January 13, 2022
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for December 2021

1 Major Seismic Activities

* On December 3, there was a magnitude (M) 4.8 earthquake under the eastern and Fujigoko regions, Yamanashi prefecture. Maximum seismic intensity 5 Lower was observed in Yamanashi prefecture.

* On December 3, there was an M5.4 earthquake under the Kii Channel. Maximum seismic intensity 5 Lower was observed in Wakayama prefecture and was caused damage with injured persons and so on.

* On December 9, there was an M6.1 earthquake in the ocean near Tokara Islands (near Kodakarajima Island). Maximum seismic intensity 5 Upper was observed in Kagoshima prefecture (Toshima Village) and was caused damage with a rockfall and so on.

• Seismic Activities for All of Japan in December 2021 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On December 21, there was an M4.8 earthquake at a depth of approximately 40 km under the central Hidaka region. The focal mechanism showed a reverse fault type with a compression axis in an NE-SW direction. This event occurred within the continental plate.

* On December 21, there was an M5.0 earthquake at a depth of approximately 95 km near Kunashiri Island. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquake Under the Central Hidaka Region on December 21
• Earthquake near Kunashiri Island on December 21

to supplementary reports

(2) Tohoku Region

* On December 8, there was an M5.2 earthquake at a depth of approximately 15 km, offshore Fukushima prefecture. The focal mechanism showed a normal fault type with a tension axis in an NW-SE direction. This event occurred within the continental plate.

* On December 8, there was an M5.0 earthquake at a depth of approximately 55 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the Pacific plate.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on December 8 at 02:29
• Earthquake Offshore Fukushima Prefecture on December 8 at 16:22

to supplementary reports

(3) Kanto-Chubu Region

* 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 is an M5.1 earthquake occurring on September 16, 2021. The high seismic activity continues in December: an M4.3 earthquake on 31st. From December 1, 2020 till January 9, 2022, there occurred 74 earthquakes with seismic intensity 1 or over: 13 earthquakes from December 1, 2021 till January 9, 2022.
According to the GNSS observation, since about December 2020, the following crustal deformation has been observed in the Noto Peninsula: the cumulative 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 over 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 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 under the eastern and Fujigoko regions, Yamanashi prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. Also, in the vicinity of the 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.

* On December 12, there was an M5.0 earthquake at a depth of approximately 50 km under the southern Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

• Kanto - Chubu Region
• Seismic Activity in the Noto Region, Ishikawa Prefecture
• Observed Data at the Time of the Seismic Activity in the Noto Region, Ishikawa Prefecture (provisional)
• Observed Data at the Time of the Seismic Activity in the Noto Region, Ishikawa Prefecture (provisional)
• Earthquake Under the Southern Ibaraki Prefecture on December 2
• Earthquake Under the Eastern and Fujigoko Regions, Yamanashi Prefecture on December 3
• Observed Data Before and After the Earthquake Under the Eastern and Fujigoko Regions, Yamanashi Prefecture (The Largest Earthquake December 5, M4.8) (provisional) (1)
• Observed Data Before and After the Earthquake Under the Eastern and Fujigoko Regions, Yamanashi Prefecture (The Largest Earthquake December 5, M4.8) (provisional) (2)
• Earthquake Under the Southern Ibaraki Prefecture on December 12

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On December 3, there was an M5.4 earthquake at a depth of 18 km under the Kii Channel. 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.

• Kinki-Chugoku-Shikoku Region
• Earthquake Under the Kii Channel on December 3
• Observed Data Before and After the Earthquake Under the Kii Channel (December 3, M5.4) (1)
• Observed Data Before and After the Earthquake Under the Kii Channel (December 3, M5.4) (2)

to supplementary reports

(5) Kyushu-Okinawa Region

* On December 9, there was an M6.1 earthquake in the ocean near Tokara Islands (near Kodakarajima Island). The focal mechanism showed a normal fault type 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 31^st there occurred 308 earthquakes with seismic intensity 1 or over: 18 earthquakes with seismic intensity 3 or over.
According to the result of the GNSS observation, no significant crustal deformation associated with the event was observed.
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 30^th there occurred 265 earthquakes with seismic intensity1 or over.

* On December 26, there was an M6.1 earthquake at a depth of approximately 20 km (CMT solution) under the ocean near Miyakojima Island. The focal mechanism showed a normal fault type with a tension axis in an ENE-WSW direction. This event occurred within the continental plate.

• Kyushu Region
• Seismic Activity in the Ocean near Tokara Islands (near Kodakarajima Island)
• Seismic Activity in the Ocean near Tokara Islands (near Kodakarajima Island) (Comparison with the past seismic activities, M2.5 or over) (1)
• Seismic Activity in the Ocean near Tokara Islands (near Kodakarajima Island) (Comparison with the past seismic activities, M2.5 or over) (2)
• Table of Number of Earthquakes with Maximum Seismic Intensity 1 or Over in the Recent Seismic Activity by Day and by Maximum Seismic Intensity (December 4 ~ December 31, 2021)
• Observed Data Before and After the Seismic Activity in the Ocean near Tokara Islands (the Largest Earthquake, December 9, M6.1) (provisional)
• Okinawa Region
• Earthquake Under the Ocean near Miyakojima Island on December 26

to supplementary reports

(6) Around the Nankai Trough

* 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.

• Vertical Movement of GNSS-Based Control Stations - Omaezaki
• Secular Variation of Benchmark 2595 (Omaezaki City)
• Vertical Movements of GNSS-Based Control Stations - Kii Peninsula and Around Muroto Cape

to supplementary reports

Supplement (Seismic Activity Since January 1)

* On January 3, there was an M6.3 earthquake at a depth of approximately 25 km near Taiwan. The focal mechanism had a compression axis in an N-S direction.

* On January 4, there was an M6.1 earthquake at a depth of approximately 60 km (CMT solution), under the ocean near Chichijima Island. Maximum seismic intensity 5 Upper was observed in Tokyo prefecture (Ogasawara Village). The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate.
According to the result of the GNSS observation, no significant crustal deformation associated with the event was observed.

• Earthquake near Taiwan on January 3
• Earthquake Under the Ocean near Chichijima Island on January 4
• Observed Data Before and After the Earthquake Under the Ocean near Chichijima Island (January 4, M6.1) (provisional)

Note: GNSS is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for December 2021

January 13, 2022
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during December 2021.

• M4.0 or greater: 149 (86 events in November)
  M5.0 or greater: 14 (7 events in November)
  M6.0 or greater: 2 (2 events in November)
  

(Reference)

• M4.0 or greater: 81 / month (69-104)
• (Median monthly number from 1998 to 2017, half of data are included in the interval shown in parentheses)
• M5.0 or greater: 10 / month (7-14)
• (Median monthly number from 1973 to 2017, half of data are included in the interval shown in parentheses)
• M6.0 or greater: 1 / month (0-2)
• (Median monthly number from 1919 to 2017, half of data are included in the interval shown in parentheses)
• M6.0 or greater: 16 / year (12-21)
• (Median annual number from 1919 to 2017, half of data are included in the interval shown in parentheses)

The following events for the period from December 2020 to late November 2021 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

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(2) Tohoku Region

There is no supplementary information for Tohoku region.

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(3) Kanto-Chubu Region

– 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. In the vicinity of the event, till December 31, there occurred 54 earthquakes of M4.0 or over: from December 1 till 31^st, 46 earthquakes.

– 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 till 17^th there occurred 25 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.

– In the northern Kii Peninsula, in the period from November 27 till December 6, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the observation of strain and tilt, slight crustal deformations were observed in the surrounding area, almost synchronous with the deep low-frequency earthquake (tremor) activity. These phenomena are considered to be caused by the short-term slow slip along the deep part of the boundary between the Philippine Sea and the continental plates.

• Earthquake in the Ocean near Torishima Island on November 29
• Seismic Activity in the Ocean near Izu-Oshima Island
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Northern Kii Peninsula
• Fig.2 Time Variation of Strain, Tilt and Underground Water Level (1) (2021/11/22 00:00 - 2021/12/08 00:00 (JST))
• Fig.2 Time Variation of of Strain, Tilt and Underground Water Level (1) (2021/11/22 00:00 - 2021/12/08 00:00 (JST))
• Fig.3 Fault Model Explaining the Variation of Strain, Tilt and Underground Water Level (Fig.2[A]) During 2021/11/27-30
• Fig.4 Fault Model Explaining the Variation of Strain, Tilt and Underground Water Level (Fig.2[B]) During 2021/12/01-02
• Fig.5 Fault Model Explaining the Variation of Strain, Tilt and Underground Water Level (Fig.2[C]) During 2021/12/03-04

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(4) Kinki-Chugoku-Shikoku Region

– In the area extending from the eastern to the western Shikoku, in the period from November 27 till December 14, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the observation of strain and tilt, slight crustal deformations were observed in the surrounding area, almost synchronous with the deep low-frequency earthquake (tremor) activity. These phenomena are considered to be caused by the short-term slow slip along 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 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.

• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in Shikoku
• Fig.12 Time Variation of Strain and Tilt (1) (2021/11/27 00:00 - 2021/12/17 00:00 (JST))
• Fig.12 Time Variation of Strain and Tilt (2) (2021/11/27 00:00 - 2021/12/17 00:00 (JST))
• Fig.13 Fault Model Explaining the Variation of Strain and Tilt (Fig.12[A]) During 2021/12/03PM-05
• Fig.14 Fault Model Explaining the Variation of Strain and Tilt (Fig.12[B]) During 2021/12/08-10
• Non-Stationary Horizontal Crustal Deformation of the Central Shikoku (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Central Shikoku (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Central Shikoku (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (provisional)
• Non-Stationary Horizontal Crustal Deformation of the Western Kii Peninsula and the Eastern Shikoku (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and the Eastern Shikoku (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and the Eastern Shikoku (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Kii Channel Estimated from GNSS Data (provisional)

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(5) Kyushu-Okinawa Region

– 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. Recently this crustal deformation seems to stagnant.

• Non-Stationary Horizontal Crustal Deformation of the Kyushu Region (After Removal of Linear Trend)
• Time Series of GNSS Continuous Observation: the Kyushu Region
• Long-Term Slow Slip Around the Southern Hyuganada Estimated from GNSS Data (provisional)

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(6) Around the Nankai Trough

–"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 11 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 11, 2022)
"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)"

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