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Earthquake Fault Under Tokyo Closer Than Expected, Study Finds

Stefan Lovgren
for National Geographic News
July 14, 2005
 
Scientists have found that a major, earthquake-producing fault that runs beneath metropolitan Tokyo is closer to the surface—and potentially more dangerous—than 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.

Layered Plates

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.

The researchers used air guns and explosives to send seismic signals into the ground, and then listened for seismic waves to bounce back. This allowed them to create an image of the plate.

"I still feel it is amazing that we can get a clear image down to 26 kilometers [16 miles] below Tokyo, where it is like a total hell of geophysics," Sato said.

Revising Kanto

The team's findings change some assumptions about the city's past earthquakes. Knowing the depth of the plates has led scientists to revise where the tectonic slips occurred that caused the 1923 Kanto earthquake, which had a magnitude of 7.9 and claimed 105,000 lives.

The research could also alter estimates of future seismic hazards, since shallower plates produce more intense shaking on the surface.

But several other factors, like the makeup of the sediment basin on which Tokyo sits, also determine how strong a quake's effects could be. The team is working on new models to estimate the exact strength of future quakes as felt on the surface.

"The change [in the depth] of the source fault generates more intense shaking to the [Tokyo] metropolitan area," Sato said. "However, how intense is the future's problem to answer."

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