March 9, 2017
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities After the 2011 Off the Pacific Coast of Tohoku Earthquake

* Aftershocks of the 2011 off the Pacific Coast of Tohoku Earthquake (hereinafter referred to as the Tohoku-Oki Earthquake, Mw9.0) occur in a wide area from the region offshore Iwate prefecture through the region offshore east of Chiba prefecture (hereinafter 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 2016, 5 years after the Tohoku-Oki Earthquake (March 2016~February 2017) became under one fifteenth smaller than that during about one year just after the Tohoku-Oki Earthquake, and thus the aftershock activity has been generally decaying. However, the seismic activity within the aftershock area is about twice the mean activity before the Tohoku-Oki Earthquake, and the seismic activity still remains in a brisk state centered on coastal regions where the seismic activity is high. Note that there was a large magnitude earthquake (M7.4) accompanying tsunami offshore Fukushima prefecture on November 22, 2016.

* According to the GNSS continuous observation, 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. Both the horizontal and vertical deformations decayed to about 1cm during the recent one month, but not yet returned to the state of pre-Tohoku-Oki Earthquake period.

* According to the ocean bottom crustal deformation observation, in the ocean area offshore Miyagi prefecture, the general westward deformation of 10 cm per year 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, Mw8.4 after about two and a half years, Mw7.8 after about five and a half years, and Mw8.6 and Mw7.8 after about seven and a half and about 11 years, respectively, in the region of outer rise of the trench axis. Thus, there have been large earthquakes for long period within the source area and its surroundings.

* As stated above, although the aftershock activity is gradually decaying as a whole, the seismic activity is still in higher state than that before the Tohoku-Oki Earthquake. In addition, there are overseas examples for gigantic earthquakes, and there were many earthquakes accompanying damage and tsunami occurring off the Pacific coast of Tohoku district. Synthetically judging from these facts, within the aftershock area and its surroundings, there is fear that large earthquakes will occur from now on, and there is a possibility of strong shaking and high tsunami. It is necessary to pay attention continuously for it.

On The 2011 Off The Pacific Coast of Tohoku Earthquake ~Seismic Activity during 6 Years~