Tokyo Bay quake map predicts strong shaking

Parts of the Tokyo metropolitan area could be shaken to the maximum level of 7 on the Japanese seismic intensity scale in the event of a major quake with its focus under northern Tokyo Bay, according to a new map by a science ministry research team.

The map, released Wednesday by the team of the Education, Culture, Sports, Science and Technology Ministry, shows that strong seismic waves would spread underground in coastal areas of Tokyo and Kanagawa Prefecture in the event of a quake with a magnitude of 7.3.

The team has significantly expanded areas that would register at least upper 6 compared to the 2004 estimates by the government's Central Disaster Management Council. Under the latest prediction, the maximum intensity of 7 could be measured in central Tokyo.

Seismic waves would move at a maximum speed of 40 to 80 centimeters per second in the firm underground regions just above the quake's focal area or where plate boundaries stick strongly to each other, according to the latest estimates.

Compared to the 2004 estimates, the research team has also expanded areas where seismic waves are predicted to run underground at a maximum of 20 to 40 centimeters per second.

Generally, if underground seismic waves move faster than 35 to 50 centimeters per second, the surface areas over them experience tremors of an intensity around upper 6 - the point at which wooden houses begin collapsing.

If the waves move at 50 to 75 centimeters per second, the corresponding surface areas would register the maximum intensity level of 7, according to the research team.

The latest projection by the team, made up of experts from the University of Tokyo's Earthquake Research Institute and other organizations, come as a result of underground structure research.

In compiling latest estimates, the team changed the depth of a plate boundary beneath Tokyo Bay by making it five to 10 kilometers shallower than in previous calculations. The team also assumed that there are two locations where plate boundaries stick strongly to each other.

Based on these factors, the research team also said it will release another map on seismic intensity scales on the surface by the end of this month.

Meanwhile, the government will review from next fiscal year the current estimates on intensity scales, tsunami heights and damage when a major quake occurs in the Tokyo metropolitan area, officials said.

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