Outline of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) —Earthquake Early Warning and observed seismic intensity—

The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) that occurred on March 11, 2011, caused strong ground motion around northeastern Japan. Before the strong ground motion hit cities, the Japan Meteorological Agency (JMA) issued Earthquake Early Warning (EEW) announcements to the general public of the Tohoku district and then the warning was automatically broadcast through TV, radios and cellular phone mails. The EEW was earlier than the S wave arrival and more than 15 s earlier than the strong ground motion (intensity 5-lower or greater on the JMA scale) everywhere in the district. Seismic intensity 7 was observed for only the second time since JMA introduced instrument-based observation for intensity measurements in 1996. Intensities of 6-upper and 6-lower were widely observed at many stations in the Tohoku and Kanto districts, over an area of approximately 400 km × 100 km. The duration of strong ground motions was quite long. For the Tokyo region, JMA EEW expected intensities of 4, which was an underestimation of the observed intensity (5-upper). This underestimation can probably be attributed to the large extent of the fault rupture.