[S-CG56] Crustal fluids and deformation

[SCG56-P01] Introduction of new AIST integrated groundwater observation station (Saiki-Kamae observation site) in Oita Prefecture

*Yuichi Kitagawa¹, Tadafumi Ochi¹, Norio Matsumoto¹ (1.Tectono-Hydrology Research Group, IEVG, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

Keywords:strain meter, groundwater, seismometer, Nankai Trough, deep slow slip

AIST plans to construct an integrated groundwater observation network consisting of 20 observation stations for monitoring the Nankai Trough megathrust earthquake. 18 observation stations were established from 2006 to FY 2022. In FY 2023, the Saiki-Kamae observation station was constructed as the 19th station in Saiki City, Oita Prefecture. The Saiki-Kamae observation station is located near the southwestern edge of an anticipated focal region of the largest class of the Nankai Trough megathrust earthquake and monitors the plate boundary around the Hyuga-Nada. In addition, it is located southwest of the area where deep low-frequency tremors and deep slow slip events (deep SSEs) occur from western Shikoku to the Bungo Channel, therefore it is an important station for obtaining a more accurate understanding of the deep slow SSEs from western Shikoku to the Bungo Channel. At the station, three observation wells were drilled to depths of 554.5 m, 200 m, and 30 m. And nine types of geophysical logging were conducted, with the main purpose of determining the appropriate depth to install various observation instruments inside the wells. In February 2024, the Ishii-type borehole strainmeter was installed near the bottom of the observation well with a depth of 554.5 m. From February to March of the same year, an instrument consisting of seismometer and tiltmeter was installed near the bottom of the 200 m deep observation well, and then, the water level sensors and the other instruments were installed at three wells. The station was completed in March 2024 and observations began. Changes in groundwater levels and crustal strains were observed with the Hyuga-nada earthquake (M 7.1) on August 8, 2024 and the Hyuga-nada earthquake (M 6.6) on January 13, 2025. We will introduce an overview of the observation station and observation results for about one year starting from April 2024.