3:30 PM - 3:45 PM
[SCG55-30] Initiation of GNSS-Acoustic seafloor geodetic observation along the Japan trench off Boso
Keywords:GNSS-Acoustic observation, Tsunami earthquake, Seafloor geodetic observation, Off Boso
Along the Japan Trench off Boso, the 1677 Enpo Boso tsunami earthquake is known to have occurred. Based on the tsunami deposit studies, its coseismic slip has been estimated up to 16 m at the very shallow portion on the plate interface (Yanagisawa et al., 2016). However, many aspects of this earthquake have remained unclear: uncertainty of the spatial extent of the source region, the potential involvement of a subducting seamount in strain accumulation for a tsunami earthquake (Lee et al., 2023), the possibility of short-term slow slip events (SSEs) occurring within the main rupture area (Nishimura, 2020), and the lack of information on the tsunami earthquakes prior to 1677 (Higaki et al., 2021). The main rupture area estimated by Yanagisawa et al. (2016) is currently a seismic gap exhibiting no significant seismic activity. Additionally, the main rupture area is also a gap of geodetic observational networks. Although the Japan Coast Guard has a GNSS-Acoustic (GNSS-A) observation site, CHOS, located at the landward side of the main rupture area, no geodetic observation that has high sensitivity to the strain accumulation near the trench. Consequently, the current interplate coupling condition along the Japan Trench off Boso, where the 1677 Enpo Boso tsunami earthquake is thought to have occurred, remains unknown.
We launched a joint project of Tohoku University and Tokyo University of Marine Science and Technology to clarify the current interplate coupling condition along the Japan Trench off Boso. In October 2024, we installed two GNSS-A observation sites, namely G26 and G27, on the seafloor directly above the main rupture area and started to observe there.
The G26 site (142.04689°E, 35.72755°N) is located directly above the trailing flank of a subducted seamount (Mochizuki et al., 2008). While the trailing flank is prone to cause aseismic slips due to the fluid-rich condition (Sun et al., 2020), the frontal flank of the seamount, subjected to high normal stress, may bring locally strong interplate locking that could trigger tsunami earthquakes over long timescales (Lee et al., 2023). G26 was established to also evaluate the influence of the subducting seamount. The G27 site (141.899412°E, 34.94610°N) is located above the center of the main rupture area. In this region, Nishimura (2020) reported the occurrence of the short-term SSEs although the accuracy of their location is not particularly high. We aim to utilize G27 to evaluate the coupling condition, whether it indicates strong interplate coupling that could lead to a future tsunami earthquake or weak interplate coupling due to the repeated occurrence of the short-term SSEs.
The installment of the two GNSS-A sites and the first campaign surveys were conducted using the training vessel, Shioji-maru. Each GNSS-A site is composed of totally four seafloor acoustic transponders: three arranged in a triangle configuration and one at the center of the triangle. We collected observational data correctly from all four transponders at G26, while one of the seafloor transponders at G27 failed to respond. Therefore, an alternative seafloor transponder will be installed at G27 during 2025, and practical surveys at G27 will commence after the additional installation. We plan to conduct campaign GNSS-A surveys once a year over the next 5 years and estimate their current displacement rates.
This presentation will provide a detailed report on the project itself, the installation of the observation sites, and the data status of the initial campaign observation.
We launched a joint project of Tohoku University and Tokyo University of Marine Science and Technology to clarify the current interplate coupling condition along the Japan Trench off Boso. In October 2024, we installed two GNSS-A observation sites, namely G26 and G27, on the seafloor directly above the main rupture area and started to observe there.
The G26 site (142.04689°E, 35.72755°N) is located directly above the trailing flank of a subducted seamount (Mochizuki et al., 2008). While the trailing flank is prone to cause aseismic slips due to the fluid-rich condition (Sun et al., 2020), the frontal flank of the seamount, subjected to high normal stress, may bring locally strong interplate locking that could trigger tsunami earthquakes over long timescales (Lee et al., 2023). G26 was established to also evaluate the influence of the subducting seamount. The G27 site (141.899412°E, 34.94610°N) is located above the center of the main rupture area. In this region, Nishimura (2020) reported the occurrence of the short-term SSEs although the accuracy of their location is not particularly high. We aim to utilize G27 to evaluate the coupling condition, whether it indicates strong interplate coupling that could lead to a future tsunami earthquake or weak interplate coupling due to the repeated occurrence of the short-term SSEs.
The installment of the two GNSS-A sites and the first campaign surveys were conducted using the training vessel, Shioji-maru. Each GNSS-A site is composed of totally four seafloor acoustic transponders: three arranged in a triangle configuration and one at the center of the triangle. We collected observational data correctly from all four transponders at G26, while one of the seafloor transponders at G27 failed to respond. Therefore, an alternative seafloor transponder will be installed at G27 during 2025, and practical surveys at G27 will commence after the additional installation. We plan to conduct campaign GNSS-A surveys once a year over the next 5 years and estimate their current displacement rates.
This presentation will provide a detailed report on the project itself, the installation of the observation sites, and the data status of the initial campaign observation.