Summary
The Nankai Trough seafloor cable seismic observation network DONET is used for earthquake early warning and tsunami prediction. A thermometer is attached to each of these ocean bottom seismometers to measure the temperature near the seafloor. In this study, by investigating long-term ocean bottom temperature data, a research group of Yusuke Yokota (Associate Professor at Institute of Industrial Science, The University of Tokyo), Tomohiro Inoue and Tatsuya Kubota (National Research Institute for Earth Science and Disaster Resilience) detected for the first time ocean waves fluctuating synchronously over an area of more than 100 km along the Nankai Trough. In oceanography, this may be a "Coastal Trapped Wave (CTW)", and this is a rare observation case that shows the possibility of new oceanographic observations using the ocean bottom seismic observation network. The temperature change is very small, a phenomenon that cannot be detected by ocean bottom pressure gauges, and the signal is of a size that does not affect earthquake observations or slow slip observations in the Nankai Trough In the future, if more accurate slow slip detection is implemented, it may become noise.

Background of the research
Changes in seafloor temperature not only affect measurements such as seafloor crustal deformation and seafloor pressure but also reflect oceanographic phenomena. Although the DONET seafloor thermometers have accumulated a large amount of data over a long period of time, they have not been investigated in detail until now.

Description of research
When a research group of Yusuke Yokota (Associate Professor at Institute of Industrial Science, The University of Tokyo), Tomohiro Inoue and Tatsuya Kubota (National Research Institute for Earth Science and Disaster Resilience) investigated the correlation values and frequencies of ocean bottom temperature data at each point, the research group found that the correlation dependent on depth was higher than the correlation dependent on distance, and that signals in the long period band of about 10-100 days in particular fluctuated synchronously. This conclusion suggests that a long period ocean phenomenon such as "coastal trapped waves" may be the cause.

Expected future
The detailed cause of the ocean bottom temperature synchronization detected this time is not clear. Most of the data up to now was during the Kuroshio large meandering, but this large meandering is expected to end. The research group believes that the cause will become clear by continuing to pay attention to whether the behavior of the data changes due to such global changes in the ocean field.
The detected signal is estimated to have a very small effect on ocean bottom pressure gauges and other instruments. There is almost no effect on the measurement of the vertical movement of the ocean bottom through ocean bottom pressure observations, but if more precise observations are realized in the future, the ocean waves observed this time may become noise and interfere with the measurement of small signals such as slow slips.

The article, "In-phase variation along the Nankai trough in the ocean bottom temperature records of DONET" was published in Journal of Oceanography at DOI: 10.1007/s10872-025-00770-y.

Research Contact

Yusuke Yokota, Associate Professor
Institute of Industrial Science, the University of Tokyo
Tel：+81-3-5452-6188
E-mail：yyokota (Please add "@iis.u-tokyo.ac.jp" to the end)