Initial evaluation of pore fluid pressure of the new long-term borehole monitoring system in the Nankai Trough, Japan

The Japan Agency for Marine-Earth Science (JAMSTEC) is operating some long-term borehole monitoring systems (LTBMSs) in the Nankai Trough seismogenic zone, by which slow slip events (SSEs) had been detected using pore fluid pressure monitoring (e.g., Araki et al. 2017 and Ariyoshi et al., 2021). The latest LTBMS named as C9038B has been installed in 2023, and the in-situ dataset have been available in the early 2024 by connecting with the Dense Ocean-floor Network System for Earthquakes and Tsunamis (DONET). The in-situ pore fluid pressure is measured by two absolute pressure gauges deployed at the seafloor, which is connected to the borehole and processed by calculation difference between the borehole pressure and the seafloor pressure to cancel the effect of ocean tide. Three pressure gauges, i.e., two pressure gauges and one pressure gauge respectively measuring borehole pressure and seafloor pressure are deployed at the seafloor, making it possible to replace the sensor itself in the case of degradation. Prior to deployment at the seafloor, we pressurized 20 MPa, almost equivalent to 2,000 m depth to three pressure gauges by using a pressure balance with ambient temperature of 2 °C for one month to evaluate the sensor’s drift, and determined which sensors are suitable to measure the borehole pressure and the seafloor pressure based on the experimental results. The pressure gauges showing the first and the second smallest sensor drift were used for seafloor and the borehole pressure measurements, respectively. Comparing the sensor drift of the experiment with that of the in-situ measurement for the initial one month, our laboratory evaluation could support the in-situ observation. The recent pressure measurement suggests that the pore fluid pressure is rising up at a rate of 0.3 hPa per day in the borehole over time. Although the permeability of the pore fluid pressure section in the borehole can be changing, further discussions may be needed to clarify the reason.