March 9, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On February 13 at 23:07 (JST), there was a magnitude (M) 7.3 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 February 28 there occurred 93 earthquakes with maximum seismic intensity 1 or over, and 7 earthquakes with maximum seismic intensity 3 or over. There occurred 44 earthquakes of M4.0 or over and the largest earthquake was that of M5.5 which occurred on February 15 at 21:26 (JST). The focal mechanism of the M5.5 event showed a reverse fault type with a compression axis in a WNW-ESE direction. The M5.5 event occurred at the boundary between the Pacific and the continental plates.
* 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 this event was observed around Fukushima prefecture: a little less than 2 cm westward movement at Odaka and S Minamisoma A observatories, Minamisoma City, and south-westward movement at Naraha A observatory, Naraha Town, Fukushima prefecture.
* The earthquake fault estimated from the focal mechanism, the distribution of the seismic activity, and the analyzed result of GNSS observation is a reverse fault dipping to south-east, which extends about 40 km in an NE-SW direction.
* 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 aftershock activity is decaying as a whole, as 10 years have passed since the occurrence of the Tohoku-Oki Earthquake, 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.
* [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 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 trench-type earthquakes. Also, after the Tohoku-Oki Earthquake, the earthquakes within the subducting plate occur with higher frequency and the probability is possibly higher.
* 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, respectively. Thus, there have been large earthquakes for a 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.
# : 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.