A research group including the Institute of Industrial Science at the University of Tokyo has researched the interlocking condition of the plate boundary within the focal region of the anticipated Nankai Trough mega-earthquake. The group revealed that the focal region consists of: "regions that remain largely unchanged over long periods" and "regions that fluctuate over time." This discovery is expected to advance the understanding of the potential magnitude and rupture processes of a devastating megathrust earthquake, and it holds promise for utilization in pre-disaster mitigation strategies.
The group consisted of Associate Professor Yusuke Yokota from the Institute of Industrial Science at the University of Tokyo, Chief Marine Disaster Research Officer Shun-ichi Watanabe from the Hydrographic and Oceanographic Department of the Japan Coast Guard (JCG), as well as researchers from the Meteorological Research Institute of the Japan Meteorological Agency and the Japan Coast Guard Academy.
The Nankai Trough is a massive submarine trench extending from off the coast of Shizuoka Prefecture to the Hyuga-nada Sea off the coast of Kyushu, where the Philippine Sea plate subducts beneath the continental plate on the Japanese archipelago side. When the plates lock tightly together, strain accumulates along the boundary interface. A megathrust earthquake is thought to occur when this strain reaches its physical limit.
However, this interplate coupling is uneven and exhibits variations. In areas where the coupling is rigid, the overriding continental plate is dragged downward, heavily accumulating strain that will eventually release simultaneously to cause a huge earthquake. Conversely, in areas where coupling is weak, subduction proceeds smoothly with minimal strain accumulation. While these fundamental principles were already understood, details regarding how this coupling changes over time had remained unresolved.
Provided by the Earthquake Research Committee, the Headquarters for Earthquake Research Promotion
Provided by the Japan Coast Guard
To address this, Yokota and his team utilized data from the JCG 's Seafloor Geodetic Observation Array (SGO-A). This network employs GNSS-Acoustic (GNSS-A) technology, which combines satellite positioning with underwater acoustic ranging to determine seafloor positions with high precision, thereby capturing crustal deformation. The team analyzed an 11-year observation records spanning from 2013 to 2023 to investigate the "slip deficit rate" indicating the degree of interplate coupling. This metric shows how much the regular plate movement is obstructed.
The analysis revealed that beneath the seafloor close to the southern coast of the Japanese Archipelago, such as off the Kii Peninsula and off Shikoku, there are distinct zones where the tightly coupled state barely fluctuated throughout the entire observation period. The research group classified these as "stable coupled zones." On the other hand, in the deeper offshore regions further south (more than 50 km away from the coastline), they confirmed zones where the coupling strength varied depending on the period. This suggests that a recurring cycle of partial slip events followed by re-coupling may be taking place in these areas.
Provided by the Japan Coast Guard
(Right) The red areas indicate the zone where the coupling strength fluctuated year by year.
Provided by the Institute of Industrial Science of the University of Tokyo
The research group has previously reported the occurrence of slow slip events along the Nankai Trough. Unlike ordinary earthquakes characterized by rapid fault rupture, slow slips occur over several days to months as the fault moves at an extremely gradual pace, affecting the delicate balance of forces along the plate boundary.
The regional variations in coupling uncovered by this study may be linked to these slow slip phenomena, suggesting that strain is not accumulating uniformly across the entire anticipated focal zone. The confirmed "stable coupled zone" is identified as the specific region poised to release massive amounts of energy during a future mega-earthquake.
The JCG has established new observation stations to augment its existing sites, enhancing long-term, continuous monitoring capabilities. The research group anticipates that their future research can clarify the conditions at the westernmost and southernmost edges of the focal zone, which could not be fully resolved in the current study. These findings were published in the international scientific journal Earth, Planets and Space on June 3.
Regarding the Nankai Trough mega-earthquake, the government released a revised damage estimation in March of last year forecasting a "death toll exceeding 290,000." In September of last year, the government's Earthquake Research Committee revised the probability of an earthquake occurring within the next 30 years. While they adjusted the conventional metric of "around 80%" to "60% to 90% or higher," they also concurrently presented an alternative calculation of "20% to 50%." However, from the standpoint of driving disaster prevention measures forward, the government emphasized the higher projection of "60% to 90% or higher."
Furthermore, the Ministry of Land, Infrastructure, Transport and Tourism revised its disaster countermeasure plan in January of this year. The updated policy heavily emphasizes "measures to connect and sustain life" aimed at reducing disaster-related deaths, in addition to traditional "measures to protect life" focused on minimizing immediate casualties from building collapses and tsunamis.
Original article was provided by the Science Portal and has been translated by Science Japan.