April 10, 2019
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)
Now 9 years have passed after the occurrence of the Tohoku-Oki Earthquake
and the aftershock activity is gradually decaying as a whole, but the number
of earthquakes per one year is larger than that of pre-the Tohoku-Oki Earthquake
period. In addition, "The Long-Term Evaluation of the Seismic Activity
Along the Japan Trench" (published on February 26, 2019) evaluated
that the probability of occurrence of large scale earthquakes is high,
because the stationary seismic activity in the region along the Japan Trench
is larger than those along other trenches. Synthetically judging from these
facts, including examples for gigantic earthquakes, 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 large tsunami.
It is necessary to pay attention continuously for it.
The situation of seismic activities and crustal deformations after the
Tohoku-Oki Earthquake is as follows.
* Within the aftershock area, the number of earthquakes of M4.0 or over, during one year, 8 years after the Tohoku-Oki Earthquake (March 2019~February 2020) became under one thirtieth smaller than that during about one year just after the Tohoku-Oki Earthquake (March 11, 2011), and thus the aftershock activity has been generally decaying. However, on coastal regions, near the axis area and the east side of the Japan Trench, the seismic activity is still higher than that before the Tohoku-Oki Earthquake.
* 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 5cm to the east and 4 cm upward during the recent one year, but not yet returned to the state of pre-the 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 maximum 9 cm and offshore Fukushima prefecture, the general eastward deformation of maximum 4 cm per year (mean value of the recent 4 years) still continues. Mean deformation per year has become smaller for the recent 4 years, compared to that for the 4 years just after the Tohoku-Oki Earthquake.
* In case of the Earthquake (Mw9.1) offshore west of the northern Sumatra
Island in 2004, there were Mw8.6 earthquake after three months, Mw8.4 one
after about two and a half years, Mw7.8 one after about five and a half
years, Mw8.6 one after about seven and a half years, and Mw7.8 one after
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.
Note: GNSS is a general name of satellite positioning system such as GPS.
On "the 2011 Off The Pacific Coast of Tohoku Earthquake" ~Seismic Activity during 9 Years~