[S-TT36] Applying optic fiber sensing to earth science

[STT36-P03] Pore fluid pressure change observed by a fiber optic strainmeter in the 20-m borehole of the Kamioka Mine after the 2024 Noto Peninsula Earthquake

*Hiroyuki Matsumoto¹, Eiichiro Araki¹, Shuhei Tsuji¹, Yuya Machida¹, Takashi Yokobiki¹, Keisuke Ariyoshi¹ (1.Japan Agency for Marine-Earth Science and Technology)

A fiber optic straimenter and a pore fluid pressure sensor were installed in the 20-m borehole in the Kamioka Mine, and their continuous dataset have been acquired since April 2023 (Machida et al., 2023). The fundamental design and sensors’ composition of the Kamioka Mine observatory is the same as the 500-m borehole observatory in the Nankai Trough installed in November 2023 for monitoring slow motion earthquakes known as slow slip events (SSEs) (Araki et al., 2023), and therefore the evaluation of response of strain against pore fluid pressure in the Kamioka Mine can help for interpretation of the in-situ observation in the Nankai Trough seismogenic zone. In the meantime, an earthquake (M7.6) occurred in the Noto Peninsula on 01 January, 2024 was the largest quake to the sensors during the long-term observation in the Kamioka Mine. The unusual pore fluid pressure change was observed just after the earthquake; pore fluid pressure was decreased by approximately 80 hPa for a duration of a few days, followed by the increasing pore fluid pressure gradually. The level of pore fluid pressure was recovered to the original level in one month with some small long-period fluctuations. Strain by the borehole fiber optic strainmeter was also changed accordingly, which correlated to the pore fluid pressure over time. The present observation suggests that the fiber optic strainmeter can measure the long-period pore fluid pressure change in the borehole. Additionally, it should be noteworthy that the earth tide with the amplitude of 50 nano-strain can be identified by the fiber optic strainmeter in the borehole.