[1] We show fine-scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high-pressure fluids released by progressive metamorphic dehydration reactions in the subducting Philippine Sea plate in Tokai district, Japan. These zones have a strong correlation with the distribution of slow earthquakes, including long-term slow slip (LTSS) and low-frequency earthquakes (LFEs). Overpressured fluids in the LTSS region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore-arc, and impede intraslab earthquakes therein. In contrast, fluid pressures are reduced in the LFE zone, which is deeper than the centroid of the LTSS, because there fluids are able to infiltrate into the narrow corner of the mantle wedge, leading to mantle serpentinization. The combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary, which in turn control the occurrence of slow earthquakes. Citation: Kato, A., et al. (2010), Variations of fluid pressure within the subducting oceanic crust and slow earthquakes, Geophys.

The temporal variation of seismic velocity near the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake), was detected using an accurately controlled routine-operated seismic source (ACROSS). The source generates elastic waves by a centrifugal force of an eccentric mass rotating around an axis. The mass is driven with an AC servomotor whose angular position is accurately controlled with reference to a very accurate global positioning system (GPS) clock. The error of the mass' position is less than 0.002 radian and does not accumulate. As a result, the source generates sinusoidal waves of very narrow spectral peaks enabling their detection with an excellent signal-to-noise ratio. Although the stability of the rotation is quite excellent, a large daily variation was found, which seems to be caused by changes in atmospheric temperature. The daily variation was 10% in amplitude and 0.1 radian in phase of the signal observed at the 800 m borehole seismometer. A significant variation was found to be due to that of coupling between the rotational source and the foundation made of reinforced concrete in which the source was situated. In order to make a correction on the signal of the 800 m borehole seismometer, the vibration of the foundation was measured and modeled assuming a rigid body movement. The correction successfully reduced the daily variation by approximately 90%, resulting in a variation of 1% in amplitude and 0.01 radian in phase. The phase variation of 0.01 radian corresponds to 100 ms and less than 0.1% in velocity over 1000 m between the source and the receiver.

The 1986 fissure eruption of Izu‐Oshima volcano was preceded and followed by strong volcanic tremor activities. The post‐eruption tremor activity consists of an alternating sequence of a tremor episode lasting about 10 min and the subsequent quiescence of about 40–50 min. We observed this post‐eruption activity by a 24‐channel seismic array deployed on the caldera floor. The semblance analysis of the recorded data revealed that high frequency (>5Hz) components of the tremor were radiated from two separated sources beneath the caldera. The tremor first occurred at one source and then migrated to the other source in a transition time of about a few minutes during each tremor episode. This transition suggests that the two sources are dynamically linked with each other in a magma‐filled crack or conduit.

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.

First testing of volcanic activity monitoring with a system of continuously operatable seismic sources, named ACROSS, was started at Sakurajima Volcano, Japan. Two vibrators were deployed on the northwestern flank of the volcano, with a distance of 3.6 km from the main crater. We successfully completed the testing of continuous operation from 12 June to 18 September 2012, with a single frequency at 10.01 Hz and frequency modulation from 10 to 15 Hz. The signal was detected even at a station that is 28 km from the source, establishing the amplitude decay relation as a function of distance in the region in and around Sakurajima Volcano. We compare the observed amplitude decay with the prediction that was made before the deployment as a feasible study. In the prediction, we used the existing datasets by an explosion experiment in Sakurajima and the distance-dependent amplitude decay model that was established for the ACROSS source in the Tokai region. The predicted amplitude in Sakurajima is systematically smaller than that actually observed, but the dependence on distance is consistent with the observation. On the basis of the comparison of the noise level in Sakurajima Volcano, only 1-day stacking of data is necessary to reduce the noise to the level that is comparable to the signal level at the stations in the island.