Seismologists anticipated a very large quake would strike in the same place as the 1923 Great Kantō earthquake — in the Sagami Trough , southwest of Tokyo . The Japanese government had tracked plate movements since 1976 in preparation for the so-called Tokai earthquake , predicted to take place in that region . However , occurring as it did north east of Tokyo , the Tōhoku earthquake came as a surprise to seismologists . While the Japan Trench was known for creating large quakes , it had not been expected to generate quakes above an 8.0 magnitude . The Headquarters For Earthquake Research Promotion set up by Japanese government then reassessed the long term risk of trench-type earthquakes around Japan , and it was announced in November 2011 that research on the 869 Sanriku earthquake indicated that a similar earthquake with a magnitude of 8.4 – 9.0 would take place off the Pacific coast of northeastern Japan , on average , every 600 years . Also , a tsunami-earthquake with a tsunami magnitude scales ( Mt ) between 8.6 and 9.0 ( Similar to the 1896 Sanriku earthquake , the Mt for the 2011 Tōhoku earthquake was 9.1 – 9.4 ) had a 30 % chance to occur within 30 years
1923 Great Kantō earthquake
1923 Great Kantō earthquake
2011 Tōhoku earthquake and tsunami