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Editorial: “Time to rethink megaquakes“

The Tohoku earthquake is likely to be the best-studied of all quakes, and the first analyses suggest we may have to rethink how megathrust quakes happen

THE Tohoku earthquake that rocked Japan last month has sent geologists reeling. As the first analyses of what may well become the best studied earthquake in history start to filter through, there is already talk of rewriting the rule book on how “megathrust” quakes happen. And all countries around the world that sit on subduction zones may now need to reconsider whether they are at risk of a similar devastating event.

At the annual meeting of the Seismological Society of America in Memphis, Tennessee, on 14 April, geologists from around the world presented early analyses of the magnitude-9 megathrust earthquake. “Few earthquakes of this size have been subjected to this kind of intense post-mortem,” says in Evanston, Illinois. “The Japanese have an incredibly dense network of GPS and seismic coverage.”

The findings, which describe the event in unprecedented detail, have convinced some that they must throw away standard theoretical frameworks, which simply cannot explain how the Earth flinched and heaved beneath the ocean floor near Japan. The event, they say, demands that we change not only our scientific understanding of large subduction quakes, known as “megaquakes”, but also our assessment of the regions around the world at risk of such events.

“There are many things we thought we knew and it’s now painfully clear we just don’t,”says Okal. , director of the Berkeley Seismological Laboratory at the University of California, agrees: “A lot of ideas will be shattered because of this quake.”

of Harvard University and his colleagues studied measurements by the – a fleet of 400 high-end seismographs dotted around North America – as the energy released by the quake rippled around the world. They found the quake’s rupture behaviour to be far more complex than any other.

Typically a subduction earthquake – in which one tectonic plate pushes beneath another – rips in one or two directions along a fault: on a north-south fault line, say, a rupture heads north, south, or north and south at the same time. But Kiser and his colleagues found that the Tohoku quake ripped left, right and centre along the fault like the starbursts of a fireworks display (see diagram).

“When we imaged the main shock, the propagation of energy was all over the place,” says Kiser. “We believe this is the most complex rupture behaviour ever observed.” The team reckons the pattern may partially explain why the quake was so ferocious.

Some fault zones are highly heterogeneous, with patches where the rock catches like Velcro and others where it slips as though oiled, says of Cornell University in Ithaca, New York. It is possible that the Japanese fault consists of a dangerous mixture of Velcro and “oily” patches. The Velcro keeps the plates glued together and absorbs the strain of subduction, but when they gave way in March, oily patches allowed the plates to slip all over the place – accounting for the complex rupture and boosting the quake’s energy release.

Kiser’s results have revealed that on 11 March, the bursts of energy ripped four separate patches that have all individually generated quakes in the past. More than any other factor, he thinks, this aggregate rupture accounts for the quake’s phenomenal size.

He points out that the entire rupture zone within which the starburst rips took place was about 40,000 square kilometres, far smaller than what might typically produce a magnitude-9 quake. That area becomes much larger – roughly 100,000 square kilometres – if you include the region covered by the hundreds of aftershocks that hit in the weeks following 11 March. Had this entire area ruptured on 11 March, the quake would have easily exceeded magnitude 9, Kiser says.

Back in Memphis, of the California Institute of Technology in Pasadena says that the relatively small zone that ruptured on 11 March can be split into two areas. His analysis has highlighted one rupture zone along the Pacific Ocean’s Japan trench, which he believes was largely responsible for the tsunami, and another rupture deeper along the fault line and closer to the coast that caused most of the shaking. “We have never seen anything like this,” he says (see diagram).

Takeshi Sagiya of Nagoya University in Japan and of the University of California at Santa Barbara presented analyses that point to another factor which contributed to the vast amounts of energy released by the event. Their latest estimates of how far the tectonic plates slid past one another suggest that at its maximum the slip was 60 metres – a figure so big that every researcher 91av contacted asked in astonishment for the figure to be repeated. Such a massive shift is unprecedented in the recorded history of earthquakes.

Kanamori says it’s possible that a structure on the sea floor, like a seamount, locked the plates together allowing stress to build up for thousands of years before it was released in one huge burst.

The huge slip happened on a fault that was not a candidate for a megathrust quake. Traditionally, young, hot, swiftly subducting plates are considered far more likely to produce megathrust earthquakes than their older, cooler, denser and more sluggish counterparts. The ocean crust off the north-east coast of Japan is about 140 million years old. It’s hardly the Usain Bolt of tectonic plates, yet it generated Japan’s largest recorded quake.

More remarkable still – the event may not be an exception, but could define a new rule. The subducting India plate that caused the 2004 Indian Ocean earthquake and tsunami is 80 to 90 million years old and is not particularly swift, says Okal. Yet it generated the third-largest earthquake ever recorded – a magnitude-9.2 event. “The standard models say give me the age of the subducting plates and the rate of subduction and I will tell you the maximum magnitude of an earthquake in that area. It says an old plate moving slowly can’t produce much beyond a magnitude 7,” says Okal. “This is now a model we essentially have to abandon.”

“What we have to realise now,” says Pritchard, “is that pretty much any subduction zone is a candidate for a magnitude-9 quake.” Romanowicz agrees: “As prepared as Japan was for earthquakes, it did not expect such a large quake in that particular place. This is the lesson to learn. It’s not just about Japan: many other places in the world could generate giant earthquakes in ways that people just aren’t paying attention to.”