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

中文翻译：

---

南海海槽俯冲带海底钻孔倾斜仪精确测量倾斜

在这项研究中，使用安装在日本西南部南海海槽熊野盆地海底钻孔中的两台倾斜计观测了俯冲带中发生的大地测量和地震现象。我们使用一台电解式倾斜计和一台摆式倾斜计分别安装在海底以下 6 米和 19 米处。这两台倾斜仪分别自 2019 年和 2021 年以来一直持续运行。两个倾斜计的记录显示了微震、正常地震的地震信号以及与 M2 潮汐（潮汐响应）密切相关的变化。通过比较地震频带内倾斜仪和宽带地震仪的功率谱密度，评估了相对于现有海底观测站的噪声环境。两台倾斜仪和地震仪在微震频带中显示出相似的频谱峰值，并且在低于 10^–2 Hz 的较低频率下，摆锤倾斜仪具有噪声水平比宽带地震仪低 20 dB。分析倾斜计的潮汐响应以揭示相应的机制，因为潮汐响应的振幅较大，难以检测大地现象。对于两个倾斜计，在西北-东南方向观察到相当大的方位角依赖性。主方向与当地测深的下坡方向大致相同。因此，这种趋势被解释为是由于沿斜坡方向沉积层厚度的差异造成的。此外，通过原位加载实验证明了两个倾斜仪倾斜测量的有效性，其中计算了弹性半空间介质的理论响应并与实验结果进行了比较。加载实验的结果可以用理论值和一组实际弹性模量来解释。

图形概要