January 13, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On December 12, there was a magnitude (M) 5.6 earthquake offshore Iwate prefecture. The maximum seismic intensity 5 Lower was observed in Aomori prefecture.
* On December 18, there was an M5.0 earthquake in the ocean area near Niijima and Kozushima Islands. The maximum seismic intensity 5 Lower was observed (at Toshima Village) in Tokyo prefecture.
* On December 21, there was an M6.5 earthquake offshore east of Aomori prefecture. The maximum seismic intensity 5 Lower was observed in Iwate prefecture and was caused damage with injured persons and so on.
* On December 16, there was an M5.0 earthquake at a depth of approximately 65 km, offshore Kushiro. The focal mechanism had a compression axis in an NNE-SSW direction. This event occurred within the Pacific plate.
* On December 12, there was an M5.6 earthquake at a depth of approximately 50 km, offshore Iwate 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 continental plates. According to the GNSS observation, no significant crustal deformation accompanied with this event was observed.
* On December 21, there was an M6.5 earthquake at a depth of approximately 45 km, offshore east of Aomori 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. According to the GNSS observation, no significant crustal deformation accompanied with this event was observed.
* On December 17, there was an M4.6 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 December 18 at 18:09 (JST), there was a M5.0 earthquake at a depth of approximately 10km in the ocean area near Niijima and Kozushima Islands (#1). The focal mechanism showed a strike-slip fault type with a compression axis in an NNW-SSE direction. This event occurred within the Philippine Sea plate. In the vicinity of this event, there occurred an M4.6 earthquake on the same day at 20:36 (JST), and an M4.7 one on January 1, 2021 at 01:53 (JST). In the period from December 15 to January 3, there were 39 earthquakes with seismic intensity 1 or over. According to the GNSS observation, no significant crustal deformation accompanied with this event was observed.
* On December 30, there was an M5.2 earthquake at a depth of approximately 55 km, offshore Ibaraki prefecture (#2). 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.
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 December 1, there was an M6.7 earthquake at a depth of approximately 620 km, offshore west of Sakhalin. The focal mechanism had a tension axis in an NNW-SSE direction. This event occurred within the Pacific plate. Seismic intensity 1 or over was observed in the area extending from Hokkaido to Kanto regions: shaking was larger in the Pacific side of Hokkaido and Tohoku regions due to the seismic wave propagating through the subducting Pacific plate (abnormal seismic intensity area).
* On December 10, there was an M6.3 earthquake at a depth of approximately 85 km, near Taiwan. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate.
#1: "The ocean area near Izu-Oshima Island" is the region name used in the announcement of Japan Meteorological Agency.
#2: "The northern Ibaraki prefecture" 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.
January 13, 2021
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during December 2020.
(reference)
The following events for the period from December 2019 to late November 2020 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – Offshore East of Aomori Prefecture | M5.5 on December 19, 2019 (Depth of approximately 50 km) |
|
| – Noto Region, Ishikawa Prefecture | M5.5 on March 13, 2020 (Depth of approximately 10 km) |
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| – Offshore East of Chiba Prefecture | M6.1 on June 25, 2020 (Depth of approximately 35 km) |
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| – 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) |
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, has been observed from 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 northern Kyushu has been observed from summer 2020. This phenomenon is considered to be caused by a long-term slow slip around the northern Hyuganada 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 southern Kyushu, has been observed from summer 2020. This phenomenon 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.
– According to the GNSS observation, the crustal deformation different from the ordinary trend near Tanegashima Island, has been observed from summer 2020. This phenomenon is considered to be caused by a long-term slow slip around Tanegashima Island at the deeper part of the boundary between the Philippine Sea and the continental plates.
–"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 January 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, January 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: