A magnitude-7 class earthquake in Tokyo Bay could cause much stronger shocks than previously thought, according to the science ministry.
A team investigating the risk of earthquakes hitting the Tokyo metropolitan area announced March 7 that a plate boundary that could generate earthquakes was about 10 kilometers closer to the ground in some places than previous research had indicated. A shallower seismic source means a greater chance of strong shocks on the surface.
The scientists said that the discovery indicated that broad areas along Tokyo Bay could experience shocks of a maximum seismic intensity of 7 on the Japan Meteorological Agency (JMA) scale if a magnitude-7 earthquake struck in the north of the bay.
The JMA warns that shocks of that strength can cause some wooden houses with low earthquake resistance to collapse and increases the risk of concrete buildings with low resistance falling down. Even wooden-framed houses with high resistance are at greater risk of leaning.
The science ministry project team said parts of Tokyo, Kanagawa and Chiba prefectures, especially areas facing Tokyo Bay, were at risk.
It is unclear where a magnitude-7 class earthquake would strike beneath the Tokyo metropolitan area. In 2004, the Central Disaster Management Council simulated 18 cases involving different seismic source locations.
The latest science ministry analysis chose a seismic source beneath the north of Tokyo Bay because many central and local government disaster prevention programs are based on that scenario.
The Central Disaster Management Council in 2004 envisaged shocks of up to "upper 6" in the JMA seismic intensity scale in the case of an earthquake originating under the northern Tokyo Bay and shocks of up to seismic intensity 7 in the case of a temblor along the Tachikawa fault in western Tokyo.
The council estimated the death toll at about 11,000 and economic damage at 112 trillion yen ($1.39 trillion) for a seismic source beneath northern Tokyo Bay.
The science ministry team’s results were to be presented March 8 at the University of Tokyo.
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