Outline of the 2011 off the Pacific coast of Tohoku Earthquake (M w 9.0) —Seismicity: foreshocks, mainshock, aftershocks, and induced activity—

Abstract

A massive earthquake of a magnitude ( M ) of 9.0 occurred on March 11, 2011, off the Pacific coast of the northeastern part of Honshu, Japan. Centroid Moment Tensor analysis of the mainshock indicates that it was the reverse fault type, with a WNW-ESE compressional axis. The earthquake occurred on the plate boundary between the island arc and the Pacific plates. Three aftershocks exceeding M 7 occurred within 40-min after the mainshock, and the aftershock area covered a wide range of 500-km × 200-km. Seismicity became active one month before the mainshock, and it continued for two weeks in an adjacent area northeast of the mainshock. Furthermore, foreshock activity with maximum M 7.3 started in the same area two days before the mainshock. Seismic activities increased in almost the entire area of the Japanese Islands after the mainshock. We infer that these earthquakes were induced by the mainshock. JMA displacement-amplitude magnitude of the mainshock was determined to be 8.4, which was smaller than the moment magnitude of 9.0.