November 10, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On October 6, there was a magnitude (M) 5.9 earthquake offshore Iwate prefecture. Maximum seismic intensity 5 Upper was observed in Aomori prefecture and was caused damage with injured persons.
* On October 7, there was an M5.9 earthquake under the northwestern Chiba prefecture. Maximum seismic intensity 5 Upper was observed in Saitama and Tokyo prefectures. In Chiba and Tokyo prefectures long-period seismic intensity 2 was observed. Also was caused damage with injured persons.
* On October 10, there was an M4.7 earthquake at a depth of approximately 50 km under the southern Tokachi region. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Pacific and the continental plates.
* On October 6, there was an M5.9 earthquake at a depth of approximately 55 km, offshore Iwate prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction. This event occurred within the Pacific plate.
Then in the vicinity of the event, till October 31 there were 3 earthquakes with seismic intensity 1 or over. The largest aftershock was an M4.7 earthquake on October 9.
According to the result of GNSS observation, no significant crustal deformation associated with the recent event was observed.
* On October 19, there was an M5.4 earthquake at a depth of approximately 40 km, offshore east of Aomori prefecture. The focal mechanism showed a normal fault type with a tension axis in an NNE-SSW direction. This event occurred within the Pacific plate.
* On October 27, there was an M5.0 earthquake at a depth of approximately 25 km, offshore Fukushima prefecture. The focal mechanism had a compression axis in an ENE-WSW direction.
* On October 7, there was an M5.9 earthquake at a depth of approximately 75 km under the northwestern Chiba prefecture. 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 in the area including the M5.9 event, extending about 10 km length in E-W and N-S directions, till October 31 there occurred 7 earthquakes with seismic intensity 1 or over. The largest aftershock was that of M4.2 which occurred on October 29.
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 about M6, such as an M6.0 one on September 25, 1980. The largest was an M6.3 earthquake on September 30, 1956.
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 which is 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.
* On October 21, there was an M5.6 earthquake at a depth of approximately 360 km, offshore south of Tokaido. The focal mechanism had a compression axis in an NNW-SSE direction. This event occurred within the Pacific plate. In the area extending from Tohoku, Kanto-Chubu regions, and Wakayama prefecture, 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 October 28, there was an M4.5 earthquake at a depth of approximately 45 km under the southern Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Philippine Sea and the continental plates.
* On October 15, there was an M3.3 earthquake at a depth of approximately 5 km in the northern Wakayama prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the crust.
* 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.
* 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 October 12, there was an M6.0 earthquake in the northwestern Pacific Ocean (near the Kurile Islands).
* On October 24, there was an M6.3 earthquake at a depth of approximately 75 km near Taiwan. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate.
* On November 1, there was an M5.3 earthquake at a depth of approximately 55 km under the northern Ibaraki prefecture. 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 Philippine Sea plates.
#: 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.
November 10, 2021
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during October 2021.
(Reference)
The following events for the period from October 2020 to late September 2021 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – 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 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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| – In the Noto Region, Ishikawa Prefecture | M5.1 on September 16, 2021 (Depth of approximately 15 km) |
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There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– In the crust of the Noto region, Ishikawa prefecture, the number of earthquakes is increasing since about 2018, and the seismic activity has become high since December 2020. The largest earthquake is an M5.1 occurring on September 16, 2021. The high seismic activity continues in October: an M4.3 on October 3 and an M4.0 on 19th. From December 1, 2020 till November 4, 2021, there occurred 57 earthquakes with seismic intensity 1 or over: 19 earthquakes from October 1 till November 4.
According to the GNSS observation, since about December 2020, the following crustal deformation has been observed in the Noto Peninsula: 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.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the central Shikoku has been observed from about 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 has been observed from about summer 2020. This phenomenon 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.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the southern Kyushu, being observed from about 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 since about spring 2021 and the state continues 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 November 8 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, November 8, 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: