April 9, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On March 20 at 18:09 (JST), there was a magnitude (M) 6.9 earthquake at a depth of approximately 60 km, offshore Miyagi prefecture. Maximum seismic intensity 5 Upper was observed in Miyagi prefecture 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 at the boundary between the Pacific and the continental plates.
* The seismic activity became high in the area of 30 km east-west and 30 km north-south, including the M6.9 event, and till March 31 there occurred 29 earthquakes with maximum seismic intensity 1 or over. The largest earthquake till March 31 was that of M4.2 which occurred on 20th at 18:13 (JST).
* According to the result of GNSS observation, the crustal deformation associated with the event was observed in a wide area, centering on Miyagi prefecture: a little more than 2 cm east-southward movement at Minamikata station, Tome City, and at Takashimizu station, Kurihara City, Miyagi prefecture.
* From the distribution of seismic activity and the analyzed results of GNSS observation and seismic waves, the source region is the area extending from the coasts of Oshika Peninsula to offshore. It is considered that this source region overlaps a part of the western source region of the 1978 Miyagi-Ken-Oki Earthquake (M7.4), but it doesn't cover the whole source regions of the mentioned earthquake and the earthquake (M7.2) offshore Miyagi prefecture in 2005.
* The 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 aftershock activity is approaching to state of the pre-the Tohoku-Oki Earthquake period 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 state 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 stationarily higher than that of the areas along the other trenches in Japan and the probability of occurrence of large earthquakes is high. The recent event is situated at the western end of the offshore Miyagi prefecture, which is the target area of the long-term evaluation. In the landward area of the offshore Miyagi prefecture, the probability of occurrence of a one order small inter-plate earthquake, which is called generally as the earthquake offshore Miyagi prefecture with M7.4 or so and happens repeatedly, within 30 years is evaluated as the rank III (#), and classified as a higher probability group in the subduction-zone earthquakes. Beside this in the area offshore Miyagi prefecture, the probability of occurrence of another one order small inter-plate earthquake with M7.0~7.5 within 30 years is estimated as the rank III, and the probability of an inter-plate earthquake with M7.9 or so is estimated as the rank II. Also, the variation of stress due to the postseismic slip of the Tohoku-Oki Earthquake influences the occurrences of these earthquakes and the probability is possibly higher.
* Synthetically judging from the above situations, we should pay attention for a long time, to the probability of a large earthquake occurring in the aftershock area of the Tohoku-Oki Earthquake 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 in the outside of the trench axis, respectively. Thus, there have been large earthquakes for long period within the source area and its surroundings. In addition, the number of earthquakes of M5.0 or over is still higher than before the earthquake, even 16 years after the 2004 Mw9.1 earthquake.
# : Rank of the probability of occurrence of a subduction-zone earthquake within 30 years is expressed as follows; rank: probability, III: greater than 26%, II: 3% ~ 26%, I: smaller than 3%, X: unknown (an immediate occurrence of an earthquake cannot be denied).
Note: GNSS is a general name of satellite positioning system such as GPS.