11:15 〜 11:30
[SCG45-45] Temporal variation in seismic velocity at the Nankai accretionary prism: Implication to fluids, stress field and plate coupling
キーワード：地震波速度、時間変化、流体、応力場、プレートカップリング
In the Nankai subduction zone, a permanent cabled network of ocean bottom seismometers has been deployed since 2010 (Dense Oceanfloor Network system for Earthquakes and Tsunamis: DONET). Using ambient noise records captured by the DONET, previous studies have attempted to monitor temporal variations in seismic velocity at the accretionary prism (Ikeda and Tsuji, 2018; Tonegawa et al. 2022). In particular, Tonegawa et al. (2022) found that the seismic velocity gradually increases over 10 years at the accretionary prism toe, and interpreted that such increases are caused by fluid drainages due to the horizontal compression of the sediments. Because the previous study investigated the temporal variation in seismic velocity until the beginning of 2021, I extended the analyzed period until November 2024 in this study.
As a result, the velocity increases that are observed at the prism toe have stopped since 2022, and some station pairs may show gradual velocity decreases. For other regions, the seismic velocities in the Kumano Basin (node KMA) and near Muroto south of Shikoku (node MRA) increased, while those in accretionary prism toe off Shikoku and Kii Peninsula do not vary significantly. If I compared this pattern to the plate coupling distribution (e.g., Takemura et al. 2022), the distributions of the velocity increase and large shear stress changes are partly coincided each other. Also, referring to the distributions of large shear stress changes, I estimated the areal changes (compression or extension) at the seafloor using Okada model (Okada, 1985). The results show the compressional field at the seafloor above the strong coupling, indicating that the plate coupling distribution potentially influence the temporal variation in seismic velocity of the accretionary prism through both the fluid amount affected by the stress field.
Acknowledgement
This study was supported by JSPS KAKENHI Grant No. JP21H05202 in Scientific Research on Transformative Research Areas “Science of Slow-to-Fast earthquakes”. We used records from DONET (DOI:10.17598/NIED.0008) which is operated by National Research Institute for Earth Science and Disaster Resilience (NIED).
As a result, the velocity increases that are observed at the prism toe have stopped since 2022, and some station pairs may show gradual velocity decreases. For other regions, the seismic velocities in the Kumano Basin (node KMA) and near Muroto south of Shikoku (node MRA) increased, while those in accretionary prism toe off Shikoku and Kii Peninsula do not vary significantly. If I compared this pattern to the plate coupling distribution (e.g., Takemura et al. 2022), the distributions of the velocity increase and large shear stress changes are partly coincided each other. Also, referring to the distributions of large shear stress changes, I estimated the areal changes (compression or extension) at the seafloor using Okada model (Okada, 1985). The results show the compressional field at the seafloor above the strong coupling, indicating that the plate coupling distribution potentially influence the temporal variation in seismic velocity of the accretionary prism through both the fluid amount affected by the stress field.
Acknowledgement
This study was supported by JSPS KAKENHI Grant No. JP21H05202 in Scientific Research on Transformative Research Areas “Science of Slow-to-Fast earthquakes”. We used records from DONET (DOI:10.17598/NIED.0008) which is operated by National Research Institute for Earth Science and Disaster Resilience (NIED).