[S-CG40] Science of slow-to-fast earthquakes

[SCG40-06] Toward a comprehensive understanding of shallow slow earthquakes: a review of key studies along the Nankai Trough and future perspectives

★Invited Papers

*Shunsuke Takemura¹, Yohei Hamada², Hanaya Okuda², Yutaro Okada³, Kurama Okubo⁴, Takeshi Akuhara¹, Akemi Noda⁵, Takashi Tonegawa² (1.Earthquake Research Institute, the University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Kyoto University, 4.National Research Institute for Earth Science and Disaster Resilience, 5.Meteorological Research Institute)

Keywords:Slow earthquake, Nankai Trough, Shallow plate boundary

Because of recent developments in offshore instruments, shallow slow earthquakes have been observed in offshore regions of Ryukyu, Nankai, Mexico, Costa Rica, and Hikurangi subduction zones. Along-strike/dip separations of shallow slow earthquakes and frictionally locked zones were identified in some subduction zones (e.g., Nishikawa et al., 2019; Baba et al., 2020, 2021; Plata-Martinez et al., 2021). In the Nankai subduction zone, decade-scale catalogs of shallow slow earthquakes and extensive studies around the Nankai Trough exist. We briefly reviewed seismological, geodetical, geological, and experimental results in shallow slow earthquake areas along the Nankai Trough for a unified understanding of shallow slow earthquakes. Details were described in our review paper (Takemura Hamda et al., 2023 EPS). Despite a belt-like distribution of deep slow earthquakes, shallow slow earthquakes occur in three specific spots: Hyuga-nada, off Cape Muroto, and southeast off the Kii Peninsula. These regions are also characterized by transitional regions between frictionally locked and stable sliding regions (e.g., Takemura, Noda et al., 2019). However, the frictional properties expected from experiments under temperature-pressure-mineral conditions of shallow plate boundary exhibit frictionally stable, i.e., positive (a–b) values in most cases (e.g., Okuda et al., 2023). In addition, although lithology from drilling core samples is not perfectly uniform along the Nankai Trough, other factors should be required for occurrences of shallow slow earthquakes. Seismic structural surveys imaged low V_S and high V_P/V_S volumes within shallow slow earthquake zones (e.g., Kitajima & Saffer 2012; Akuhara et al., 2020; Tonegawa et al., 2017). These volumes could be interpreted as high pore-fluid pressure conditions. High-pore fluid conditions could temporally fluctuate during shallow slow earthquake episodes (Tonegawa et al., 2022). Thus, we consider that episodic pore-fluid migration around regions with high pore-fluid pressure causes local pore pressure build-up and then promotes occurrences of shallow slow earthquakes.
A modeling study also demonstrated slow earthquakes in shallow depths with a velocity-strengthening system through pore pressure variations (Perez-Silva et al., 2023). However, the detailed mechanism of shallow slow earthquake occurrence within the volumes with high-pore fluid pressure is still an open question. Continuous monitoring of fluid migration around plate boundaries is critical for a more quantitative understanding of shallow slow earthquake mechanisms in the future. The relationships between megathrust and shallow slow earthquakes are also still unclear. Integration of greater numbers of seismological, geodetic, geological, and experimental studies will be indispensable to address these issues.