A Disaster and a Warning—But of What?

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Science  23 Dec 2011:
Vol. 334, Issue 6063, pp. 1634
DOI: 10.1126/science.334.6063.1634

The great Tohoku earthquake has everyone, seismologists included, wondering where the next blow will come from.

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When it struck on 11 March, the great Tohoku earthquake jolted seismologists as much as it did the Japanese mainland. Historical records and instrumental observations had painted a picture of frequent but tolerable seismicity. Earthquakes of magnitude 7.5 or so had recurred every 30 to 40 years on that part of the offshore fault. More of the same was presumably in store.

There was, however, a hint of worse to come. Geologists digging in coastal areas had found a thin layer of sand washed 3 to 4 kilo meters inland by a tsunami in the year 869; they estimated that a magnitude-8.4 quake had produced the tsunami. But a decade after the initial discovery, researchers assessing the seismic risk of the region were only beginning to seriously consider that quakes much larger than any on record might be striking the coast at very long intervals.

Japan is the most intensively monitored region in the world, but its vaunted geophysical network gave no clear warning of the impending catastrophe. Instruments detected only the barest hint of a slow buildup of crustal stress that might be released in an earthquake. That is understandable in hindsight: The segment of fault that was accumulating the strain that would power the magnitude-9.0 (M9) quake was 150 kilometers offshore, in effect beyond the reach of land-based instruments.

The unforeseen cataclysm has everyone, seismologists included, wondering where the next blow will come from. Some seismologists are eyeing the next fault segment to the south, closer to Tokyo, which is now more highly stressed because of the March event. Japan's long historical record mentions no earthquakes in the region, and there is no offshore monitoring there, either.

Indeed, in November, an official Japan earthquake assessment committee announced that it foresees a 30% chance of an M9 quake occurring along that stretch of the fault in the next 30 years. It was the first time the committee had ever recognized the possibility of an M9 event occurring near Japan, although some seismologists question the accuracy and utility of assigning percentages and years to the forecast events. It is worrying that this section of the fault is closer to shore than it is along the Tohoku region. That means more intense shaking for buildings and bridges and less time to escape a tsunami.

Keeping the flame.

Lanterns memorializing victims of Japan's March earthquake and tsunami float on a river near Minami-Soma, Fukushima Prefecture, on 11 August.


Farther afield, seismologists are now wondering where else mega-quakes will strike. Clues could come from the suddenly energized study of paleotsunamis. By recognizing that buried sand layers were left by ancient tsunamis, geologists opened a new window on past tectonic activity. In Japan, by comparing deposits left by the March tsunami with those left in 869, scientists are coming to realize that the extent of the sand layer is not necessarily the same as the inland run of the waves, as previously thought. Investigations on the Sendai plain after the March tsunami found that sand extended only 62% of the distance the waves washed inland. This means the size of the 869 tsunami and the earthquake that caused it may have been grossly underestimated.

Despite its magnitude, the focus of the March quake was so far offshore it caused surprisingly little damage to structures on land. Built to exacting codes, buildings and bridges generally stood up to the shaking while occupants and contents got tossed about. The experience at Tohoku University in Sendai was a lesson in earthquake preparedness: The chemistry department deliberately made hallways safe havens by insisting that no chemicals, heavy objects, or obstructions be stored there. Following disaster plans, researchers dashed to the halls when the first tremors hit and waited there while fume hoods, air conditioners, and chemicals rained down on lab benches. None of the buildings on the university's main Sendai campuses collapsed, although several suffered damage. Research resumed fairly quickly.

But, as throughout the region, it was the tsunami that exacted a toll. The two buildings of Tohoku's Marine Science Laboratory were among the 80% of structures in the coastal town of Onagawa either washed away or rendered useless by the tsunami. Decades' worth of culture collections and research data—backed up within the center—disappeared.

The tsunami also washed away Japan's faith in the safety of nuclear power as multiple backup safety systems at the Fukushima Daiichi Nuclear Power Plant proved no match for the waves. The massive release of radiation resulting from core melting and hydrogen explosions necessitated the evacuation of over 100,000 area residents and required heroic efforts from thousands of emergency workers to bring the crippled reactors under control.

The fallout reached Tokyo, both literally and figuratively. Japan is rethinking its nuclear-centric energy policy and its commitment to research projects such as the Monju fast breeder reactor, located in Japan, and the ITER fusion reactor, now being built in France.

The disaster exposed another fault line in society: between the public and the scientific community. There is a gnawing feeling among scientists that they failed to provide the advice policymakers and the public needed and wanted, both in advance of the catastrophe and in the heat of the crisis. Japanese scientists are now mulling how to bridge the gap.

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