March 13, 2011
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On March 11 at 14:46 (JST), there was a M9.0 (preliminary) earthquake at a depth of approximately 25km, off-shore of Sanriku. The magnitude of the main shock is the largest in Japan. This event had a maximum seismic intensity 7 observed in Kurihara City, Miyagi prefecture. This earthquake caused a tsunami on the Pacific coast from the Hokkaido, Tohoku and Kanto regions, with tsunami heights over 7.3m in Soma, of 4.2m in Oarai, and over 4.1m in Kamaishi.
* The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Pacific and the continental plates.
* The maximum aftershock was a M7.5 (preliminary) one that occurred at 15:08 on March 11, as of 15:00 on March 13. There were three earthquakes of M7.0 or over and 40 (preliminary) aftershocks of M6.0 or over, off-shore of the Pacific coast from Iwate through Ibaraki prefectures. The aftershock area extends approximately 500km in a N-S direction. There is fear that large aftershocks will hereafter occur.
* According to the GPS data, crustal movements associated with this event were observed and there were movements of approximately 4.4m toward the east-southeast at the Shizugawa observation point (Miyagi prefecture). In addition, on the coast from Iwate through Fukushima prefectures, the maximum subsidence of approximately 75cm was observed. There are the regions that keep inundated after the tsunami ceased.
* The hypocenter area extends from the region off-shore of Iwate to Ibaraki prefectures. Various analyses of seismic waves, crustal movement and other data suggest that the extent is approximately 400km in length and approximately 200km in width, and that the maximum slip is over approximately 20m. It is thought that the area covers the regions off-shore of Miyagi prefecture, to the east off-shore south of Sanriku along the trench, off-shore of Fukushima prefecture and off-shore of Ibaraki prefecture. In addition, there is the possibility that the area extends to the middle region off-shore of Sanriku and to a part of the region along the trench, off-shore north of Sanriku to Ibaraki prefecture. All of these regions were evaluated by the Earthquake Research Committee.