地震が連動しなかったのだから、ひずみエネルギーが残っていようと、蓄積されていようと、すぐには開放されるような規模のものではないと考えるのが自然だと思うが。とかく目立ちたがり屋が多そうなので、今ひとつ信頼できない。
数百年にも亘って溜まったエネルギーがなにを切っ掛けに地震を発生させるかが、ポイントだろう。それを類推するのでなければ、何の役にもたたない。

だから地震予測はほとんど意味がなく、事前の備えと早期検出が重要

Science 20 May 2011:
Vol. 332 no. 6032 p. 911
DOI: 10.1126/science.332.6032.911
NEWS FOCUS
SEISMOLOGY
New Work Reinforces Megaquake's Harsh Lessons in Geoscience
Richard A. Kerr
High-tech analyses of Japan's March earthquake overturn long-held views of fault behavior and warn that another disaster may be looming.

The moment the Tohoku-Oki earthquake struck off northern Japan on 11 March, many researchers knew their expectations had been shattered. The great offshore fault could not be counted on to behave at all predictably. And using onshore observations to gauge whether an offshore fault is building toward failure has grave limitations.

Now three papers (http://scim.ag/MSimons, http://scim.ag/S-Ide, and http://scim.ag/M-Sato) published online this week in Science help show why the inevitable release of seismic energy failed to play out as expected and why monitoring from afar fell short. The papers also point to a possible huge quake to the south, closer to Tokyo. Seismologists are concerned, says Mark Simons of the California Institute of Technology in Pasadena, but they are also now acutely aware of their limitations. “We have no idea what's going on” to the south, he says, but they're anxious to find out.

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A game of ring toss. March's huge quake (yellow contours) and past smaller quakes (colored loops) have left a patch of threatening fault (question mark).
CREDIT: ADAPTED FROM MARK SIMONS ET AL., SCIENCE (2011)
Many seismologists had thought that the offshore fault north of Tokyo was fairly simple and uniform. The ocean plate diving beneath Japan, the thinking went, should stick and slowly build up enough stress to rupture the fault. And the fault should fail segment by segment in large but not huge earthquakes. That's how the fault seemed to have behaved in recent centuries, with quakes of magnitude 7 to 8 or so popping off on any one segment every few decades or few centuries.

But it turns out that the fault has more than one mode of operation. The three Science papers gauge where and by how much the fault slipped in the 11 March magnitude-9.0 quake. Simons and his colleagues combined seismic data recorded around the world, crustal movements on Japan recorded by GPS, and tsunami waves recorded at buoys at sea. Satoshi Ide of the University of Tokyo and colleagues compared the seismic signature of the magnitude 9 with that of its largest foreshock, a magnitude 7.3. And Mariko Sato of the Japan Coast Guard in Tokyo and colleagues actually measured the motion of the sea floor before and after subsea GPS observations. “You can't have a better recorded earthquake,” says David Wald of the U.S. Geological Survey in Golden, Colorado, who was not involved in any of the studies.

The three studies and unpublished estimates by other groups suggest that during the quake, the descending ocean plate and the overlying plate carrying Japan slipped past each other by as much as 50 to 60 meters. “Those are enormous slips,” Wald says, running about two to three times the maximum slip reported for the magnitude-8.8 Maule, Chile, quake of last year. But the pattern of slip is equally striking. Five contiguous segments of the fault spanning more than 600 kilometers broke at once in the quake, rather than one or at most two, as scientists had assumed. But only two central segments experienced extreme slip, and that high slip was concentrated far offshore on the shallower part of the fault (within the figure's yellow contours). Historic quakes had broken short segments of the deeper part of the fault nearer land (the loops of various colors).

Obviously, the fault is more complicated than most researchers had assumed. Simons and his colleagues suggest that some irregularity on the fault is to blame. Something―perhaps a seamount on the sinking plate―pinned the high-slip patch of fault in place for 500 or 1000 years, they argue, while patches around it failed repeatedly in smaller quakes. The apparent absence of quakes in the stuck patch led many seismologists to assume that the fault there could be slowly but steadily slipping without building up any strain. And their only means of monitoring the buildup of strain on the fault―GPS measurements of slow ground movement on land―was greatly handicapped by having the stuck patch 150 kilometers offshore. With such a limited perspective on the past release and the current buildup of strain, a magnitude-9 quake caught researchers by surprise.

Learning that most of the March megaquake's slip was concentrated on two segments makes scientists more worried about other faults around the Pacific. “If you can get a 9 that is this compact,” Wald says, “it increases the number of places you can [fit in] a 9 where you may not have expected one.”

All eyes are now on the southern portion of the length of fault that broke in the Tohoku quake. Neither historical quakes nor the Tohoku quake has broken the offshore, shallow portion of the fault there. And the Tohoku rupture transferred stress southward along the fault, abruptly increasing the stress there. As had been the case to the north, researchers can't say for sure whether that portion of the fault (marked by the question mark) has been freely slipping without generating quakes or locked and building strain toward a quake.

If the offshore southern portion is indeed stuck, Simons and colleagues see “the possibility of a sibling to the 2011 event” that could be “similar to what just occurred offshore,” but half as far from Tokyo. So researchers are anxious to find out whether the stress transferred southward from the 9 has accelerated slow slip on the fault and thus defused the threat of a quake. If the fault isn't slipping, another quake would be in the works. Speed is of the essence: A magnitude-8.7 sibling quake followed the 2004 Indian Ocean megaquake by 3 months.

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Displacement Above the Hypocenter of the 2011 Tohoku-Oki Earthquake
Mariko Sato, Tadashi Ishikawa, Naoto Ujihara, Shigeru Yoshida, Masayuki Fujita, Masashi Mochizuki, and Akira Asada
Science 19 May 2011: 1207401Published online 19 May 2011
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The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries
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Science 19 May 2011: 1206731Published online 19 May 2011
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Shallow Dynamic Overshoot and Energetic Deep Rupture in the 2011 Mw 9.0 Tohoku-Oki Earthquake
Satoshi Ide, Annemarie Baltay, and Gregory C. Beroza
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Published Online 19 May 2011
< Science Express Index
Science DOI: 10.1126/science.1206731
REPORT
The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries
Mark Simons1,*, Sarah E. Minson1, Anthony Sladen1,2, Francisco Ortega1, Junle Jiang1, Susan E. Owen3, Lingsen Meng1, Jean-Paul Ampuero1, Shengji Wei1, Risheng Chu1, Donald V. Helmberger1, Hiroo Kanamori1, Eric Hetland4, Angelyn W. Moore3, and Frank H. Webb3
+ Author Affiliations

1Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
2Geoazur, Observatoire de la Côte d’Azur, Université de Nice–Sophia Antipolis, CNRS, IRD, Valbonne, 06103 Nice Cedex 2, France.
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
4Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
*↵To whom correspondence should be addressed. E-mail: simons@caltech.edu
ABSTRACT

Geophysical observations from the 2011 Mw 9.0 Tohoku-Oki, Japan, earthquake allow exploration of a rare large event along a subduction megathrust. Models for this event indicate that the distribution of coseismic fault slip exceeded 50 m in places. Sources of high-frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip. Relative to the Mw 8.8 2010 Maule, Chile, earthquake, the Tohoku-Oki earthquake was deficient in high-frequency seismic radiation―a difference that we attribute to its relatively shallow depth. Estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event.

Published Online 19 May 2011
< Science Express Index
Science DOI: 10.1126/science.1207401
BREVIA
GEOPHYSICS
Displacement Above the Hypocenter of the 2011 Tohoku-Oki Earthquake
Mariko Sato1,*, Tadashi Ishikawa1, Naoto Ujihara1, Shigeru Yoshida1, Masayuki Fujita1, Masashi Mochizuki2, and Akira Asada2
+ Author Affiliations

1Hydrographic and Oceanographic Department, Japan Coast Guard, 5-3-1 Tsukiji, Chuo-ku, Tokyo 104-0045, Japan.
2Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.
*↵To whom correspondence should be addressed. E-mail: eisei@jodc.go.jp
ABSTRACT

The moment magnitude (Mw) = 9.0 2011 Tohoku-Oki megathrust earthquake occurred off the coast of northeastern Japan. Combining Global Positioning System (GPS) and acoustic data, we detected very large seafloor movements associated with this event directly above the focal region. An area with more than 20 meters of horizontal displacement―that is, four times larger than those detected on land―stretches several tens of kilometers long along the trench; the largest amount reaches about 24 meters toward east-southeast just above the hypocenter. Furthermore, nearly 3 meters of vertical uplift occurred, contrary to observed terrestrial subsidence.

Published Online 19 May 2011
< Science Express Index
Science DOI: 10.1126/science.1207020
REPORT
Shallow Dynamic Overshoot and Energetic Deep Rupture in the 2011 Mw 9.0 Tohoku-Oki Earthquake
Satoshi Ide1,*, Annemarie Baltay2, and Gregory C. Beroza2
+ Author Affiliations

1Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan.
2Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, CA 94305–2215, USA.
*↵To whom correspondence should be addressed. E-mail: ide@eps.s.u-tokyo.ac.jp
ABSTRACT

Strong spatial variation of rupture characteristics in the Mw 9.0 Tohoku-Oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed. Finite-source imaging reveals that the rupture consisted of a small initial phase, deep rupture for up to 40 s, extensive shallow rupture at 60 to 70 s, and continuing deep rupture lasting over 100 s. A combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench. Normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault. Despite prodigious total slip, shallower parts of the rupture weakly radiated at high frequencies, whereas deeper parts of the rupture radiated strongly at high frequencies.

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すきだねぇ。