Earth Planets Space, Vol. 63 (No. 7), pp. 547-551, 2011
Mitsuyuki Hoshiba1, Kazuhiro Iwakiri1, Naoki Hayashimoto1, Toshihiro Shimoyama2, Kazuyuki Hirano2, Yasuyuki Yamada2, Yuzo Ishigaki2, and Haruyuki Kikuta2
1Meteorological Research Institute, Nagamine 1-1, Tsukuba 305-0031, Japan
2Japan Meteorological Agency, Ohtemachi 1-3-4, Chiyoda-ku, Tokyo 100-8122, Japan
(Received April 12, 2011; Revised May 21, 2011; Accepted May 22, 2011; Online published September 27, 2011)
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.
Key words: The 2011 off the Pacific coast of Tohoku Earthquake, strong ground motion, seismic intensity, Earthquake Early Warning.