoori Fault, which is about 31 km long, is located southwest of the Lake Ohara fault. However, it is unclear whether it constitutes a series of fault zones with the Lake Ohara Fault (HERP, 2016). In addition, the Jifuku fault is distributed in a northeast-southwest direction to the northwest of the Lake Ohara fault. We report here the characteristics of gravity anomaly around the Lake Ohara Fault based on the gravity anomaly distribution.
Data and Method
We conducted a total of 51 gravity measurements in the area around the Lake Ohara fault from September 10 to 12, 2021. We used a Scintrex CG-3M gravimeter owned by Kanazawa University. The latitude, longitude, and elevation of the measurement points were determined by GNSS. In addition to the existing gravity data from Kanazawa University, we used the gravity data from Geospatial Information Authority of Japan (2006), Southwest Japan Gravity Group (2010), Yamamoto et al. (2011), and Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (2013) for the analysis. The assumed density used for the topographic and Bouguer corrections was 2300 kg/m3, referring to the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (2006). In addition to the usual corrections, the gravity data were topographically corrected by 10 m DEM, low-pass filtered, and a plain trend removed. To detect the characteristics of the fault structure against the gravity anomaly, we calculated the directional derivative (in the direction of the fault strike and perpendicular to the strike), horizontal first derivative, and vertical first derivative.
Results and Discussion
In general, density structure boundaries in the subsurface are highlighted as areas of high horizontal first derivative values and zero vertical first derivative lines. However, no such features were found along the Lake Ohara fault. This fact suggests that the vertical displacement at the basement beneath the Lake Ohara fault is small, although the sparse distribution of gravity measurement points along the Lake Ohara fault may have caused the features. In the central part of the Lake Ohara fault, the spatial pattern of the horizontal first derivative appears to reflect right lateral displacement, which is consistent with the fact that the Lake Ohara fault is a right-lateral fault. The Lake Ohara fault and the Jifuku fault are located in a region of low gravity anomaly and low vertical first derivative, suggesting a relationship between the regional basement structure and the fault length.
Acknowledgments: We used the gravity data from GSJ (), Yamamoto et al. (2011), and Geological Survey of Japan, AIST (2013).
The Lake Ohara fault is a right-lateral active fault that extends northeast-southwest from southwestern Shimane Prefecture to central Yamaguchi Prefecture (e.g., Mizuno et al., 2003), and is about 42 km long (HERP, 2016). Furthermore, the Ogoori Fault, which is about 31 km long, is located southwest of the Lake Ohara fault. However, it is unclear whether it constitutes a series of fault zones with the Lake Ohara Fault (HERP, 2016). In addition, the Jifuku fault is distributed in a northeast-southwest direction to the northwest of the Lake Ohara fault. We report here the characteristics of gravity anomaly around the Lake Ohara Fault based on the gravity anomaly distribution.
Data and Method
We conducted a total of 51 gravity measurements in the area around the Lake Ohara fault from September 10 to 12, 2021. We used a Scintrex CG-3M gravimeter owned by Kanazawa University. The latitude, longitude, and elevation of the measurement points were determined by GNSS. In addition to the existing gravity data from Kanazawa University, we used the gravity data from Geospatial Information Authority of Japan (2006), Southwest Japan Gravity Group (2010), Yamamoto et al. (2011), and Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (2013) for the analysis. The assumed density used for the topographic and Bouguer corrections was 2300 kg/m3, referring to the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (2006). In addition to the usual corrections, the gravity data were topographically corrected by 10 m DEM, low-pass filtered, and a plain trend removed. To detect the characteristics of the fault structure against the gravity anomaly, we calculated the directional derivative (in the direction of the fault strike and perpendicular to the strike), horizontal first derivative, and vertical first derivative.
Results and Discussion
In general, density structure boundaries in the subsurface are highlighted as areas of high horizontal first derivative values and zero vertical first derivative lines. However, no such features were found along the Lake Ohara fault. This fact suggests that the vertical displacement at the basement beneath the Lake Ohara fault is small, although the sparse distribution of gravity measurement points along the Lake Ohara fault may have caused the features. In the central part of the Lake Ohara fault, the spatial pattern of the horizontal first derivative appears to reflect right lateral displacement, which is consistent with the fact that the Lake Ohara fault is a right-lateral fault. The Lake Ohara fault and the Jifuku fault are located in a region of low gravity anomaly and low vertical first derivative, suggesting a relationship between the regional basement structure and the fault length.
Acknowledgments: We used the gravity data from GSJ (), Yamamoto et al. (2011), and Geological Survey of Japan, AIST (2013).