Tilt Measurement along the Nankai Trough Subduction Zone by Borehole Type Tiltmeters

We found azimuthal dependence in a component of tilt variation correlated with M2 tide by borehole type tiltmeters at Kumano basin along the Nankai trough, Japan.

In the Kumano basin, the Philippine Sea plate is subducting beneath the Amour plate in NW-SE direction. M8 class earthquakes are repeatedly occurred with interval of 100 - 200 years. VLFEs and short-term SSEs are also observed.

To detect short-term SSEs and other geodetic events, we developed a new tiltmeter and installed it into a 20m-deep borehole (Kimura et al., 2021, JpGU Meeting). In this region, a high-precision electrolytic tiltmeter in 6m-deep borehole is under operation. Considering the noise level and sensitivity, we developed more sensitive tiltmeter based on optical geodetic sensor (Zumberge et al., 2018). The new tiltmeter expected to measure broadband signals from several tens of Hz to several month with higher sensitivity than the previous one. In addition, we installed that into deeper borehole to decrease the noise level.

In the record of the tilt meter, microseisms, seismic signals of natural earthquakes, and a variation well correlated with M2 tide (hereinafter we call this component tidal response) are observed. However, step-like changes are also observed once to twice a day. Most of the steps are less than one micro radians, and cumulative movements are -30 and 50 micro radians for North and East direction. Intervals and amplitudes of the steps become longer and smaller with time, respectively. After removing of the steps, almost linear trend with rate of approximately 10 micro radian/year were revealed.

We focused on the tidal response and analyzed azimuthal dependence. The tidal response is observed in 200 nano radians at maximum for both NS and ES directions. In particle motion, larger amplitude is shown in NW-SE direction. In addition, a variation of 15 days period with amplitude of 100 nano radians is also observed. The variation also shows azimuthal dependence in NW-SE. To discuss the azimuthal dependency, we analyzed the electrolytic tiltmeter. As a result, the tidal response is also observed with maximum amplitude of 2500 and 1000 nano radians for NS and EW directions, respectively, and the same azimuthal dependence was found. The sensitive direction of NW-SE may reflect difference in elasticity made by local geodesy in the subduction zone.