After the eruption of Mt. Ontakesan Volcano in 2014, Ontakesan Volcano Laboratory, Nagoya University was established in 2017 to keep and develop the face-to-face relationship between the local ...community and volcano experts. In 2018, the Ontakesan Volcano Meister System also started to undertake activities for volcanic disaster management and promotion of the regional economy. Additionally, two visitor centers opened in Kiso Town (at the foot of Mt. Ontakesan) and Otaki Village (at the entrance of the trail to the summit) in 2022. We compared these activities in the Ontakesan area with other volcanic areas (Usuzan, Bandaisan, Hakoneyama, Fujisan, Asosan, Unzendake, and Sakurajima) from the perspective of literacy enhancement on volcanic disaster management. We made an interview survey of the organizations/facilities responsible for volcanic disaster prevention education in these volcano areas to evaluate the activity of the Ontakesan Voclano Meisters. We considered common and specific issues among them to clarify the characteristics of literacy enhancement for volcanic disaster reduction in the Ontakesan area. In all the organizations that we surveyed, there is a common emphasis on the education for children to transfer disaster memories to the next generation and to raise their awareness of disaster prevention. Though the Ontakesan Volcano Meisters have less interaction with the local residents than other areas, they exceed in the enlightenment for climbers and have made efforts to raise the safety awareness of climbers on site since their establishment.
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.
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.
Temporal variations in the P‐ and S‐wave travel times were continuously monitored using ACROSS, which is a newly developed system for utilizing accurately controlled continuous sinusoids for seismic ...exploration. We made an experiment lasting for 15 months at a site near the Nojima fault which ruptured during the 1995 Kobe earthquake (M7.2). The elastic waves emitted from the ACROSS vibrators located at the surface were observed with seismometers at 800 m and 1700 m depth. The long‐term variations of about 2 ms for both the P and S waves were observed during the 15‐month experiment. The major cause of these variations is identified to be that of a near‐surface property which changes the near‐source vibration. The variations in the deeper structures estimated after the reduction of the near‐source variation were about 0.5 ms. An S‐wave delay of about 1 ms was detected due to the 2000 Western Tottori earthquake (M6.6).
Abstract
We report the results of continuous monitoring—using a seismometer array—of the travel time of seismic waves generated by an ACROSS artificial seismic source. The seismometer array, which ...was deployed in a surface vault located 2.4 km from the source, recorded both direct
P
- and
S
-waves and refracted
P
- and
S
-waves that traveled along a velocity boundary between the granite basement and overlying sedimentary rocks. We analyzed temporal variation in differential travel time and apparent velocity for these phases for a period of 1 month and found significant temporal variation in the differential travel time. Most of the variation can be attributed to changes in environmental conditions, such as atmospheric temperature and rainfall. Variation is even observed in the seismogram that is located 50 m from the vibration source, although much smaller variation is observed in the vibration of the foundation to which the source is attached. The spectral study revealed that the effects of temperature and rainfall depend strongly on the frequency range used by ACROSS and that a large variation occurs in the 15- to 20-Hz range, especially between 17 and 20 Hz. The environmental effect on the temporal variation is comparable to the record of refracted
S
waves and that of a distance of 50 m, whereas a larger variation was observed in the direct
S
wave. This result shows that the signal is affected by the environmental change near the vibration source. The environmental effect can be drastically reduced when the signal from the 15- to 20-Hz range is eliminated in the analysis.
NIED Hi-net is the nation-wide high sensitivity seismograph network which has been constructed by the National Research Institute for Earth Science and Disaster Prevention. The high quality ...observation data of the Hi-net brought us outstanding contribution to seismology. Recently, the detection of minute changes of seismic travel times with the use of the Hi-net data is tried by various methods such as seismic ACROSS (Accurately Controlled, Routinely Operated, Signal System), seismic interferometry and so on. Although time accuracy is essential for precise measurement of temporal variations of seismic travel times, reliability evaluation of the sampling timing of the Hi-net data has been not always sufficient so far. This paper reports the temporal variations of the sampling timings at the twenty Hi-net stations for the eight years from 2004 to 2012 and proposes an approach to detect minute changes in seismic travel times in sub-millisecond order. The sensor check signal which is usually used to calibrate the seismometer, is enabled on 1PPS of GPS receiver independently of the sampling clock. Therefore the sampling clock can be calibrated by the GPS clock using the delay time of the sensor check signal. The delay times are usually stable, but stepwise changes were observed at restarts of ADC (Analog-to-Digital Converter). This method was applied to the TOKI seismic ACROSS signals observed by Hi-net Yaotsu (11.3 km distance from TOKI). Consequently, sub-millisecond to several milliseconds stepwise travel time changes were removed, and travel time changes owing to large earthquakes, and annual and secular travel time changes became clear. This correction method is useful to not only the seismic source-receiver system such as ACROSS but also other methods to detect seismic travel time changes of utilizing arrival time difference between observation stations.
Temporal variations of Green functions associated with the eruptive activity at Sakurajima Volcano, Japan, were estimated using an accurately controlled routinely operated signal system (ACROSS). We ...deconvolved 400 s waveforms of the ACROSS signal at nearby stations by a known source time function and stacked the results based on the time relative to individual eruptions and the eruption intervals; the quantities obtained by this procedure are Green functions corresponding to various stages of the eruptive activity. We found an energy decrease in the later phase of the Green functions in active eruptive periods. This energy decrease, localized in the 2–6 s window of the Green functions, is difficult to explain by contamination from volcanic earthquakes and tremors. The decrease could be more reasonably attributed to a subsurface structure change caused by the volcanic activity.
Key Points
We computed and stacked the Green functions using ACROSS at Sakurajima
We compared the Green functions with the eruptive activity
A decrease of the later phase energy in active periods was observed
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