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Tokyo's fault line just below the surface

July 14, 2005 By Michael Hopkin This article courtesy of Nature News.

New survey shows earthquake danger is closer than thought.

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The fault line that runs beneath Tokyo is kilometres closer to the surface than seismologists realized, according to a new survey. That could be worrying news to residents waiting for the next 'big one' to hit the city, as quakes occurring at a shallow depth tend to cause more shaking and damage to buildings than deeper ones.

Tokyo is among the most densely populated earthquake zones in the world, with some 33 million people living and working in the conurbation. In 1923, around 105,000 people in the city were killed by a huge magnitude 7.9 quake.

Shallower geometry tends to produce more intense shaking.
Hiroshi Sato
University of Tokyo, Japan
Occasional quakes continue to plague the city, thanks to the fact that it sits on a fault line where two rocky plates meet: one holds Honshu Island, while another - the Philippine Sea plate - slides underneath it.

As a result of the seismic activity, the area is perhaps the best-monitored earthquake region in the world, with an intricate network of global positioning sensors that detect tiny seismic shifts.

But before now, seismologists had not accurately determined the depth of Tokyo's fault line. This could be because jolts occurring at the interface where the two plates grind together are quite rare, says Hiroshi Sato of the University of Tokyo's Earthquake Research Institute, who led the new study.

Analyses suggest that the many of the big quakes in the region instead have epicentres embedded more deeply in the lowermost plate, causing researchers to surmise that the fault line itself was deeper than it is.

Shake it up

Sato and his team built up a picture of the Tokyo fault line by sending vibrations into the ground from trucks, air guns and explosives. They examined the pattern of the waves that bounced back to determine the composition of the rocks beneath.

The fault line itself lies between 4 and 26 kilometres below the city, the researchers report in this week's Science1. Previous estimates had put the depth at between 20 and 40 kilometres.

This could mean that an earthquake occurring at the fault line could be more damaging than previously thought. "It is expected that shallower geometry produces more intense shaking," says Sato. However, he adds, ground motion is also controlled by factors such as soil type and the precise direction in which the rock slips.

Other experts also caution that the study will have to be validated. It could be that Sato saw waves bouncing off an offshoot of the fault running closer to the surface, for example, rather than the main fault itself.

Ready for the jolt

But knowing more about the fault will certainly help in predicting the effects of future quakes, Sato adds. "Before our survey, in spite of the sophisticated monitoring system, the basic structure was not well determined," he says. "Now we know its precise geometry, we will be able to better analyse its behaviour."

The study may even help to predict the timing of the next large jolt, Sato hopes. By calculating how far the fault slipped in the 1923 event, and comparing this to the Philippine plate's average speed of advance and rate of stress build-up, researchers could estimate the average frequency of quakes.

"We are advancing step by step," Sato says.


  1. Sato H., et al. Science, 309. 462 - 464 (2005).


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