9:15 AM - 9:30 AM
[SSS12-14] Stress field in eastern part of Hokkaido
Keywords:Hokkaido, Stress field, Earthquake, Eastern part of Hokkaido
The Pacific Plate is subducted to Hokkaido (the northern part of the Japan island), and the plate convergence direction is approximately N66°W (e.g., Itoh and Nishimura, 2016). Stress field in eastern Hokkaido estimated by previous studies favors strike-slip faulting (Terakawa and Matsu'ura, 2010; Uchide et al., 2022). The stress field in the Kutcharo caldera favors normal faulting (Terakawa and Matsu'ura, 2010). There are several active and Quaternary volcanoes (e.g., the Atosanupuri volcano, and the Meakan-dake volcano) in eastern Hokkaido (AIST, 2024). Also, several active faults with NE-SW strike are in this region. The Rausu-dake segment is defined as a normal fault and the others are defined as reverse faults (AIST, 2024). Few earthquakes of sufficient magnitude to determine focal mechanism have occurred in eastern Hokkaido. Therefore, few studies discussed the stress field in this region. In this study, we aim to investigate the stress field in eastern Hokkaido. To achieve this, we re-estimated the stress field using currently available focal mechanism data and found spatial stress variations in eastern Hokkaido.
Data and Method
We used moment tensor inversion method (Michael 1984; 1987) to estimate the stress field. F-net moment tensor solutions (1997-2024, Mjma 3.5-, VR > 60) and the focal mechanisms determined by Uchide et al. (2020) (2003-2020, Mjma 0.5-3.1, quality A and B) were used for our estimations. We set the east of the Hidaka Mountains (143.3°E-146.0°E, 43.0°N-44.0°N, depth 0-30 km) for the target region.
Results
Focal mechanisms distribution
64 focal mechanisms were distributed widely in the area, including 29 reverse fault and 25 strike-slip fault types. Reverse fault types were mostly located in the south of Meakan-dake volcano. Normal fault types were in the Kutcharo caldera and the Lake Mashu. 57 % of focal mechanisms showed the P-axis direction N60°W-N80°W, which is approximately consistent with the plate convergence direction. P-axis varied in the Kutcharo caldera and the Lake Mashu.
Stress field
We set an equidistant area (1° latitude × 0.5° longitude) and estimated the stress field for each area. The estimated stress field favors reverse faulting in the western part and strike slip faulting in the eastern part of eastern Hokkaido. The maximum horizontal compressional direction (SHmax) was 118.84° (N61.16°W), consistent with the plate convergence direction. Stress field in an area including the Kutcharo caldera showed large SHmax uncertainty.
The Kutcharo caldera and the Lake Mashu region
Due to the small number of focal mechanisms, the stress field was estimated including focal mechanisms quality C (Uchide catalog; Uchide et al., 2022). Estimated stress field showed intermediate between strike-slip and normal fault type with large uncertainty. We removed focal mechanisms located in this local region from the relevant area and re-estimated the stress field. Re-estimated stress field showed the optimum SHmax 104.93° (N75.07°W) approximately consistent with the plate convergence direction with small uncertainty.
The south of Meakan-dake volcano region
Stress field in this region favors pure reverse faulting with stress ratio approximately 0.5. The SHmax direction is consistent with the plate convergence direction (118.84°; N61.16°W). We removed focal mechanisms located in the south of Meakan-dake volcano region from the relevant area and re-estimated the stress field and re-estimated the stress field. Re-estimated stress field showed intermediate type of reverse and strike-slip fault type (transpression).
Discussions
To verify that the estimated stress field is consistent with the actual stress field, we computed the slip angle (calculated rake) for the estimated stress field. The strike-slip types of stress field reproduced the rake similar to the strike-slip focal mechanisms. Furthermore, this stress field reproduced the rake similar to the normal fault in Lake Mashu. The reverse fault types of stress field reproduced the rakes of reverse fault focal mechanisms located in the south of Meakan-dake volcano.
Conclusions
Transpressive and strike slip stress fields are distributed in the west and east sides of eastern Hokkaido, respectively. We found that stress field favors reverse faulting is locally distributed in south of Meakan-dake volcano. Since the SHmax direction is consistent with the plate convergence direction, the stress field in the south of Meakan-dake volcano is considered to be caused by plate motion. In the region from the Kutcharo caldera to Lake Mashu, P-axis of focal mechanisms were varying. Volcanic activities and topography have possibly affected the stress field.
Data and Method
We used moment tensor inversion method (Michael 1984; 1987) to estimate the stress field. F-net moment tensor solutions (1997-2024, Mjma 3.5-, VR > 60) and the focal mechanisms determined by Uchide et al. (2020) (2003-2020, Mjma 0.5-3.1, quality A and B) were used for our estimations. We set the east of the Hidaka Mountains (143.3°E-146.0°E, 43.0°N-44.0°N, depth 0-30 km) for the target region.
Results
Focal mechanisms distribution
64 focal mechanisms were distributed widely in the area, including 29 reverse fault and 25 strike-slip fault types. Reverse fault types were mostly located in the south of Meakan-dake volcano. Normal fault types were in the Kutcharo caldera and the Lake Mashu. 57 % of focal mechanisms showed the P-axis direction N60°W-N80°W, which is approximately consistent with the plate convergence direction. P-axis varied in the Kutcharo caldera and the Lake Mashu.
Stress field
We set an equidistant area (1° latitude × 0.5° longitude) and estimated the stress field for each area. The estimated stress field favors reverse faulting in the western part and strike slip faulting in the eastern part of eastern Hokkaido. The maximum horizontal compressional direction (SHmax) was 118.84° (N61.16°W), consistent with the plate convergence direction. Stress field in an area including the Kutcharo caldera showed large SHmax uncertainty.
The Kutcharo caldera and the Lake Mashu region
Due to the small number of focal mechanisms, the stress field was estimated including focal mechanisms quality C (Uchide catalog; Uchide et al., 2022). Estimated stress field showed intermediate between strike-slip and normal fault type with large uncertainty. We removed focal mechanisms located in this local region from the relevant area and re-estimated the stress field. Re-estimated stress field showed the optimum SHmax 104.93° (N75.07°W) approximately consistent with the plate convergence direction with small uncertainty.
The south of Meakan-dake volcano region
Stress field in this region favors pure reverse faulting with stress ratio approximately 0.5. The SHmax direction is consistent with the plate convergence direction (118.84°; N61.16°W). We removed focal mechanisms located in the south of Meakan-dake volcano region from the relevant area and re-estimated the stress field and re-estimated the stress field. Re-estimated stress field showed intermediate type of reverse and strike-slip fault type (transpression).
Discussions
To verify that the estimated stress field is consistent with the actual stress field, we computed the slip angle (calculated rake) for the estimated stress field. The strike-slip types of stress field reproduced the rake similar to the strike-slip focal mechanisms. Furthermore, this stress field reproduced the rake similar to the normal fault in Lake Mashu. The reverse fault types of stress field reproduced the rakes of reverse fault focal mechanisms located in the south of Meakan-dake volcano.
Conclusions
Transpressive and strike slip stress fields are distributed in the west and east sides of eastern Hokkaido, respectively. We found that stress field favors reverse faulting is locally distributed in south of Meakan-dake volcano. Since the SHmax direction is consistent with the plate convergence direction, the stress field in the south of Meakan-dake volcano is considered to be caused by plate motion. In the region from the Kutcharo caldera to Lake Mashu, P-axis of focal mechanisms were varying. Volcanic activities and topography have possibly affected the stress field.