Predict (Simulation Study)

We developed a database and a sophisticated model of time-space development of the slips at the plate boundary based on the survey data obtained from global positioning, leveling, etc. We simulated the cycle of earthquakes throughout the Nankai Trough area that extends from Tokai to Hyuga-nada and clarified the fracture pattern of both interrelated and non-interrelated earthquakes under various conditions. To clarify the diversity of earthquake cycles, we need to learn the history of earthquakes over a span of thousands of years. We tried to identify the time and scale of past tsunamis through the investigation of tsunami deposits, which are a kind of archive that records the situations in past tsunamis.

Verifying the Slip along a Plate Boundary

Based on the geodetic surveying data from GPS, leveling, triangulation, and tide levels and using a new analytical method, we predicted the distribution of slips and slip deficits along a plate boundary and the change in slips over time.

Based on this, we have clearly identified the locked zone corresponding to the source region of the Great East Japan Earthquake, as well as the same zone of a huge plate extending to around Hyuga-nada along the Nankai Trough. These have become one of the tools to review the traditional predictions about massive earthquakes along the Nankai Trough.

Predicting a Seismic Cycle from past Tsunami Deposit

By clarifying the time and scale of past tsunamis from deposits, we can identify the correlation between the historical and geological records. If we can predict the time and scale of each tsunami in various locations, we can predict the locations of the source areas of past Nankai earthquakes more accurately, and this helps with empirical research to elucidate the cycles of interrelated/non-interrelated earthquakes.

At the bottom of an untouched pond, the sand carried from the sea floor by tsunamis remains as it was. We investigated more than 30 lakes and marshes from Miyazaki to Lake Hamana (Hamana-ko). From the research results, it was revealed that tsunamis that were large enough to leave such deposits struck at intervals of 300 years. It was proven from the survey results that the notable and historic Hoei Tsunami was a size that could be seen only once in hundreds or a thousand years but we could not conclude that it was the largest ever. We found a sign in the Ryujin Pond located on the east coast of Kyushu that suggests that a tsunami 3,300 years ago might have been larger. From Tosa Bay to the east coast of Shikoku, there was also a mark from a large tsunami that occurred 2,000 years ago.

We are now conducting an investigation on the Kii Peninsula to further clarify the difference by region and explore the cause.

Deposit of Ryujin Pond, Kanigaike Pond and Kamoda-oike Pond

Predicting Seismic Cycles from a Simulation

We simulated a possible future earthquake using a super computer, based on data from a sophisticated 3D model that reflected the result of crust investigations and a database on the slips at the plate boundary.

We also developed a more sophisticated simulation method that takes into account plate substances and friction characteristics changes, which have not been considered in the existing simulation, to predict the cycle of the interrelated earthquakes, phenomena that might take place if the Nankai, Tonankai, and Tokai earthquakes occurred sequentially, the cycle of occurrence.