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
- Changes in fluid pressure were calculated near the plate boundary causing large subduction zone earthquakes
- Contrary to the conventional models, it was discovered that the drop in fluid pressure near the plate boundary after earthquakes was negligible
- The researchers propose long-term monitoring of fluid pressure near the plate boundary, a factor of large Nankai Trough earthquakes
(a) Schematic diagram of drainage from cracks formed after large earthquakes (b) Relationship between fluid pressure and conditions before and after repeated earthquakes
Earthquakes are thought to occur near plate boundaries when the nearby fluid pressure builds up to around the maximum level. Current monitoring techniques enable monitoring of fluid pressure fluctuation of approximately 10 MPa, as revealed though this research
Multiple plates are in contact with each other around the Japanese Islands, and in estimating the risk of earthquake disasters, it is essential to elucidate the mechanism by which large earthquakes occur at plate boundaries such as the Nankai Trough.
Earthquakes that occur over long periods, called slow earthquakes, are thought to be induced by fluid at plate boundaries. Slow Nankai Trough earthquakes are monitored by comprehensive observation facilities for groundwater and other phenomena installed by AIST from the Chubu region to the Shikoku region. As slow earthquake observation continues while the mechanism for the occurrence of large earthquakes at plate boundaries is studied from geodetic and seismological perspectives, this mechanism must also be elucidated from a material science perspective.
Based on study and analysis of the outcrop of the Nobeoka Thrust in Miyazaki Prefecture in collaboration with the United States Geological Survey, the University of Tokyo, and Tokyo University of Marine Science and Technology, the researchers found a possibility that negligible amount of fluid near the plate boundary at a depth of about 8 km is lost even if cracks are formed in the rock after earthquakes, and that high pressure is maintained and continues to build up.
Increases in fluid pressure near plate boundaries are known to be directly linked to earthquakes. Once earthquakes occur, the fluid pressure near plate boundaries is reduced by drainage through cracks formed in the rock. Subsequently, it is thought that fluid pressure gradually builds up when cracks close. A certain amount of time is required until fluid pressure increases again, but the actual extent of change in fluid pressure that decreases after earthquakes and the time until the fluid pressure builds up again before the next earthquake are not well understood. The researchers performed sophisticated calculation and modeling of changes in time for fluid pressure deep underground. They revealed a smaller decline in fluid pressure after earthquakes than a decline shown with a previous model. After this drop, the pressure builds up over time. The results both show the importance of long-term monitoring of fluid pressure near plate boundaries in predicting the occurrence of large earthquakes and suggest a framework for the investigation.