[S-CG59] Crustal fluids and deformation

[SCG59-01] Relationship between dynamic strains of earthquakes and post-seismic crustal strains changes or groundwater level changes in the case of the TSS observation site of AIST

*Yuichi Kitagawa¹, Norio Matsumoto¹ (1.Tectono-Hydrology Research Group, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

Keywords:strain, groundwater, post-seismic change, dynamic strain

In order to research short- and intermediate-term forecast of the Nankai megathrust earthquake, we observe crustal movement and groundwater at observation sites in Aichi Prefecture, Kii Peninsula and Shikoku, in particular focus on monitoring of short-term SSEs on the plate boundary. At the Tosashimizu-Matsuo (TSS) observation site of AIST, crustal strain changes and groundwater level changes due to earthquakes have been repeatedly observed. The crustal strain changes were post-seismic slow changes. Most of the crustal strain changes were observed as principal strain fields with contraction in the northeast-southwest direction and extension in the northwest-southeast direction. Regarding the groundwater level changes, the groundwater levels at the hole-2 observation well (250 m deep) were step-like or slowly decreased after the earthquakes. There are many cases in which crustal strain changes and groundwater level changes occurred simultaneously for the same earthquake, but there were a few earthquakes in which crustal strain changed but groundwater level did not change, and vice versa. We investigated the relationship between hypocentral distance and magnitude of earthquakes, and the presence or absence of change, and it is found that the crustal strain change tends to occur for distant large earthquakes and the groundwater level change tends to occur easily for nearby small earthquakes. It is often thought that crustal strain changes and groundwater level changes triggered by distant earthquakes are caused by seismic waves (dynamic strain) of earthquakes. Looking at the relationship between the amplitude of dynamic strain of earthquake and the presence or absence of change, it was found that groundwater level change tends to occur even if the dynamic strain is small. Since crustal strain change tends to occur more easily for distant earthquake, it is thought that it depends not only on the amplitude of the dynamic strain but also on its frequency.