Big One ‘likely to be shallower’

by Minoru Matsutani

Staff Writer

The focus of any major earthquake that strikes beneath the Tokyo metropolitan area is likely to be shallower than previously thought, a government project team said Wednesday.

The edge between the ground plate and the Philippine Sea plate underneath Tokyo is 5 or 10 km shallower than earlier projections, which could lead to Tokyo suffering a quake with an intensity of 7, greater than previously anticipated, the team said.

The team of scientists from the University of Tokyo and other organizations determined the correct depth of the tectonic plate junction by using a new quake-observing technique. The team buried seismic-measuring devices 20 meters deep at 296 points, in Tokyo, Chiba, Kanagawa, Saitama, Ibaraki and other prefectures.

The science ministry appointed the team in 2007 to conduct the research for five years.

A shallower tectonic plate junction means a shallower focus of a quake, thus greater shaking.

Last month, the team announced the plate junction is actually shallower than the 30 to 40 km previously thought. Thus, Tokyo could experience “shindo,” or 7, the top of the Japanese intensity scale, if a magnitude-7.3 quake strikes in the northern part of Tokyo Bay, greater than the previously anticipated upper 6.

Following that announcement, the science ministry said the country needs to strengthen antidisaster measures.