Response of the Deep-Seafloor Borehole Fiber-Optic Strainmeter Installed in the Nankai Trough to the Vertical Crustal Deformation

A borehole fiber-optic strainmeter and pore-pressure gauges were installed at the C9038B deep-seafloor borehole observatory at the Nankai trough subduction zone for the precise measurement of seafloor crustal deformation. To monitor activities associated with subduction processes, such as slow-slip events, through continuous observations at the borehole observatory, the characteristics of these observations must be clarified. In the first two years of observations after installation, clear tidal and tsunami responses were recorded. To understand the physical origins of these observational characteristics, we performed finite-element method (FEM) simulations. The response characteristics of observations were modeled by considering both the crustal structure around the observatory and the instrument used for observations. The simulation sequentially solved two elastic models comprising the crustal structure around the observatory and sensor structure. The sedimentary layer was modeled as V-shaped, with the major components of the sensor units being included in the simulations. The elastic properties assigned to these models were based on drilling parameters and seismic surveys. Consequently, the simulation reproduced the tidal response of strain and pore pressure observations. The loading response coefficient derived from the FEM-based tidal analysis also successfully explained the signals associated with the tsunami generated by the Kamchatka earthquake on 29 July 2025. This demonstrated that the numerical results accurately represented the response of the observatory to external loading events. Furthermore, to investigate the general tendencies of the borehole observations, similar simulations were performed using models that considered various elastic parameters and sedimentary layer geometries. Regarding changes in the elastic parameters, the strain and pore pressure observations were sensitive to Young’s modulus and Poisson’s ratio, respectively. Changes in the shape of the sedimentary layer caused perturbations in the strain observations; however, these effects were relatively smaller than those associated with variations in the elastic parameters within realistic ranges.