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[SSS11-12] Regional Stress Field in the Central-north Kinki District Investigated by Dense Seismic Observation
Keywords:Kinki district, fault, stress field, σ1, rotation, depth
In the central-north Kinki district, active fault zones such as the Arima-Takatsuki, the Mitoke/Kyoto-Nishiyama, the Hanaore, and the Lake Biwa West Coast are distributed, and this district is one of the major areas in Japan where active faults are concentrated. Moreover, microseismic activity has been continuing for a long period and over a wide range in the Hokusetsu and Tanba areas, north of the Arima-Takatsuki fault zone. This seismicity is different from the aftershock of a major earthquake. The central-north Kinki district is part of the southwestern edge of the Niigata-Kobe Tectonic Zone, so it is an important place to consider tectonics in a wide area.
The stress field in this district has already been analyzed by Fujino and Katao (2009), Aoki et al. (2012) and Iio (2021), and many results have been obtained. The purpose of this study is to clarify the stress field in more detail by focusing on the vertical variation of the stress field, which has been difficult to analyze so far.
Analysis area and research methods are the same as Tanaka et al. (2022). The coordinate origin is the epicenter of the 2018 Osaka earthquake (June 18, 2018 Mj6.1). The analysis area is a rectangular region of 40 km each to the east and west, 70 km to the north, and 30 km to the south. The stress field was analyzed using a large number of seismic data obtained from the dense seismic observation network named the Manten system (Miura et al., 2010) and the stationary network in this area. The mechanism solution was obtained from the earthquake waveforms that occurred between November 17, 2008 and March 29, 2018. The stress field was estimated by stress tensor inversion. In stress tensor inversion, we set the grid size to be 10 km for both E-W and N-S, and 2.5km deep. The depths in the previous studies were about 10 km or 5 km.
We were able to obtain detailed stress fields. The orientation of the surface displacement near the main part of the Arima-Takatsuki fault zone is N80ºE and that of the Mitoke fault zone is N110ºE. Tanaka et al. (2022) revealed that σ1 directions in these fault zones are N95-110ºE and N65-75ºE, respectively. In both regions, the rotation of σ1 is toward an orientation that could cause lateral sliding motion.
Aoki et al. (2012) described the characteristics of the Tanba area as follows: "The west coast of Lake Biwa is under a reverse fault type stress field, while the Tanba area is dominated by regions where σ2 and σ3 are not clearly distinguishable and cannot be taken as either a reverse fault or a strike-slip fault." The results of this study are similar, but in some places the stress field near the surface is the strike-slip fault type, while it tends to change to the reverse fault type as it gets deeper.
As pointed out by Aoki et al. (2012), there is a tendency for the reverse fault type stress field to prevail around the Hanaore fault zone and the Lake Biwa West Coast fault zone. However, the change toward the west is not uniform, and there are places where both the fault type and the σ1 direction change with depth.
Within the entire analysis area, there are places where the stress field changes with depth, while in other places the fault type and the σ1 direction do not change significantly with depth. These results suggest that this district is under a variety of stress fields, even though the surface area is only 80 x 100 km2.
For each location, we intend to continue our research on the dynamics that cause changes in the stress field, taking into account geological characteristics and the structures of the lower and upper crust.
The stress field in this district has already been analyzed by Fujino and Katao (2009), Aoki et al. (2012) and Iio (2021), and many results have been obtained. The purpose of this study is to clarify the stress field in more detail by focusing on the vertical variation of the stress field, which has been difficult to analyze so far.
Analysis area and research methods are the same as Tanaka et al. (2022). The coordinate origin is the epicenter of the 2018 Osaka earthquake (June 18, 2018 Mj6.1). The analysis area is a rectangular region of 40 km each to the east and west, 70 km to the north, and 30 km to the south. The stress field was analyzed using a large number of seismic data obtained from the dense seismic observation network named the Manten system (Miura et al., 2010) and the stationary network in this area. The mechanism solution was obtained from the earthquake waveforms that occurred between November 17, 2008 and March 29, 2018. The stress field was estimated by stress tensor inversion. In stress tensor inversion, we set the grid size to be 10 km for both E-W and N-S, and 2.5km deep. The depths in the previous studies were about 10 km or 5 km.
We were able to obtain detailed stress fields. The orientation of the surface displacement near the main part of the Arima-Takatsuki fault zone is N80ºE and that of the Mitoke fault zone is N110ºE. Tanaka et al. (2022) revealed that σ1 directions in these fault zones are N95-110ºE and N65-75ºE, respectively. In both regions, the rotation of σ1 is toward an orientation that could cause lateral sliding motion.
Aoki et al. (2012) described the characteristics of the Tanba area as follows: "The west coast of Lake Biwa is under a reverse fault type stress field, while the Tanba area is dominated by regions where σ2 and σ3 are not clearly distinguishable and cannot be taken as either a reverse fault or a strike-slip fault." The results of this study are similar, but in some places the stress field near the surface is the strike-slip fault type, while it tends to change to the reverse fault type as it gets deeper.
As pointed out by Aoki et al. (2012), there is a tendency for the reverse fault type stress field to prevail around the Hanaore fault zone and the Lake Biwa West Coast fault zone. However, the change toward the west is not uniform, and there are places where both the fault type and the σ1 direction change with depth.
Within the entire analysis area, there are places where the stress field changes with depth, while in other places the fault type and the σ1 direction do not change significantly with depth. These results suggest that this district is under a variety of stress fields, even though the surface area is only 80 x 100 km2.
For each location, we intend to continue our research on the dynamics that cause changes in the stress field, taking into account geological characteristics and the structures of the lower and upper crust.