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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 developed a method to detect attenuation changes during seismic wave propagation excited by precisely controlled artificial seismic sources, namely Accurately Controlled Routinely Operated Signal System (ACROSS), and applied it to monitor the temporal changes for in situ data collected by previous studies. Our method, together with the use of the ACROSS sources, is less susceptible to noise level changes, from which conventional methods such as envelope calculation suffer. The method utilizes the noise level that is independently estimated in the frequency domain and eliminates the influence of the noise from the observed signal. For performance testing, we applied this method to a dataset that was obtained in an experiment at Awaji Island, Japan, from 2000 to 2001. We detected a change in amplitude caused by rainfall, variation in atmospheric temperature, and coseismic ground motions. Among them, coseismic changes are of particular interest because there are limited studies on coseismic attenuation change, in contrast to many studies on coseismic velocity decrease. At the 2000 Western Tottori earthquake (MW = 6.6, epicenter distance of 165 km), a sudden decrease in amplitude of up to 5% was observed. The coseismic amplitude reduction and its anisotropic characteristics, which showed a larger reduction in the direction of the major axis of velocity decrease, were consistent with the opening of fluid-filled cracks, as proposed by previous studies. The Δ Q - 1 corresponding to the amplitude change gives similar values to those reported in previous studies using natural earthquakes. [ABSTRACT FROM AUTHOR]

We analyze temporal variations in the travel times of both P and S waves (Vp and Vs) for 14 months at Toyohashi (central Japan) with a continuously operating vibration source that could efficiently produce both P and S waves. Seismic waves produced by the source, which is called the accurately controlled routinely operated signal system (ACROSS), are recorded by three nearby seismic stations, and the travel time variation at each station is estimated using the transfer function calculated from the recorded data. Long-term variations in Vp and Vs are observed and can be interpreted by the change in fluid saturation and crack density of the subsurface rocks. The variation in fluid saturation and crack density are consistent with that in the groundwater level, which is measured at the station nearest to the ACROSS. Short-term responses to rainfalls are observed at the station nearest to the ACROSS system; the interpretation of the changes in crack density and saturation is inconsistent with the ground water observation, partly owing to the initial response to rainfall. This can be interpreted as an air–water mixture within pores or cracks on a fine scale. [ABSTRACT FROM AUTHOR]

We discovered a secular change in the travel time of direct S-waves over a 10-year observation period by means of continuous operation of an artificial and stable seismic source, called Accurately Controlled Routinely Operated Signal System (ACROSS), which is deployed in the central part of Japan along the Nankai Trough. We used 13 High Sensitivity Seismograph Network Japan (Hi-net) stations around the ACROSS source to monitor the temporal variation in travel time. Green’s functions were calculated for each station daily from March 29, 2007, through October 30, 2017. Secular advance in the temporal variation in travel time was seen for the whole operation period, in addition to a steplike delay associated with the 2011 Tohoku earthquake. We estimated the rate of secular change and the amount of coseismic step by modeling the transfer function of S-waves with a linear trend and the coseismic step of the 2011 Tohoku earthquake. Distance dependences of the travel time changes can be explained as a combination of common bias and dispersion for each station, for both the secular and coseismic changes. This can be interpreted as a randomly distributed change in seismic velocity over the range of the observation region. An azimuthal dependence exists for both changes and shows larger changes in the NE-SW direction than in the NW-SE direction from the ACROSS source. [ABSTRACT FROM AUTHOR]