March 9, 2018
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) 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 2017, 6 years after the Tohoku-Oki Earthquake (March 2017~February 2018) became under one twenty-fifth 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 1.5 times the mean activity before the Tohoku-Oki Earthquake. The seismic activity still remains in a brisk state: On coastal regions there occurred an earthquake with maximum seismic intensity 5 Lower, such as the earthquake (M5.9) offshore Fukushima prefecture on October 6, 2017. Near the axis area and the east side of the Japan Trench, there occurred 3 earthquakes with M6.0 or over, such as the earthquake (M6.3) offshore Sanriku on September 21, 2017.
* According to the GNSS continuous observation, crustal deformation which is thought to be post-seismic deformation has still been observed in a wide area from the Tohoku through the Kanto-Chubu regions. During about one year just after the Tohoku-Oki Earthquake, at the maximum value, the horizontal movement was 77 cm generally to the east, and the vertical subsidence and upheaval were 12 cm and 17 cm, respectively. Both the horizontal and vertical deformations decayed to about 6cm to the east and 5 cm upward during the recent one year, but not yet returned to the state of pre-Tohoku-Oki Earthquake period.
* According to the ocean bottom crustal deformation observation, crustal deformation which is thought to be post-seismic deformation has still been observed, in the ocean area offshore Miyagi prefecture, the general westward deformation of 11 cm (mean value of the recent 3 years) at the maximum per year still continues. Mean deformation per year has become smaller for the recent 3 years, compared to that for the 4 years just after the Tohoku-Oki Earthquake.
* In case of the earthquake (Mw 9.1) offshore west of the northern Sumatra Island in 2004, there were Mw8.6 after three 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 7 Years~