In faults called the decollement zone located in shallow areas of plate subduction boundaries, we observed slow earthquakes in which slip failure proceeds more slowly than regular earthquakes. At the time of an earthquake, if the decollement zone slips fast together with the earthquake, the slip might evolve into a large earthquake that causes a large tsunami. Large Nankai megathrust earthquakes are expected to occur, so it is urgently necessary to clarify the causes of the unstable behavior of the ocean's crust where slow earthquakes can develop in the decollement zone.

Around plate boundaries, clay minerals are known to alter due to high temperature and pressure. Therefore, we aimed to develop a numerical analysis method that expresses the dynamic changes in soil quality that are accompanied by such clay mineral alteration, and to investigate the dynamic behaviors of the ocean's crust which alters while bearing shearing force.

The results of analyses in which plate subduction was assumed (bottom figure) showed that gap water pressure increases in the upper areas of the decollement zone, and in response, the strength of the decollement zone tended to soften after temporary hardening. Hardening causes sticking between plates and softening causes the development of slow earthquakes. This suggests that clay mineral alteration is a cause of crust variance resulting in tsunamis.

Going forward, we will focus on changes in ion density and temperature in decollement zones as these affect the speed of alteration, and will then improve the model.