March 17, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On March 16 at 23:36 (JST), there was a magnitude (M) 7.4 (provisional) 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 seismic intensity 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.
* The active seismic activity continues in the area including the M7.4 event, extending about 40 km length in an NNE-SSW direction, such as an M5.6 (preliminary) earthquake occurred on 17th at 00:52 (JST). In this area there occurred an M6.1 (provisional) earthquake on 16th at 23:34 (JST) just before the M7.4 event. After this M6.1 event till 17th at 18 o'clock there occurred 26 earthquakes with seismic intensity 1 or over.
* 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.
* This event caused tsunami observed along the coasts of Miyagi and Fukushima prefectures, such as that of 0.3 m (preliminary) at Ishinomaki Port observatory (Ports and Harbours Bureau), Ishinomaki City, Miyagi prefecture.
* This event caused large accelerations observed such as 1,233 gal (3 components synthesized) at KiK-net Kawasaki Observatory, Kawasaki Town, 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 (provisional) northward movement 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.
* In the area of strong shaking, we should pay attention for a week or so, to the possibility of an earthquake which causes maximum seismic intensity 6 Upper or so. Especially for 2 or 3 days after the earthquake, there are many cases that a large scale earthquake occurs.
* This 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.