October 11, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On September 16, there was a magnitude (M) 5.1 earthquake in the Noto region, Ishikawa prefecture. Maximum seismic intensity 5 Lower was observed in Ishikawa prefecture.
There was no remarkable activity.
There was no remarkable activity.
* On September 14, there was an M6.0 earthquake at a depth of approximately 390 km, offshore south of Tokaido. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate. In the area extending from Tohoku to Chubu regions, seismic intensity 3 ~ 1 was observed: shaking was larger in the Pacific side of the Tohoku to Kanto regions due to seismic waves propagating through the subducting Pacific plate (abnormal seismic intensity).
* On September 16, there was an M5.1 earthquake at a depth of approximately 15 km in the Noto region, Ishikawa prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the crust. In the vicinity of the recent event, the number of earthquakes is increasing since 2018 and the seismic activity becomes higher since December 2020. The recent event was the largest since December 2020. On June 26, 2021, there occurred an M4.1 earthquake, on July 11 an M3.9, on August 14 an M4.2, September 7 an M4.2 and on October 3 an M4.3: the high seismic activity continues after September. From December 2020 till October 6, 2021 there occurred 46 earthquakes with seismic intensity 1 or over: from September 1 to October 6, 16 earthquakes.
According to the GNSS observation, no significant crustal deformation accompanying the recent event was observed. On the other hand, in the Noto Peninsula since about December 2020, the following crustal deformation is observed: the displacement of 1 cm or so in the south-southwest direction at the Noto observatory in Noto Town, Ishikawa prefecture, and the uplift of 2 cm or so at the Suzu observatory in Suzu City.
* On September 19 at 17:18 (JST), there was an M5.3 earthquake at very shallow depth in the Hida region, Gifu prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the crust. In the vicinity of the recent event the seismic activity has become high: On the same day at 17:19 (JST), there occurred an M4.9 earthquake and at 19:04 (JST) an M4.7 earthquake. From September 19 till 28 there were 32 earthquakes with seismic intensity 1 or over.
In the neighborhood (near the border of Nagano and Gifu prefectures) the seismic activity became high since April 2020 such as an M5.5 earthquake on April 23 and an M5.4 on May 19. Also, in 1998 the seismic activity became high: an M5.0 earthquake on August 12 and an M5.6 on August 16.
* On September 29, there was an M6.1 earthquake at a depth of approximately 390 km under the central Japan Sea. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate. In the area extending from Hokkaido to Chubu regions, seismic intensity 3 ~ 1 was observed: shaking was larger in the Pacific side of the Hokkaido to Kanto regions due to seismic waves propagating through the subducting Pacific plate (abnormal seismic intensity).
There was no remarkable activity.
There was no remarkable activity.
* There has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time.
* On September 21, there was an M6.6 earthquake at a depth of approximately 40 km (CMT solution) in the Kurile Islands. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction.
* On October 6, there was an M5.9 earthquake at a depth of approximately 55 km, offshore Iwate prefecture. Maximum seismic intensity 5 Upper was observed in Aomori prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the Pacific plate.
Then in the vicinity of the recent event, the seismic activity continues: till October 10 there occurred 3 earthquakes with seismic intensity 1 or over. The largest aftershock till October 10 is the M4.7 earthquake on October 9.
According to the result of GNSS observation, no significant crustal deformation associated with this event was observed.
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 5 Upper or so.
* On October 6, there was an M5.4 earthquake at a depth of approximately 45 km, offshore east of the Osumi Peninsula. The focal mechanism showed a strike-slip fault type with a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.
* On October 7, there was an M5.9 earthquake at a depth of approximately 75 km under the northwestern Chiba prefecture. Maximum seismic intensity 5 Upper was observed in Saitama and Tokyo prefectures. Also, in Chiba and Tokyo prefectures long-period seismic intensity 2 was observed. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the Philippine Sea plates.
Then the seismic activity continues in the area including the M5.9 event, covering about 10 km length both in E-W and N-S directions, and till October 10 there occurred 4 earthquakes with seismic intensity 1 or over. The largest aftershock till October 10 was that of M3.5 which occurred on October 8.
According to the result of GNSS observation, no significant crustal deformation associated with this event was observed.
In the vicinity of the recent event, on July 23, 2005, there was an M6.0 earthquake and maximum seismic intensity 5 Upper was observed in Tokyo prefecture. There have sometimes been earthquakes of M6 or so, such as an M6.0 one on September 25, 1980. The largest was an M6.3 earthquake on September 30, 1956.
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 5 Upper or so.
In the southern Kanto region where the recent event occurred, the Pacific plate is subducting from the Japan Trench in the east, under the Philippine Sea plate subducting in the south, and there have occurred many earthquakes of M7 or so. In the "Long-Term Evaluation of the Seismic Activity Along the Sagami Trough (2nd ed.) (Published on April 25, 2014)" the probability of the occurrence of an earthquake of M7 or so (M6.7~M7.3) within 30 years was evaluated as rank III (#), classified as the group of the high probability in the subduction zone earthquakes.
#: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.
October 11, 2021
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during September 2021.
(Reference)
The following events for the period from September 2020 to late August 2021 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – Reihoku, Fukui Prefecture | M5.0 on September 4, 2020 (Depth of approximately 5 km) |
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| – Offshore Ibaraki prefecture | M5.7 on November 22, 2020 (Depth of approximately 45 km) |
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| – Offshore Iwate Prefecture | M5.6 on December 12, 2020 (Depth of approximately 50 km) |
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| – Ocean Area Near Niijima and Kozushima Islands | M5.0 on December 18, 2020 (Depth of approximately 5 km) |
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| – Offshore East of Aomori Prefecture | M6.5 on December 21, 2020 (Depth of approximately 45 km) |
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| – Offshore Fukushima Prefecture | M7.3 on February 13, 2021 (Depth of approximately 55 km) |
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| – In Kermadec Islands | Mw8.1 on March 5, 2021 | ||
| – In the Northern Wakayama M4.6 on March 15, 2021 Prefecture | M4.6 on March 15, 2021 (Depth of approximately 5 km) |
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| – Offshore Miyagi Prefecture | M6.9 on March 20, 2021 (Depth of approximately 60 km) |
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| – Offshore Miyagi Prefecture | M6.8 on May 1, 2021 (Depth of approximately 50 km) |
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There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
There is no supplementary information for Kanto-Chubu region.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the central Shikoku has been observed from spring 2019. This phenomenon is considered to be caused by a long-term slow-slip around the central Shikoku at the deeper part of the boundary between the Philippine Sea and the continental plates.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the western Kii Peninsula and the eastern Shikoku, being observed from summer 2020, is considered to be caused by a long-term slow-slip around the Kii Channel at the deeper part of the boundary between the Philippine Sea and the continental plates. This crustal deformation seems to have slowed down in spring 2021 and continue at present.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in southern Kyushu, being observed from summer 2020, is considered to be caused by a long-term slow-slip around the southern Hyuganada at the deeper part of the boundary between the Philippine Sea and the continental plates. This crustal deformation seems to have slowed down in spring 2021 and continue at present.
–"There has been observed no unusual change which indicates
that the possibility of occurrence of great earthquakes along the Nankai
Trough has relatively raised compared to the normal time.":
(This evaluation is in agreement with the views presented at the regular meeting on October 7 of the Assessment Committee for Earthquakes Along the Nankai Trough (reference below).)
(Reference)
Recent Crustal Activity Around the Nankai Trough (Seismological and Volcanological Department, Japan Meteorological Agency, October 7, 2021)
"In the present, there has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time. (Excerpted)"
There is no supplementary information for other regions.
Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: