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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. [ABSTRACT FROM AUTHOR]

We have developed a new magnetometer system specialized to multipoint and long-term observations on the seafloor to promote marine or ocean-bottom experiments mainly for the electromagnetic sounding of the Earth’s interior. In situ magnetic field observation on the seafloor is an essential geophysical technique to investigate structures of the oceanic crust and the upper mantle, many of which are still frontier as to observational evidences. The in situ and long-term observations require long-term-operable and small-size magnetometer systems, which are placed on the seafloor over a year in pressure-resistant cases without external power supply and communication. We have designed and developed a new electric circuit board of small size and lower power consumption for the magnetometer system. Our new magnetometer system, what we call “DOKODEMO MAG,” is suitable to be installed in a pressure-resistant cylinder of 36 mm diameter and can operate independently over 2 years with a smaller amount of batteries than the conventional system because its power consumption was saved to ~ 33 mW. This magnetometer system is capable of observing orthogonal three-axis magnetic fields continuously with sampling frequency of 5 Hz at maximum and an accuracy of ~ 0.1 nT. The system also records tilt and temperature of the system and voltage of the batteries. Prototype models of this magnetometer system were tested for in situ operation for 5 months on the seafloor around the Kikai caldera in the south of the Kyusyu Island, SW Japan. The results of the test showed sufficient performance of our new magnetometer system and its potential of future usage for every type of marine or ocean-bottom operations. [ABSTRACT FROM AUTHOR]