Precise tilt measurement by seafloor borehole tiltmeters at the Nankai Trough subduction zone

• Published in: Earth, planets, and space Vol. 75; no. 1; pp. 188 - 14
• Main Authors: Tsuji, Shuhei, Araki, Eiichiro, Yokobiki, Takashi, Nishida, Shuhei, Machida, Yuya, Zumberge, Mark, Takahashi, Keisuke
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.12.2023; Springer; Springer Nature B.V; SpringerOpen
• Subjects: 6. Geodesy; Analysis; Bathymetry; Boreholes; Broadband; Drilling; Earth and Environmental Science; Earth Sciences; Earthquakes; Elastic half spaces; Environmental aspects; Frequencies; Geology; Geophysics/Geodesy; Japan; Long-term observation; Measurement; Microseisms; Modulus of elasticity; Nankai trough; Noise levels; Observatories; Ocean floor; Optical sensing; Pendulums; Seafloor borehole; Seismic activity; Seismographs; Seismology; Seismometers; Sensors; Slopes; Subduction (geology); Subduction zones (Geology); Thickness; Tide; Tilt; Tiltmeters; Japan; Azimuthal dependence; Tide; Long-term observation; Tidal response; Tilt; Seafloor borehole; Nankai trough; Optical sensing
• ISSN: 1880-5981; 1343-8832; 1880-5981
• DOI: 10.1186/s40623-023-01946-w

In this study, geodetic and seismic phenomena occurring in the subduction zone were observed with two tiltmeters that were installed in seafloor boreholes drilled in the Kumano Basin in the Nankai Trough, southwest Japan. We used one electrolytic-type tiltmeter and one pendulum-type tiltmeter installed 6 and 19 m below the seafloor, respectively. The two tiltmeters have been continuously operated since 2019 and 2021, respectively. The records of the two tiltmeters showed microseisms, seismic signals of normal earthquakes, and variations that correlated well with the M2 tide (tidal response). The noise environment relative to existing seafloor observatories was assessed by comparing the power spectral densities of the tiltmeters and broadband seismometers in the seismic frequency band. The two tiltmeters and the seismometers showed similar spectral peaks in the microseism band, and at lower frequencies below 10 –2  Hz, the pendulum tiltmeter had a noise level that was up to 20 dB lower than that of the broadband seismometers. The tidal responses of the tiltmeters were analyzed to reveal corresponding mechanisms because a large amplitude of the tidal response makes it difficult to detect geodetic phenomena. Considerable azimuthal dependence was observed in the NW–SE direction for both tiltmeters. The principal direction was approximately the same as the down-slope direction of the local bathymetry. This tendency was thus interpreted to be caused by the differences in the thickness of the sedimentary layer along the direction of the slope. Furthermore, the validity of the tilt measurements for the two tiltmeters was demonstrated by in-situ loading experiments, where the theoretical response of an elastic half-space medium was computed and compared with the experimental results. The results of the loading experiments can be explained using theoretical values with a set of realistic elastic moduli. Graphical Abstract