Tsunami Propagation Observed via Seafloor Fiber Optics ― A Case Study of Tsunami Near Torishima in October 2023

1. Key Points

• Distributed acoustic sensing (DAS)^※1, which measures the strain on optical fibers installed on the seafloor, has enabled earthquakes to be observed along fiber optic cable transects, in contrast to the conventional observations using ocean bottom instruments. DAS observations were conducted on seafloor fiber optic cables offshore of Muroto, Japan to observe slow earthquakes^※2 in the Nankai Trough region.

• On October 9, 2023, high-frequency tsunamis with epicenters near Torishima occurred, causing changes in tidal levels across southern Japan. Our DAS observation offshore of Muroto successfully captured the propagation of these tsunamis from approximately 60 km offshore to the coast.

• A method of quantitatively evaluating the factors underlying optical fiber distortion during tsunami propagations was developed to calculate tsunami wave heights from strain data. Tsunami wave heights may be estimated from seafloor optical cable data in the future, and the use of optical cables with DAS observations should facilitate both high-density earthquake and wide-area tsunami observations, thereby contributing to coastal disaster prevention measures.

2. Overview

High-frequency tsunamis occurred on October 9, 2023, near Torishima, Japan. By applying DAS technology to seafloor fiber optic cables installed offshore of Muroto, scientists from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Research Institute for Marine Geodynamics — Takashi Tonegawa of the Subduction Dynamics Research Center and Eiichiro Araki of the Research and Development Center for Earthquake and Tsunami Forecasting — successfully captured the propagation of these tsunamis along the fiber optic cable from approximately 60 km offshore to the coast. The tsunami epicenter was identified as Torishima. DAS observation data as well as data from a water pressure gauge installed near the optical cables were separately used to estimate the time series (i.e., propagation) of tsunami generation near the epicenter, thereby independently confirming the validity of the DAS results. These findings suggest that the time series of tsunami generation could be estimated using only DAS observation records, even in circumstances where there are no other geophysical instruments near seafloor optical cables. DAS also measures the strain on optical fibers, and the JAMSTEC scientists were able to use DAS data to determine the causes of strain during tsunami propagation. In the future, it may be possible to directly convert strain data from DAS observations into tsunami wave heights.

The results of the study summarized here will be published in Geophysical Research Letters on June 6, 2024 (JST). The research was supported by the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (KAKENHI) (A) “Slow-to-Fast Seismology,” grants JP21H05202 and JP21H05204.