Predicting where quakes will strike

EVEN AS THE Japanese authorities struggle to cope with the humanitarian disaster caused by last Friday’s major earthquake, that event may have set the stage for an even worse disaster. The possibility of a major tremor under the seabed close to Tokyo has become a real danger, according to an Irish geophysicist.

“The quake has made a major earthquake more likely for Tokyo,” the University of Ulster’s professor of geophysics Prof John McCloskey said this week as the current tragedy played out before us in the media. “For some time people have been afraid that Tokyo was due, or at least was a high risk area, for another earthquake.

“Preliminary work has shown that possibility has become more likely as a result of the [Sendai] earthquake.”

Prof McCloskey, of UU’s school of environmental sciences, isn’t into making idle predictions. He studies in detail how geological fault lines change as a result of an earthquake and then isolates where the next big event is most likely to occur.

He has a record of success in being able to do this following the terrible St Stephen’s Day quake in 2004 off the coast of Sumatra. He analysed the fault lines afterwards and, in a report in the journal Nature, said that the stresses had been moved further along the Sumatran fault line.

He predicted that another large quake, probably up to a magnitude 8.5, would occur at a given point along the fault, with the report going out in the journal on 17th March, 2005. Just 11 days later an 8.7 magnitude quake occurred where he had predicted.

He is currently sharing data with colleagues in the US and elsewhere both to define what happened geologically with the Sendai quake and to analyse where the stresses have changed along the massive fault line that runs south deep under the seabed off Japan’s east coast.

Last Friday’s quake and the hundreds of sometimes quite powerful aftershocks that followed were clustered around a stress point about 100km off the large city of Sendai and about 35km under the sea floor, he says.

The stress arises because the Earth’s crust is made up of large plates that move across the planet’s surface, crushing and grinding against one another. Two of the world’s biggest plates, the Eurasian and Pacific, meet along a fault line that runs from Alaska all the way down to the Philippines.

The Pacific plate is subducting, or slowly sliding, under the Eurasian and stress builds as they advance. Friction between rock surfaces stops movement but only until the stress becomes so great that they break free and lurch forward. This movement was huge in the case of the Sendai quake.

“I expect it was somewhere between 25 and 30 metres,” with early figures suggesting a slip along hundreds of kilometres along the fault, Prof McCloskey says. Once the fault breaks, the movement is very quick, running along the fault at two or three kilometres a second. And because movement on this fault line usually has a vertical component it is always likely to kick off an associated tsunami.

Prof McCloskey and other geophysicists use information from the results of a quake to understand what happens underground. This includes knowing its strength but also the nature of the tsunami it kicked off. Only a major quake could have thrown up 10m waves of such power, he suggests.

They can also measure where movement has not occurred, indicating that stress at that point will have increased rather than diminished because of movement further up along the fault line. This new stress point, where pressure will build, is probably about 200km south of Sendai, along a part of the fault much closer to Tokyo, Prof McCloskey estimates.

More work is needed to confirm where the stresses will mount and what the pressures in the rock are like so deep under the seabed. But while they can isolate a location, geophysicists can’t make reliable predictions on when the next big earthquake will occur, Prof McCloskey says.

The movement of the plates is relentless however and the stresses will build until the next quake occurs to relieve them. If a similar magnitude 9 event occurs so close to Tokyo it will have a regional as well as a global impact given Tokyo’s central role in the world economy, Prof McCloskey says.