The fourth strongest quake recorded in history , Tohoku caused abrupt stress changes in the upper plate and triggered widespread seismic activity throughout the Japanese island . Forty-two million people live in Tokyo metropolitan area . Institute : Tokyo faces 70 % probability of M-7 quake within four years , 98 % chance within 30 years . Comment: March 11 quake and tsunami left more than 15,700 dead , government says .
Forty-two million people live in Tokyo metropolitan area . Institute : Tokyo faces 70 % probability of M-7 quake within four years , 98 % chance within 30 years . Comment: March 11 quake and tsunami left more than 15,700 dead , government says .