The fault that has twice caused major earthquakes in the Tokyo area may be shallower and more hazardous than previously thought.
The Philippine Sea plate dives beneath the Eurasian plate in this area. Newly developed seismic profiles indicate that the top surface of that plate ranges from 2.4 miles to 15.6 miles below the surface of the land. That is much shallower than earlier estimates that placed the top of the plate as much as 24 miles deep.
"In general, the amount of shaking is a function of distance from the source. Thus, it is expected that shallower geometry (would) produce more intense shaking," Hiroshi Sato of the University of Tokyo's Earthquake Research Institute said.
Sato's findings, based on seismic reflections, are being published in Friday's issue of the journal Science.
This fault has been blamed for major quakes that struck the Tokyo area in 1703 and 1923.
Today, 33 million people live in the Tokyo metropolitan area, which is home to business and financial institutions that reach worldwide.
Sato's report is based on high-resolution images of the area where the giant plates meet and move against one another.
He likened it to using a flashlight to see beneath the land surface. Such a precise survey has never been attempted around the Tokyo metropolitan area, he said in an e-mail interview.
Thus, the new images provide important and very basic data on the plate interface, he said.
How much more dangerous the shallower fault is remains to be seen.
He said many factors affect ground motion, such as the arrangement of a site, sediment deposits and direction of the quake movement. Sato said his team is working to simulate earthquake ground motion using computer models
Using the images to develop more precise estimates of ground motion in an earthquake can provide the basis for countermeasures, Sato said.
The research was funded by the Special Project for Earthquake Disaster Mitigation in Urban Areas of the Japanese Ministry of Education, Culture, Sports, Science and Technology.