February 14, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On February 13 at 23:07 (JST), there was a magnitude (M) 7.3 (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. 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 seismic activity continues in the area including the M7.3 event, extending about 40 km length in an NE-SW direction, and till 14th at 19 o'clock there occurred 3 earthquakes with maximum seismic intensity 3 or over. The largest earthquake till 14th at 19 o'clock was that of M5.2 (preliminary) which occurred on 14th at about 16:31 (JST).
* The earthquake fault estimated from the focal mechanism and the distribution of the seismic activity is a reverse fault dipping to east-south-east, which extends in an NNE-SSW direction.
* This event caused tsunami observed along the coasts of Miyagi and Fukushima prefectures, such as that of 0.2 m (preliminary) at Ishinomaki Port observatory (Ports and Harbours Bureau), Ishinomaki City, Miyagi prefecture.
* This event caused large accelerations observed such as 1,432 gal (3 components synthesized) at KiK-net Yamamoto Observatory, Yamamoto Town, Miyagi prefecture.
* According to the result of GNSS observation, the crustal deformation associated with the event was observed: a little less than 2 cm (provisional) westward movement at Odaka and S Minamisoma A observatories, Minamisoma City, Fukushima prefecture.
* In the area where strong shaking was felt, it is necessary to be prepared for an earthquake with maximum seismic intensity 6 Upper, for a week or so from the occurrence of the earthquake. Especially in a few days or so, an earthquake of a large magnitude often occurs.
* The 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 aftershock activity is decaying as a whole, the number of earthquakes per year is larger than that of the era before the Tohoku-Oki Earthquake.
* 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 stationary higher than that of the areas along the other trenches in the country 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 trench-type earthquakes. Also, after the Tohoku-Oki Earthquake, the earthquakes within the subducting plate occurr with higher frequency and the probability is possibly higher.
* We should pay attention for a long time, to the probability 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, respectively. Thus, there have been large earthquakes for 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.