April 11, 2011
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

The 2011 off the Pacific Coast of Tohoku Earthquake

* On March 11 at 14:46 (JST), there was a M9.0 earthquake at a depth of approximately 25km, off-shore of Sanriku. The magnitude of the main shock is the largest in Japan up to the present. This event had a maximum seismic intensity 7 observed in Kurihara City, Miyagi prefecture. This earthquake caused large tsunamis on the Pacific coast from the Hokkaido, Tohoku and Kanto regions, with tsunami heights over 8.5m in Miyako, over 8.0m in Ofunato, over 7.6m in Ayukawa, Ishinomaki City, and over 7.7m in Soma. According to field surveys up to the present, the run-up height over approximately 30m was measured at the Koborinai fishing port (Miyako City, Iwate prefecture).

* 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 M7.7 earthquake that occurred at 15:15 on March 11, as of April 11. There were more than 60 aftershocks of M6.0 or over. On April 7, there was a M7.1 earthquake with a seismic intensity 6 Upper observed in Miyagi prefecture. The aftershock area extends approximately 500km in an N-S direction. There is fear that large aftershocks will occur from now on, and there is a possibility that it will be hit by strong shaking and high tsunami.

* According to the GPS data, crustal movements associated with the main shock were observed in a wide area from the Tohoku through the Kanto regions. There was a horizontal displacement of approximately 5.3m to the east-southeast and a subsidence of approximately 1.2m at the Oshika station (Miyagi prefecture). Since the main shock, crustal movements to the east have been observed, which are thought to be the afterslip. Crustal movements of approximately 41cm at the Yamada station (Iwate prefecture) and approximately 27cm at the Choshi station (Chiba prefecture) were observed, as of April 5. According to the data of Synthetic Aperture Radar (SAR) from the advanced land observing satellite, "Daichi", crustal movements, in harmony with the GPS data, were observed in a wide area from the Tohoku through the Kanto regions.

* The focal area extends from the region off-shore of Iwate to Ibaraki prefectures. It is inferred that the extent is more than approximately 400km in length and approximately 200km in width, and that the maximum slip is over approximately 20m. It is thought that the area with large slip covers a part of the region along the trench off-shore south of Sanriku and the region along the trench off-shore north of Sanriku through Boso, all of which were evaluated by the Earthquake Research Committee. It is also thought that the focal area furthermore includes the middle region off-shore of Sanriku and the regions off-shore of Miyagi, Fukushima and Ibaraki prefectures.

* Associated with the main shock, crustal deformations over 5m in a horizontal direction were observed, and they have given strain changes in a wide area. There are some regions of brisk seismicity observed from the Tohoku region through the Kanto and Chubu regions, and it is thought that their seismicity has been affected by the main shock.