– Towards understanding of the mechanism of slow earthquakes in the Nankai Trough –
Researchers: OTSUBO Makoto, Senior Researcher, Geodynamics Research Group, Research Institute of Earthquake and Volcano Geology, and MIYAKAWA Ayumu, Senior Researcher, Geophysics Research Group, Research Institute of Geology and Geoinformation
The researchers calculated stress orientations within the Philippine Sea plate, investigated directions of fluid migration, and clarified that slow earthquakes occur as a result of fluid accumulating in the mantle below the Eurasian plate, in collaboration with Hiroshima University and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
Schematic figure showing the orientation of fluid migration in the subducting plate (blue arrow) at the depth where slow earthquakes occur and the relationship between the stress orientations for the rock during the earthquakes and the orientation of fluid flow in the rock. Gray arrows indicate the orientations of the three principal stress axes of the stress.
In recent years, many slow earthquakes occurred in the Nankai Trough. AIST has installed comprehensive observation facilities for groundwater and other items from the Chubu region to the Shikoku region, and monitors slow earthquakes with high precision. It is thought that slow earthquakes are related to fluid accumulations at the plate boundary, but the details of the slow earthquake mechanism were unclear.
The researchers proposed a fluid migration model by using the data of earthquakes that occurred within the Philippine Sea plate at depths of 30 to 70 km. They calculated the intra-plate stresses required for earthquake occurrences, and investigated the amount of fluid contained in the plate and directions of fluid migration. The results showed that the fluid migrated from the Philippine Sea plate accumulates in the mantle below the Eurasian plate and reduces friction between the rocks, which causes slow earthquakes.
The researchers intend to promote comprehensive investigations of the behavior of fluid near the plate boundary using the rock samples obtained through excavation by the Deep-sea Scientific Drilling Vessel “CHIKYU”.