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.
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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