December 9, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
There was no remarkable activity.
There was no remarkable activity.
* On November 9, there was a magnitude (M) 4.9 earthquake at a depth of approximately 5 km in Naka-dori, Fukushima prefecture. The focal mechanism showed a normal fault type with a tension axis in an NNE-SSW direction. This event occurred within the crust.
* In the crust of the Noto region, Ishikawa prefecture, the number of earthquakes is increasing since about 2018, and the seismic activity became high since December 2020, and it has become higher since about July 2021. The largest earthquake is an M5.1 occurring on September 16, 2021. The high seismic activity continues in November: an M4.0 earthquake on 5th. From December 1, 2020 till December 5, 2021, there occurred 63 earthquakes with seismic intensity 1 or over: 12 earthquakes from November 1 till December 5.
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 3 cm or so at the Suzu observatory in Suzu City.
Based on the situation of the seismic activity and the crustal deformation, the series of the seismic activity will continue for the time being.
* 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 continental plates.
* On November 20, there was an M4.6 earthquake at a depth of approximately 100 km, under the eastern Tama, Tokyo prefecture (#1). The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the Pacific plate.
* On November 29, there was an M6.4 earthquake at a depth of approximately 10 km (CMT solution) in the ocean near Torishima Island. The focal mechanism showed a normal fault type with a tension axis in an ENE-WSW direction. This event occurred within the Pacific plate.
There was no remarkable activity.
* On November 11, there was an M6.5 earthquake at a depth of approximately 20 km (CMT solution) in the ocean near Miyakojima Island (#2). The focal mechanism showed a normal fault type with a tension axis in an N-S 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 December 2, there was an M5.1 earthquake at a depth of approximately 65 km under the southern Ibaraki prefecture. The focal mechanism had a tension axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate.
* On December 3 at 06:37 (JST), there was an M4.8 earthquake at a depth of approximately 20 km in the Fujigoko area, eastern Yamanashi prefecture. Maximum seismic intensity 5 Lower was observed in Yamanashi prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. In the vicinity of the recent event, on the same day at 02:17 (JST) there was an M4.1 earthquake.
According to the result of the GNSS observation, no significant crustal deformation associated with the event was observed.
The vicinity of the recent event is the area where the Philippine Sea plate is colliding with the continental plate, and there have sometimes been earthquakes of M5.0 or over, such as an M6.0 earthquake in 1983. On January 28, 2012, there was an M5.4 earthquake just after an M4.9 one.
In the area of strong shaking, we should pay attention for a week or so after the earthquake, to the possibility of an earthquake which causes maximum seismic intensity 5 Lower or so.
* On December 3, there was an M5.4 earthquake at a depth of approximately 20 km under the Kii Channel. Maximum seismic intensity 5 Lower was observed in Wakayama prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the crust.
According to the result of the GNSS observation, no significant crustal deformation associated with the event was observed.
In the area of strong shaking, we should pay attention for a week or so after the earthquake, to the possibility of an earthquake which causes maximum seismic intensity 5 Lower or so.
* On December 9, there was an M6.0 (preliminary) earthquake at a depth of approximately 20 km in the ocean near Tokara Islands (near Kodakarajima Island). Maximum seismic intensity 5 Upper was observed in Kagoshima prefecture (Toshima Village). The focal mechanism showed a normal fault type (preliminary) with a tension axis in an NW-SE direction. This event occurred within the continental plate. In the ocean near Tokara Islands the seismic activity has become high since December 4, and till 9th at 12 o'clock there have occurred 233 earthquakes with seismic intensity 1 or over, 16 earthquakes with seismic intensity 3 or over: the high activity continues.
In the vicinity of the recent seismic activity area, there occurred several high seismic activities. During the activity in October 2000, on October 2 there was an M5.9 earthquake with seismic intensity 5 Upper observed, and 2 other earthquakes with seismic intensity 5 Lower observed. Recently there was a high seismic activity in April 2021; on April 10 and 12 there were M5.3 earthquakes, and from April 9 till 30th there occurred 265 earthquakes with seismic intensity1 or over.
In this area we should pay attention for the time being, to the occurrence of an earthquake which causes maximum seismic intensity 5 Upper or so.
#1: "Under the 23 Wards, Tokyo Prefecture" is the region name used in the announcement of Japan Meteorological Agency.
#2: "Offshore south of Okinawa Main Island" is the region name used in the announcement of Japan Meteorological Agency.
Note: GNSS is a general name of satellite positioning system such as GPS.
December 9, 2021
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during November 2021.
(Reference)
The following events for the period from November 2020 to late October 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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| – Offshore Iwate Prefecture | M5.9 on October 6, 2021 (Depth of approximately 55 km) |
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| – Under the Northwestern Chiba Prefecture | M5.9 on October 7, 2021 (Depth of approximately 75 km) |
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There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– In the ocean near Izu-Oshima Island there was a seismic activity
beginning on December 4, the maximum earthquake being an M3.2 one and there
occurred 24 earthquakes with seismic intensity 1 or over. In the vicinity
of the recent seismic activity area, there was a seismic activity in March
~ May 2021.
– 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.
– On November 29 in the Satsuma region, Kagoshima prefecture, there was a seismic activity, the maximum being an M3.3 earthquake, and there occurred 10 earthquakes with maximum seismic intensity 1 or over.
– 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 has slowed down since about spring 2021 and the state seems to 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 December 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, December 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)"
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: