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 (
M
W
= 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.
Graphical Abstract
Abstract
We have developed a new continuous monitoring system based on small seismic sources and distributed acoustic sensing (DAS). The source system generates continuous waveforms with a wide ...frequency range. Because the signal timing is accurately controlled, stacking the continuous waveforms enhances the signal-to-noise ratio, allowing the use of a small seismic source to monitor extensive areas (multi-reservoir). Our field experiments demonstrated that the monitoring signal was detected at a distance of ~ 80 km, and temporal variations of the monitoring signal (i.e., seismic velocity) were identified with an error of < 0.01%. Through the monitoring, we identified pore pressure variations due to geothermal operations and rains. When we used seafloor cable for DAS measurements, we identified the monitoring signals at > 10 km far from the source in high-spatial resolution. This study demonstrates that multi-reservoir in an extensive area can be continuously monitored at a relatively low cost by combining our seismic source and DAS.
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.
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.
Following the 2009 L'Aquila earthquake, the Dipartimento della Protezione Civile Italiana (DPC), appointed an International Commission on Earthquake Forecasting for Civil Protection (ICEF) to report ...on the current state of knowledge of short-term prediction and forecasting of tectonic earthquakes and indicate guidelines for utilization of possible forerunners of large earthquakes to drive civil protection actions, including the use of probabilistic seismic hazard analysis in the wake of a large earthquake. The ICEF reviewed research on earthquake prediction and forecasting, drawing from developments in seismically active regions worldwide. A prediction is defined as a deterministic statement that a future earthquake will or will not occur in a particular geographic region, time window, and magnitude range, whereas a forecast gives a probability (greater than zero but less than one) that such an event will occur. Earthquake predictability, the degree to which the future occurrence of earthquakes can be determined from the observable behavior of earthquake systems, is poorly understood. This lack of understanding is reflected in the inability to reliably predict large earthquakes in seismically active regions on short time scales. Most proposed prediction methods rely on the concept of a diagnostic precursor; i.e., some kind of signal observable before earthquakes that indicates with high probability the location, time, and magnitude of an impending event. Precursor methods reviewed here include changes in strain rates, seismic wave speeds, and electrical conductivity; variations of radon concentrations in groundwater, soil, and air; fluctuations in groundwater levels; electromagnetic variations near and above Earth's surface; thermal anomalies; anomalous animal behavior; and seismicity patterns. The search for diagnostic precursors has not yet produced a successful short-term prediction scheme. Therefore, this report focuses on operational earthquake forecasting as the principle means for gathering and disseminating authoritative information about time-dependent seismic hazards to help communities prepare for potentially destructive earthquakes. On short time scales of days and weeks, earthquake sequences show clustering in space and time, as indicated by the aftershocks triggered by large events. Statistical descriptions of clustering explain many features observed in seismicity catalogs, and they can be used to construct forecasts that indicate how earthquake probabilities change over the short term. Properly applied, short-term forecasts have operational utility; for example, in anticipating aftershocks that follow large earthquakes. Although the value of long-term forecasts for ensuring seismic safety is clear, the interpretation of short-term forecasts is problematic, because earthquake probabilities may vary over orders of magnitude but typically remain low in an absolute sense (< 1% per day). Translating such low-probability forecasts into effective decision-making is a difficult challenge. Reports on the current utilization operational forecasting in earthquake risk management were compiled for six countries with high seismic risk: China, Greece, Italy, Japan, Russia, United States. Long-term models are currently the most important forecasting tools for civil protection against earthquake damage, because they guide earthquake safety provisions of building codes, performance-based seismic design, and other risk-reducing engineering practices, such as retrofitting to correct design flaws in older buildings. Short-term forecasting of aftershocks is practiced by several countries among those surveyed, but operational earthquake forecasting has not been fully implemented (i.e., regularly updated and on a national scale) in any of them. Based on the experience accumulated in seismically active regions, the ICEF has provided to DPC a set of recommendations on the utilization of operational forecasting in Italy, which may also be useful in other countries. The public should be provided with open sources of information about the short-term probabilities of future earthquakes that are authoritative, scientific, consistent, and timely. Advisories should be based on operationally qualified, regularly updated seismicity forecasting systems that have been rigorously reviewed and updated by experts in the creation, delivery, and utility of earthquake information. The quality of all operational models should be evaluated for reliability and skill by retrospective testing, and they should be under continuous prospective testing against established long-term forecasts and alternative time-dependent models. Alert procedures should be standardized to facilitate decisions at different levels of government and among the public. Earthquake probability thresholds should be established to guide alert levels based on objective analysis of costs and benefits, as well as the less tangible aspects of value-of-information, such as gains in psychological preparedness and resilience. The principles of effective public communication established by social science research should be applied to the delivery of seismic hazard information.
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.
The 2014 Mount Ontake eruption started just before noon on September 27, 2014. It killed 58 people, and five are still missing (as of January 1, 2016). The casualties were mainly caused by the impact ...of ballistic blocks around the summit area. It is necessary to know the magnitude of the block velocity and energy to construct a hazard map of ballistic projectiles and design effective shelters and mountain huts. The ejection velocities of the ballistic projectiles were estimated by comparing the observed distribution of the ballistic impact craters on the ground with simulated distributions of landing positions under various sets of conditions. A three-dimensional numerical multiparticle ballistic model adapted to account for topographic effect was used to estimate the ejection angles. From these simulations, we have obtained an ejection angle of
γ
= 20° from vertical to horizontal and
α
= 20° from north to east. With these ejection angle conditions, the ejection speed was estimated to be between 145 and 185 m/s for a previously obtained range of drag coefficients of 0.62–1.01. The order of magnitude of the mean landing energy obtained using our numerical simulation was 10
4
J.
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