March 11, 2014
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* Aftershocks of the 2011 off the Pacific Coast of Tohoku Earthquake (hereinafter referred to as the Tohoku-Oki Earthquake, Mw9.0) occurred in a wide area from the region offshore Iwate prefecture through the region offshore east of Chiba prefecture (hereafter referred to as the aftershock area). (Mw: moment magnitude)
* Within the aftershock area, the number of earthquakes of M4.0 or over, during one year from March 2012 (March 2012～February 2013) became under one fifth smaller, and during one year from March 2013 (March 2013～February 2014) became under one tenth smaller, than the number during about one year just after the Tohoku-Oki Earthquake, and thus the aftershock activity has been decaying. However, in comparison with the mean activity before the Tohoku-Oki Earthquake, the number of earthquakes of M4.0 or over within the aftershock area is more than three times, and the aftershock activity still remains in brisk state. [Reference 1]
* Seen from the point of view of regions, the aftershock activity during the last one year was higher in coastal regions than in offshore regions. Although the aftershock activity has been decaying in the region from Hamadori, Fukushima prefecture to Northern Ibaraki prefecture, where the activity became active after the Tohoku-Oki Earthquake, it still continues.
* In case of the earthquake (Mw 9.1) offshore west of the northern Sumatra Island in 2004, there were Mw8.6 after four months, Mw8.5 after about two and a half years, Mw7.5 after about five and a half years, and Mw8.6 after about seven and a half years in the region of outer rise. Thus, there have been large earthquakes for long period within the source area and its surroundings. [Reference 2]
* According to the GNSS data, crustal deformation which is thought to be postseismic deformation has still been observed in a wide area from the Tohoku through the Kanto-Chubu regions. During about one month just after the Tohoku-Oki Earthquake, at the maximum value, the horizontal movement was 30 cm, and the vertical subsidence and upheaval were 6 cm and 5 cm respectively. The deformation is decaying: the horizontal deformation was up to around 1cm, and the vertical deformation was 1cm or so during the recent one month. [Reference 3]
* Although the aftershock activity is gradually decaying as a whole, the activity is still in higher state than the one before the Tohoku-Oki Earthquake. In addition there are overseas examples for gigantic earthquakes. Synthetically judging from these facts(items), within the aftershock area and its surroundings, there is fear that large earthquakes will occur from now on, and there is a possibility that it will be hit by strong shaking and high tsunami. It is necessary to do care continuously for it.
* Although the aftershock activity is gradually decaying as a whole, the activity is still in higher state than the one before the Tohoku-Oki Earthquake. In addition there are overseas examples for gigantic earthquakes. Synthetically judging from these facts(items), within the aftershock area and its surroundings, there is fear that large earthquakes will occur from now on, and there is a possibility that it will be hit by strong shaking and high tsunami. It is necessary to do care continuously for it.