April 11, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On March 16 at 23:36 (JST), there was a magnitude (M) 7.4 earthquake at a depth of approximately 55 km, offshore Fukushima prefecture. Maximum seismic intensity 6 Upper was observed in Miyagi and Fukushima prefectures and some damage was caused. Also, in Miyagi prefecture long-period ground motion class 4 was observed. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate.
* Although the seismic activity continues in the area including the M7.4 event, extending about 40 km length in an NNE-SSW direction, it became lower as time passed. The largest aftershock is an M5.5 earthquake which occurred on 17th. In this area there occurred an M6.1 earthquake on 16th at 23:34 (JST) just before the M7.4 event. After this M6.1 event till April 11 at 08 o'clock there occurred 117 earthquakes with seismic intensity 1 or over: 14 earthquakes with seismic intensity 3 or over.
* In the vicinity of the recent event, on March 25 there was an M5.2 earthquake. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
* In the vicinity of the recent event, on February 13, 2021 there was an
M7.3 earthquake within the Pacific plate. The seismic activity area of
this 2021 event is adjacent to the south side of that of the recent event:
the main activity areas for both events seem to be separate. Note that
at about 60 km distant in the NNE direction from the recent event, on April
7, 2011 there was an M7.2 earthquake within the Pacific plate.
The M5.2 earthquake on March 25 occurred at the southwest end of the seismic activity area of the 2021 earthquake. At the boundary between the Pacific and the continental plates near the M5.2 event, after the 2021 M7.3 earthquake, there occurred an M5.2 earthquake on February 14, 2021 and an M5.5 earthquake on February 15.
* This event caused tsunami observed along the coasts extending from Aomori to Ibaraki prefectures, such as that of 31 cm at Ishinomaki port (Ports and Harbours Bureau), Ishinomaki City, Miyagi prefecture.
* According to the result of GNSS observation, the crustal deformation associated with this event was observed wide around Miyagi and Fukushima prefectures: 3 cm or so northward movement and 2 cm or so uplift at S-Ishinomaki and Oshika observatories, Ishinomaki City, Miyagi prefecture.
* The earthquake fault estimated from the focal mechanism, the distribution of the seismic activity, and the analyzed results of GNSS observation and seismic waves, is a reverse fault dipping to east-southeast, which extends in an NNE-SSW direction. The slip on the fault plane seems to have proceeded from the origin in an NNE direction.
* This event caused large accelerations observed such as 1,233 gal (3 components
synthesized) at KiK-net Kawasaki Observatory, Kawasaki Town, Miyagi prefecture.
* Comparing the distribution of seismic intensity and maximum acceleration of the recent event and the 2021 event, there are many observatories which show larger seismic intensity and maximum acceleration for the recent event as a whole, and this tendency is strong for especially observatories in the north side of the hypocenter. This result is influenced by the fact that the recent event has a larger magnitude and is situated to the north of the 2021 event.
* This recent event occurred within the aftershock area of "the 2011
off the Pacific coast of Tohoku Earthquake" (hereinafter referred
to as the Tohoku-Oki Earthquake). Although the seismic activity in the
aftershock area is approaching to the state before the Tohoku-Oki Earthquake
as a whole, the number of earthquakes per year is larger than that of the
era before the Tohoku-Oki Earthquake, and the seismic activity of the present
level will continue for the time being. After February 2021 other than
the recent event, an M7.3 earthquake on February 13, 2021 offshore Fukushima
prefecture, an M6.9 one on March 20 offshore Miyagi prefecture, and an
M6.8 one on May 1 offshore Miyagi prefecture occurred.
* [Long-term Evaluation of the Seismic Activity along the Japan Trench (published on February 26, 2019)] (hereinafter referred to as the long-term evaluation) evaluated that the seismic activity of the area along the Japan Trench is stationarily higher than that of the areas along the other trenches in Japan and the probability of occurrence of a large earthquake is high. The recent event is considered, from the point of the location, the focal mechanism, and the value of M and so on, as the earthquake within the subducting plate (offshore east of Aomori prefecture and the northern part of offshore Iwate prefecture ~ offshore Ibaraki prefecture) as supposed by Earthquake Research Committee. Note that in the long-term evaluation, the probability of occurrence of an earthquake of M7.0 to 7.5 or so within 30 years is estimated as the rank III (#), and is classified as the group with higher probability in the subduction-zone earthquakes. Also, after the Tohoku-Oki Earthquake, the earthquakes within the subducting plate occur with higher frequency and the probability is possibly higher.
* Synthetically judging from the above situations, we should pay attention for a long time, to the possibility of a large earthquake occurring in the aftershock area and the surrounding area including the inland, and of suffering from strong shaking and high tsunami.
* In case of the earthquake offshore west of the northern Sumatra Island in 2004 (moment magnitude (Mw) 9.1), 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 outside the trench axis, respectively. Thus, there have been large earthquakes for a long period within the source area and its surroundings.
# : Rank of the probability of occurrence of a trench-type earthquake within 30 years is expressed as follows; III: greater than 26%, II: 3% ~ 26%, I: smaller than 3%, X: unknown (an immediate occurrence of earthquake cannot be denied).
Note: GNSS is a general name of satellite positioning system such as GPS.