Current and near future efforts in the Nankai Trough | Dr. Eiichiro Araki

Measuring wide dynamic range behavior of the seismogenic plate margin by optical fiber strain: Current and near future efforts in the Nankai Trough Talk provided by Dr. Eiichiro Araki from JAMSTEC on April 22, 2025 as part of the SZ4D International Webinar Series. For more information, visit www.sz4d.org Full Webinar Description: The dynamics of the Nankai Trough plate boundary zone span a very wide frequency band and dynamic range, from massive, long-interval M8-class earthquakes to shallow low-frequency tremors and shallow slow slip events. To better understand these phenomena, we believe the use of fiber-optic strain measurement is highly effective and have been actively developing relevant technologies, which we introduce here.First, we present borehole observatories in the seafloor. As part of the IODP program, three observatories have been installed in the source region of the Tonankai earthquake. These have enabled time-series observations of shallow slow slip events that occur repeatedly in the area. By expanding this observation network to cover a wider area from the Nankai earthquake source region to the Hyuganada region, our goal is to monitor the temporal progression of slow slip events across the broader Nankai Trough in real time. To achieve this, we have developed a long-term borehole observation system that includes new borehole fiber-optic strainmeter, and in 2023, we installed the first unit and began observations. In addition, we are planning to distribute arrays of seafloor fiber-optic strainmeters. In 2022, we successfully observed a shallow slow slip event that occurred in off Kii-strait. We have since been working on multi-component array observations and seafloor installations near boreholes. By 2025, we plan to deploy an array in the Hyuga-nada region and are continuing with its development. Furthermore, fiber-optic sensing technologies, such as Distributed Acoustic Sensing (DAS), have recently been applied to submarine cables, and we consider them equivalent to densely distributed seafloor fiber-optic strainmeters over a wide area. We began continuous DAS observations in 2021 using a 120 km cable off the coast of Muroto. We are also developing new fiber-optic sensing technologies using this cable, aiming to extend the frequency range of DAS and to detect crustal deformations on the seafloor based on measurement principles different from conventional DAS. Based on the knowledge gained from experiments off the coast of Muroto, we are planning to monitor entire submarine cable systems, such as DONET and N-net, using fiber-optic sensing technologies. Additionally, we have been attempting fiber-optic sensing using optical fibers laid inside boreholes. In 2024, we successfully conducted the first such measurement, and we believe this approach is highly promising for observing processes near the plate boundary zone within the Earth. Finally, data analysis from these observation methods is also a critical challenge. By analyzing fiber-optic DAS data together with seismometer data from DONET in real time, we aim to track the time-series development of crustal phenomena such as low-frequency tremors and earthquakes.