To predict the occurrence of interrelated massive earthquakes, it is essential to make a sophisticated physical model of the earthquake zone. To know the shape of the plate and what substances constitute the plate boundary in the Nankai Trough earthquake zone, the survey started in Hyuga-nada in 2008. It gradually moved eastward to reach the Tokai area in 2011 and currently off the coast of Suruga Bay. We placed ocean-bottom seismometers both extensively and densely and conducted survey and observation of natural and artificial earthquakes.

As a result of these surveys, we revealed the structural changes in the Philippine Sea Plate, which is subducting along Hyuga-nada (Kyushu-Palau Ridge) and off the coast of Shikoku, as well as the existence of the area called the transition belt, where friction characteristics change.

We also continued the observation of undersea earthquakes and water pressure off the coasts of Miyagi Prefecture and Nemuro, which are under the imminent danger of earthquakes. From the survey in Miyagi, we gathered data on the crustal movements at the time of a series of earthquakes and the change in water pressure at the bottom of sea before and after the Great East Japan Earthquake on March 11, 2011. It shows that the earthquake zone moves and the degree of concentration of seismic activities at the plate boundary changes significantly before the occurrence of an earthquake. In addition, we could confirm the vertical crustal movement at the seafloor that occurred during the period from the largest foreshock on March 9 to the main shock.

We will examine the details of the aseismic slip that occurred after the largest foreshock and the relationship between the slip event and the occurrence of the main shock.

It is known that slow tremor earthquakes (low-frequency earthquakes) occur around the rupture zone of a massive earthquake and interrelate to each other. We will further examine whether the interrelation and the pattern of occurrence of very small earthquakes can be indicators that suggest the current condition of the massive earthquake zone. At the same time, to identify the pattern of occurrence of seismic activities throughout the Nankai Trough area, we placed ocean-bottom seismometers for long-term observations and will continue watching the activities both from land and on the ocean.

Detailed findings from the research and observation are digitized and utilized to establish a more sophisticated framework (modeling) for the simulation of the past and possible future earthquakes.