2025-07-30 北海道大学
ハの字型構造をなす2断層系における、断層のずれる向きと加わる力の関係（論文中の図9を改変）。各図の右側の断層で（a）逆断層型、（b）逆断層+左横ずれ型、（c）左横ずれ型の地震が起きた場合に、断層に加わる力の変化を示す。赤い領域は断層をずれやすくする力が、青い領域はずれにくくする力がかかっていることを意味する。
＜関連情報＞
- https://www.hokudai.ac.jp/news/2025/07/post-1999.html
- https://www.hokudai.ac.jp/news/pdf/250730_pr3.pdf
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JB031225
ダブルバージェンス構造における強い断層間相互作用：台湾東部で発生した2022年玉里地震及び2022年池上地震からの教訓 Strong Fault Interaction in Double-Vergence Structure: Lessons From the 2022 Yuli Earthquake and the 2022 Chihshang Earthquake, Eastern Taiwan
Yuri Ishimaru, Youichiro Takada, Kuo-En Ching, Wu-Lung Chang
Journal of Geophysical Research: Solid Earth Published: 10 July 2025
DOI:https://doi.org/10.1029/2025JB031225
Abstract
The Longitudinal Valley Fault (LVF) and the Central Range Fault (CRF) in eastern Taiwan consist of a head-to-head double-vergence structure hosting disastrous earthquakes. It was previously proposed that the fault slip on one of these faults suppresses the earthquake generation on the other. Nonetheless, the 2022 Chihshang earthquake (Mw 7.0) on the CRF occurred soon after the 2022 Yuli earthquake (Mw 6.7) on the LVF. Here, we provide a comprehensive framework of the fault interaction consistently explaining these contradictory findings. First, we estimated the coseismic slip distribution of the Yuli earthquake from Global Navigation Satellite System (GNSS) and L-band satellite interferograms. The results indicate almost pure reverse faulting. Second, with the estimated slip distribution, we calculated changes in Coulomb failure function (ΔCFF) on the CRF due to the Yuli earthquake on the LVF. The ΔCFF reaches +0.25 MPa around the main rupture area of the Chihshang earthquake, which is equivalent to the clock advance of 36–100 years, suggesting a large impact on the earthquake generation cycles. Finally, we found that a rake angle of fault slip has a significant effect on the ΔCFF on the other fault in the double-vergence structure: it takes large positive values when 90° like the Yuli earthquake, but almost negative when 45° or less, which comprehensively explains the seismic quiescence previously reported and the positive ΔCFF on the CRF caused by the Yuli earthquake. The strong impact of the rake angle is also supported by the temporal distribution of historical earthquakes in eastern Taiwan.
Key Points
- Almost pure dip-slip of the Yuli earthquake on the east-dipping Longitudinal Valley Fault in eastern Taiwan is estimated by InSAR and GNSS
- The Yuli earthquake increased Coulomb stress in main rupture area of the Chihshang earthquake on the west-dipping Central Range Fault
- Dip-slip earthquakes on one fault promote earthquakes on the other fault while strike-slips suppress those in double-vergence fault system
Plain Language Summary
The Longitudinal Valley Fault (LVF) and the Central Range Fault (CRF) in eastern Taiwan are hosts of disastrous earthquakes. These are closely located and dip in opposite directions (inverted V-shaped). It was previously proposed that earthquakes on one of these faults suppress earthquakes on the other. Nonetheless, the 2022 Chihshang earthquake (Mw 7.0) on the CRF occurred soon after the 2022 Yuli earthquake (Mw 6.7) on the LVF. We comprehensively understand these contradictory findings by focusing on the fault slip directions. First, we revealed almost pure reverse faulting of the LVF during the Yuli earthquake from geodetic observations. According to our simulation, the Yuli earthquake imparted the stress equivalent to a steady stress accumulation of 36–100 years around the main rupture area of the Chihshang earthquake on the CRF, suggesting a large impact on the earthquake generation. Finally, we demonstrated the importance of slip directions on the fault interaction within the inverted V-shaped fault systems: reverse slip prompts earthquakes on the other fault while lateral slip suppresses. The effect of the slip direction consistently explains both the previously suggested seismic quiescence and the promotion of the Chihshang earthquake by the Yuli earthquake. This mechanism is also consistent with historical seismicity.
