March 9, 2021
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, moment magnitude
(Mw) 9.0) 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). About 10 years have passed after the occurrence
of the Tohoku-Oki Earthquake, and the seismic activity in the aftershock
area is gradually approaching to that of pre-the Tohoku-Oki Earthquake
period as a whole, but the number of earthquakes per one year is larger
than that of pre-the Tohoku-Oki Earthquake period. On the 13th of the previous month, there was a large scale earthquake (magnitude (M)
7.3) offshore Fukushima prefecture. Also, in the wide area of the eastern
Japan and at the ocean bottom of the aftershock area, crustal deformation
considered as the postseismic deformation is still observed. The postseismic
deformation is decaying as a whole with time, but have not returned to
the pre-the Tohoku-Oki Earthquake period.
Based on the transition of the seismic activities and the crustal deformations
during the period from the occurrence of the Tohoku-Oki Earthquake to the
present, and examples for other gigantic earthquakes, the seismic activity
of the current level in the aftershock area will continue for the time
being. On the other hand, "The Long-Term Evaluation of the Seismic
Activity Along the Japan Trench" (published on February 26, 2019)
evaluated that the probability of occurrence of the large scale earthquakes
within the region along the Japan Trench from offshore east of Aomori prefecture
to offshore Boso including the coasts is high, because the stationary seismic
activity in the region is larger than those along other trenches.
Synthetically judging from seismic activity, examples for other gigantic earthquakes, the long-term evaluation and so on, there is fear that large earthquakes will occur from now on within the aftershock area and its surrounding areas including inlands, 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, 9 years after the Tohoku-Oki Earthquake (March 2020~February 2021) became under 1/25 smaller than that just after the Tohoku-Oki Earthquake (March 11, 2011). 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. Also, there occur large scale earthquakes with damage, such as the earthquake (M7.3) offshore Fukushima prefecture on 13th of the previous month.
* According to the GNSS continuous observation, crustal deformation which is thought to be postseismic deformation has still been observed in a wide area of the eastern Japan. 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 6 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 postseismic deformation has still been observed. In the ocean area offshore Miyagi prefecture, the general westward deformation continues. On the other hand, in the ocean area offshore Fukushima prefecture, the general eastward deformation is converging. The postseismic deformation became small with time as a whole, but not yet returned to that of the pre-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. Also, the number of earthquakes of M5.0
or over, after 16 years of the Mw9.1 earthquake in 2004, is larger than
that of the pre-earthquake period.
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 10 Years~