for National Geographic News
Scientists have found that a major, earthquake-producing fault that runs beneath metropolitan Tokyo is closer to the surfaceand potentially more dangerousthan previously thought.
The boundary between two key tectonic plates just south of Tokyo was thought to be about 12.5 miles to 25 miles (20 to 40 kilometers) below the surface.
But Japanese researchers have found that the fault is no more than 16 miles (26 kilometers) from the surface, and in some places it's only 2.5 miles (4 kilometers) beneath the city.
This closer proximity means that Tokyo could be more susceptible to the effects of earthquakes than previously thought. Stronger ground motion is more likely when a quake occurs closer to the surface.
"The geometry of the source fault is very crucial to estimating strong ground motions," said Hiroshi Sato at the University of Tokyo's Earthquake Research Institute.
Sato led the research, which will appear in tomorrow's issue of the journal Science.
The 35 million residents of greater Tokyo feel earthquakes almost every week. Most are small and cause no damage. But some, such as the 1923 Kanto quake and the 1855 Ansei Edo quake, have devastated the Japanese capital.
The city is built atop the junction of three tectonic plates: the Eurasian plate, the Philippine Sea plate, and the Pacific plate.
They form a so-called subduction zone, in which one plate dives beneath another. Near Tokyo, the Philippine Sea plate dips below the Eurasian plate. The Pacific plate runs below the other two.
California's San Andreas Fault, by contrast, is a strike-slip fault, where the plates grind against each other sideways.
Sato and his colleagues used seismic imaging to measure the depth of the Philippine Sea plate. The Japanese government commissioned the research.
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