01. 2014年7月18日 09:00:24
Japan earthquake has raised pressure below Mount Fuji, says new study
Geological disturbances caused by 2011 tremors mean active volcano is in a 'critical state', say scientific researchers
Pierre Le Hir
Guardian Weekly, Tuesday 15 July 2014 13.59 BST
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Proximity of Mount Fuji to the epicentre of the March 2011 earthquake. Christine Oliver/Guardian Source: Le Monde
Mount Fuji, or Fujisan as it is known in Japanese, is the highest point on the archipelago (rising to 3,776 metres) and the national emblem, immortalised in countless etchings. In June last year Unesco added it to the World Heritage list as a "sacred place and source of artistic inspiration". But it is still an active volcano, standing at the junction between the Pacific, Eurasian and Philippine tectonic plates. Though it has rarely stirred in recorded history, it is still potentially explosive.
The Tohoku – or Great East Japan – earthquake on 11 March 2011 triggered a devastating tsunami, which in turn caused the Fukushima Daiichi nuclear disaster. According to a Franco-Japanese study published by Science (PDF), the magnitude-9 tremor also increased the pressure on Mount Fuji. "Our work does not say that the volcano will start erupting, but it does show that it's in a critical state," says Florent Brenguier, a researcher at the Institute of Earth Sciences (IST) in Grenoble, France, and lead author of the publication, to which the Institute of Global Physics (IPG) in Paris also contributed.
Adopting a novel approach, the scientists carried out a sort of giant echo-scan of the bowels of the Earth, based on the huge mass of data recorded after the mega-quake by Japan's Hi-net system, the densest network in the world, with 800 seismic sensors. They focused on signals commonly known as seismic noise, the result of constant interaction between ocean swell and "solid" earth. In the past such data has generally been dismissed as background interference.
By recording fluctuations in this barely perceptible subterranean noise they were able to map geological disturbances in the bedrock of Japan caused by the seismic waves following the violent quake in March 2011. "Seismic waves travel a very long way, going round the world several times," Brenguier explains. "Their movement makes the Earth's crust vibrate, and rather like a shock wave this produces breaks or cracks in the rock."
Mount Fuji summit
A snow-capped Mount Fuji. A Franco-Japanese study of seismic data suggests pressure is building for a new eruption. Photograph: The Asahi Shimbun/Getty
One might well imagine that such disturbance is greatest close to the epicentre of a quake, but this is not the case. The Franco-Japanese study shows that the area where the Earth's crust suffered the greatest damage was not around Tohoku, in the north-east of Honshu island, but in the volcanic regions, in particular under Mount Fuji, some 400km away. "The volcanic regions are the ones where the fluids trapped in the rock – boiling water, gas, liquid magma, which cause an eruption when they rise to the surface – exert the greatest pressure. The seismic waves add to this pressure, causing even more disturbance," Brenguier says.
The magnitude 6.4 quake that occurred four days after the tsunami, followed by many smaller aftershocks, was a further indication that Mount Fuji is under high pressure.
So should Japan be on red alert? "We cannot establish a direct relation of cause and effect between quakes and volcanic eruptions, even if statistically the former lead to an increase in the latter," Brenguier says. "All we can say is that Mount Fuji is now in a state of pressure, which means it displays a high potential for eruption. The risk is clearly higher."
Science, however, has no way of predicting when this might happen. But there is a precedent. The last eruption of Mount Fuji occurred in 1707. It projected almost a billion cubic metres of ash and debris into the atmosphere, some of which reached Tokyo (then called Edo) 100km away. It was preceded, 49 days earlier, by a magnitude 8.7 quake to the south of Japan that, in conjunction with the tidal wave it raised, claimed more than 5,000 lives. This time, more than three years have already passed since the Tohoku quake. But that does not mean that Mount Fuji, under the constant supervision of Japanese geologists, is slumbering.
Come what may, the method developed by the Franco-Japanese team for investigating volcanic areas should improve the accuracy of efforts all over the world to assess the risk of major volcanic eruptions.
This story appeared in Guardian Weekly, which incorporates material from Le Monde
Mount Fuji, in addition to being a picturesque landmark and an important part of Japanese culture, is also an active volcano. Its last eruption was just over 400 years ago, but its location ― where the Eurasian, Pacific, and Philippine tectonic plates meet ― mean it will always have potential for eruption. A new study (PDF) has examined the pressures around Mount Fuji in the wake of several recent earthquakes, including the magnitude 9 tremor that unleashed the destructive tsunami in 2011. The researchers now say the volcano is in a "critical state." According to the study's lead author, "The volcanic regions are the ones where the fluids trapped in the rock – boiling water, gas, liquid magma, which cause an eruption when they rise to the surface – exert the greatest pressure. The seismic waves add to this pressure, causing even more disturbance." They have no way of predicting when an eruption might happen, but the potential seems greater than ever.
Giant earthquakes help predict volcanic eruptions
Paris, 4 July 2014
Researchers at the Institut des Sciences de la Terre (CNRS/Université Joseph Fourier/Université de Savoie/IRD/IFSTTAR) and the Institut de Physique du Globe de Paris (CNRS/Université Paris Diderot/IPGP), working in collaboration with Japanese researchers, have for the first time observed the response of Japanese volcanoes to seismic waves produced by the giant Tohoku-oki earthquake of 2011. Their conclusions, published in Science on July 4, 2014, reveal how earthquakes can impact volcanoes and should help to assess the risk of massive volcanic eruptions worldwide.
To download the press release: Seismes géants
Mapping pressurized volcanic fluids from induced crustal seismic velocity drops. Brenguier, F., Campillo, M., Takeda, T., Aoki, Y., Shapiro, N.M., Briand, X., Emoto, K., & Miyake, H. Science, 4 July 2014.
Mapping pressurized volcanic fluids from induced crustal seismic velocity drops
F. Brenguier1,*, M. Campillo1, T. Takeda2, Y. Aoki3, N. M. Shapiro4, X. Briand1, K. Emoto2, H. Miyake3
+ Author Affiliations
1Institut des Sciences de la Terre, Université Joseph Fourier, CNRS, F-38041 Grenoble, France.
2National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan.
3Earthquake Research Institute, University of Tokyo, Tokyo, Japan.
4Institut de Physique du Globe de Paris, Sorbonne Paris Cité, CNRS (UMR7154), 75238 Paris Cedex 5, France.
↵*Corresponding author. E-mail: email@example.com
Volcanic eruptions are caused by the release of pressure that has accumulated due to hot volcanic fluids at depth. Here, we show that the extent of the regions affected by pressurized fluids can be imaged through the measurement of their response to transient stress perturbations. We used records of seismic noise from the Japanese Hi-net seismic network to measure the crustal seismic velocity changes below volcanic regions caused by the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki earthquake. We interpret coseismic crustal seismic velocity reductions as related to the mechanical weakening of the pressurized crust by the dynamic stress associated with the seismic waves. We suggest, therefore, that mapping seismic velocity susceptibility to dynamic stress perturbations can be used for the imaging and characterization of volcanic systems.
In Science Magazine
Shaking up volcanoes
Stephanie G. Prejean, Matthew M. Haney
Science 4 July 2014: 39.
Large volcanic eruptions are preceded by long-term pressure buildup in volcano magmatic and hydrothermal systems. Therefore, knowledge of the extent and state of these pressurized volcanic fluids at depth will help in the better anticipation of future eruptions. In particular, seismic tomography is often used to delineate volcano-feeding systems at different scales (1, 2). However, a major difficulty of traditional seismic imaging of volcanoes is that the geological contrasts of the host rock might dominate the final tomographic images, which are only partially sensitive to the content and state of volcanic fluids (3).
Recent geodetic observations have shown that volcanic areas are characterized by anomalous responses to crustal deformation induced by large earthquakes, as demonstrated by the subsidence of volcanoes in Chile and Japan after the 2010 Maule and 2011 Tohoku-Oki earthquakes (4, 5). This sensitivity to strong coseismic deformation and shaking is probably associated with the presence of pressurized hydrothermal and magmatic fluids at depth in a fractured medium. We explored the responses of volcanoes to transient stress perturbations by investigating the temporal evolution of crustal seismic velocities in Japan in response to the seismic shaking and deformation caused by the March 2011 moment magnitude (Mw) 9.0 Tohoku-Oki earthquake.
The Hi-net, Japanese high-sensitivity seismograph network, is among the densest in the world; thus, the 2011 Tohoku-Oki earthquake remains the best-recorded large earthquake to date. It was associated with large, widespread static ground deformation and ground shaking (Fig. 1). In this study, we used seismic noise-based monitoring (6) to characterize the response of the upper crust to the coseismic shaking and deformation caused by the earthquake. We analyzed 1 year of continuous seismic records from a portion of the dense Hi-net seismic network (600 stations, as shown in the inset to Fig. 1A), spanning from 6 months before to 6 months after the earthquake occurrence.
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Fig. 1 Static strain and ground shaking caused by the Tohoku-Oki earthquake.
(A) Modeled coseismic dilatation static strain at 5 km in depth (7). The red star shows the position of the epicenter of the Tohoku-Oki earthquake. (Inset) Positions of the Hi-net seismic stations (red points). (B) Averaged peak ground velocity measured using the KiK-net strong-motion network (7).
We computed the daily vertical-vertical noise cross-correlation functions using a processing scheme that minimized the effects of the strong aftershock activity that followed the Tohoku-Oki earthquake (7). To avoid the choice of an arbitrary reference cross-correlation function and to improve the precision of the measurements, we separately computed velocity changes for all of the possible daily cross-correlation functions for each station pair. Using a Bayesian least-squares inversion, we retrieved accurate daily continuous velocity change time series for every station pair (7).
We computed the seismic velocity changes averaged over the day of the Tohoku-Oki earthquake and 4 days after, relative to the seismic velocity changes time series averaged over 6 months before the Tohoku-Oki earthquake (Fig. 2A) (7). These changes mainly correspond to the response of the upper crust to the coseismic shaking and deformation. Similar to previous studies of coseismic velocity variations (6, 8), a reduction in velocity was widespread over Honshu Island. Furthermore, the strongest velocity drops were not observed in the area closest to the epicenter or within large sedimentary basins, as would be expected. The patterns of the observed velocity reductions did not correlate with the intensity of the ground shaking or with the coseismic deformation (Fig. 1); instead, the strongest coseismic velocity reductions occurred under volcanic regions. In particular, a large part of the Tohoku volcanic front and the Mt. Fuji volcanic region are well delineated.
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Fig. 2 Crustal seismic velocity perturbations caused by the Tohoku-Oki earthquake.
(A) Coseismic crustal seismic velocity changes induced by the 2011 Tohoku-Oki earthquake. (Inset) Velocity changes averaged over 5 days preceding the day of the Tohoku-Oki earthquake. (B) Seismic velocity susceptibility computed using the seismic velocity changes shown in (A). Black triangles denote Quaternary period volcanoes, and the red line depicts the main volcanic fronts.
The mechanism by which seismic velocities decrease in response to stress perturbations is commonly described as related to the opening of cracks (9, 10), which might also induce an increase in permeability and a transfer of fluids at depth and may lead to further triggering of earthquakes. Over long distances, large earthquakes are known to trigger anomalous hydrothermal activity (11), aftershocks on a global scale (12), tectonic tremor activity (13), and slow-slip events (14). The origin for this remote triggering of activity is believed to be the associated dynamic stress that is caused by the passing of the seismic waves.
We used the approach of Gomberg and Agnew (15) to estimate the level of dynamic strain Δξ and dynamic stress Δσ from the observed peak ground velocity (PGV), such that Δξ ≈ ν/c and Δσ ≈ μν/c, where μ is the mean crustal shear modulus (~30 × 109 Pa), ν is the PGV (measured by the KiK-net, strong-motion seismograph network installed in boreholes together with the Hi-net sensors), and c is the mean wave phase velocity of the Rayleigh waves that propagate within the upper crust (~3 km/s). The dynamic strain caused by the passing of the seismic waves was one to two orders of magnitude higher than the static coseismic strain for Honshu Island. We thus conclude that the dynamic stress associated with the seismic waves emitted by the Tohoku-Oki earthquake was the main cause of the large seismic velocity reductions under the volcanic regions―in particular, the Mt. Fuji area, where the static stress change can be considered negligible. We then defined the seismic velocity susceptibility as the ratio between the observed reductions in the seismic velocity and the estimated dynamic stress. The distribution of these seismic velocity susceptibilities correlates with the main volcanic areas (Fig. 2B).
The sensitivity of the seismic velocity to stress changes in the rock increases with decreasing effective pressure (16, 17). Under volcanic areas, the effective pressure in the crust can be reduced because of the presence of highly pressurized hydrothermal and magmatic volcanic fluids at depth. We thus argue that the observed strong coseismic velocity reductions delineate the regions where such pressurized volcanic fluids are present in the upper crust. An important implication of our observation is that the seismic velocity susceptibility to stress can be used as a proxy to the level of pressurization of the hydrothermal and/or magmatic fluids in volcanic areas. So far, this susceptibility is greatest in the Mt. Fuji area and along the Tohoku volcanic arc, where it reached 15 × 10−4 MPa−1, whereas it is more than 10 times smaller for the Cretaceous stiff plutonic regions of eastern Tohoku (Fig. 2B).
Fluids are also known to have important roles in earthquake nucleation (3). The volcanic areas where large seismic velocity susceptibility was observed were also characterized by large triggered seismic activity after the Tohoku-Oki earthquake (18, 19), including a particularly strong (magnitude 6.4) earthquake that occurred 4 days after the main shock, near Mt. Fuji. This confirms that the crust in these areas is quite sensitive to strong transient stress perturbations. We argue that mapping the susceptibility of seismic velocities to dynamic stress changes can be used to image and characterize regions with low effective pressure, such as volcanic systems.