Long-term Submarine seismic observation has many difficulties, such as the cost of the installation and the constant power supply. However, the recent development of new observation technology that uses submarine fiber-optic cables, which are commonly used for telephone and Internet communications, as sensors to detect ground motion is bringing significant innovation to submarine seismic observations. Research and Development Center for Earthquake and Tsunami Forecasting(FEAT) succeeded in developing a seismic monitoring system that uses an existing seafloor cable several tens of kilometers long off Cape Muroto. The monitoring system is equivalent to a dense and permanent array of the ocean bottom seismometers installed at several meter intervals.
We, Subduction Dynamics Research Center (SDR), processed the ambient noise of the data obtained through the fiber optic cable off Cape Muroto and have succeeded in extracting various seismic waves propagating under the seafloor. In addition, we have developed a new technique to visualize the complicated geological structure beneath the seafloor using the seismic waves extracted from the ambient noise observed by the fiber-optic cable. These new observation and analysis techniques have the potential to detect changes in the subseafloor structure under the existing fiber-optic cables. We will advance our observation and data analysis techniques to monitor the minute changes that occur in the subduction seismogenic zones.

The 2011 Tohoku earthquake was the largest earthquake instrumentally recorded in Japan and caused a devastating tsunami. A large tsunami inundation similar to the 2011 Tohoku earthquake is recorded in historical documents, but the recurrence of such devastating earthquakes and tsunamis is poorly understood. Marine paleoseismology, a study of prehistoric giant earthquakes and tsunamis based on the traces left by these events, is key to understanding the geohazard and risk posed by long-recurrence earthquakes.
When the seafloor is strongly shaken by earthquakes, the sediments suspended by the strong ground motion move down the slope like an avalanche, reaching the deep seafloor and depositing there, forming a layer called "turbidites". Since the turbidites layers are the traces of past earthquakes and tsunamis, the seafloor sediments sampled by coring are the key to paleoseismology. The distribution of the turbidites show the extent of strong ground motion at the seafloor.
In the Japan Trench, we succeeded in determining the extent of the strong ground motion from the paleoearthquake that occurred more than 1000 years ago, based on the sediment samples taken from the seafloor. In the Nankai Trough, we found many turbidite layers in the sediment cores, but some of them may not be related to strong ground motion. Therefore, we will develop a method to distinguish the cause of the turbidite layer formation and will reveal the historical occurrence of earthquakes and tsunamis in this area.
We will continue to conduct deep seafloor surveys and core sampling, and reconstruct earthquake occurrence records up to tens of thousands of years in the past, mainly in the Nankai Trough, Japan Trench, and Kuril Trench areas.