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[HDS12-P04] Redetermination of hypocenters along the Enasan-Sanageyama North Fault using the Double-Difference Earthquake Location Algorithm
Keywords:Enasan-Sanageyama North Fault zone, Double-Difference Earthquake Location, Fault structure
According to the long-term assessment of the Byobuyama-Enasan Fault Zone and the Sanageyama North Fault Zone (The Headquarters for Earthquake Research Promotion, 2021), the magnitude of an anticipated earthquake along the Enasan-Sanageyama North Fault Zone is estimated at about 7.7 when the entire fault zone simultaneously ruptures, and the 30-year probability is estimated to be 0-2%. The population exposure to seismic intensity 6 lower is estimated to be about 6.7 million, and the anticipated earthquake along this fault zone carry the risk of significant damage to social and economic activities. Therefore, it is necessary to urgently revise the assessment of earthquakes and strong ground motions caused by earthquakes along this fault zone.
The Enasan-Sanageyama North Fault Zone is a complex zone where several faults with different displacement and dip directions are connected. There is a possibility that these faults have separately ruptured, and it is necessary to perform the additional investigations to improve the reliability of the activity history in this fault zone. In addition, it is necessary to clarify the distribution and shape of faults at depth, and the subsurface seismic structure in the region for the assessment of strong ground motions.
On the basis of the above backgrounds, this study reports the results of more accurate relative hypocenters by the Double-Difference (DD) method (Waldhauser and Ellsworth, 2000) to clarify the distribution of faults at depth. The target regions are: 1) near the junction of the Sanageyama North Fault and the Enasan Fault (Region-1), and 2) the area of fault step on the Enasan Fault (Region-2). We selected 516 earthquakes of M0.2 or greater that occurred at depths of 20 km or less under the two regions from June 3, 2002 to December 31, 2018 referring the JMA unified catalog. The arrival time of seismic waves are manually picked at all the seismic stations prior to the DD analysis to minimize the error in reading the arrival time of seismic waves. The criteria of the selected earthquakes analyzed by the DD method were as follows: 1) seismic wave readings were available at eight or more stations, 2) the distance from the hypocenter to the stations was limited to 60 km, and 3) the distance between pairs of adjacent sources was limited to 10 km. At Region-1, the number of earthquakes to be relocated and seismic stations were 436 and 36, respectively. The numbers of P- and S-wave pairs were 369745 and 409868 (779613 in total). At Region-2, the number of earthquakes to be relocated and seismic stations were 76 and 36, respectively. The number of P- and S-wave pairs was 15226 and 16393 (31619 in total).
The result of DD analysis shows six fine plane structures of earthquakes in the NE-SW orientation at depths ranging from 10 to15 km beneath the two regions. Among these structures, the structure with the M4.4 earthquake does not exist on the Sanageyama North Fault; and the structure may be related to another fault pointed out on the geological maps, 1:200,000 Toyohashi and Irago Misaki (Makimoto et al., 2004) and the 1:50,000 Akechi (Yamazaki et al., 2020). The earthquake group located at the area of Enasan stepover has the same strike and dip as those of the Enasan Fault. The conjugate structure is found beneath the Enasan Fault. These findings are expected to be utilized in the improvement of the strong motion assessment.
Acknowledgements: This study is a part of Integrated Research for Byobuyama/Enasan Fault Zone and Sanageyama Fault Zone (Enasan-Sanageyama-Kita Fault Zone), MEXT commissioned project in FY2020. We used seismic waveforms recorded by Hi-net (doi:10.17598/NIED.0003), National Research Institute for Earth Science and Disaster Resilience, Japan Meteorological Agency, Nagoya Univresity, Disaster Prevention Research Institute, Kyoto University, and National Institute of Advanced Industrial Science and Technology.
The Enasan-Sanageyama North Fault Zone is a complex zone where several faults with different displacement and dip directions are connected. There is a possibility that these faults have separately ruptured, and it is necessary to perform the additional investigations to improve the reliability of the activity history in this fault zone. In addition, it is necessary to clarify the distribution and shape of faults at depth, and the subsurface seismic structure in the region for the assessment of strong ground motions.
On the basis of the above backgrounds, this study reports the results of more accurate relative hypocenters by the Double-Difference (DD) method (Waldhauser and Ellsworth, 2000) to clarify the distribution of faults at depth. The target regions are: 1) near the junction of the Sanageyama North Fault and the Enasan Fault (Region-1), and 2) the area of fault step on the Enasan Fault (Region-2). We selected 516 earthquakes of M0.2 or greater that occurred at depths of 20 km or less under the two regions from June 3, 2002 to December 31, 2018 referring the JMA unified catalog. The arrival time of seismic waves are manually picked at all the seismic stations prior to the DD analysis to minimize the error in reading the arrival time of seismic waves. The criteria of the selected earthquakes analyzed by the DD method were as follows: 1) seismic wave readings were available at eight or more stations, 2) the distance from the hypocenter to the stations was limited to 60 km, and 3) the distance between pairs of adjacent sources was limited to 10 km. At Region-1, the number of earthquakes to be relocated and seismic stations were 436 and 36, respectively. The numbers of P- and S-wave pairs were 369745 and 409868 (779613 in total). At Region-2, the number of earthquakes to be relocated and seismic stations were 76 and 36, respectively. The number of P- and S-wave pairs was 15226 and 16393 (31619 in total).
The result of DD analysis shows six fine plane structures of earthquakes in the NE-SW orientation at depths ranging from 10 to15 km beneath the two regions. Among these structures, the structure with the M4.4 earthquake does not exist on the Sanageyama North Fault; and the structure may be related to another fault pointed out on the geological maps, 1:200,000 Toyohashi and Irago Misaki (Makimoto et al., 2004) and the 1:50,000 Akechi (Yamazaki et al., 2020). The earthquake group located at the area of Enasan stepover has the same strike and dip as those of the Enasan Fault. The conjugate structure is found beneath the Enasan Fault. These findings are expected to be utilized in the improvement of the strong motion assessment.
Acknowledgements: This study is a part of Integrated Research for Byobuyama/Enasan Fault Zone and Sanageyama Fault Zone (Enasan-Sanageyama-Kita Fault Zone), MEXT commissioned project in FY2020. We used seismic waveforms recorded by Hi-net (doi:10.17598/NIED.0003), National Research Institute for Earth Science and Disaster Resilience, Japan Meteorological Agency, Nagoya Univresity, Disaster Prevention Research Institute, Kyoto University, and National Institute of Advanced Industrial Science and Technology.