December 9, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for November 2020

1 Major Seismic Activities

* On November 22, there was a magnitude (M) 5.7 earthquake offshore Ibaraki prefecture. The maximum seismic intensity 5 Lower was observed in Ibaraki prefecture.

• Seismic Activities for All of Japan in November 2020 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There was no remarkable activity.

• Hokkaido Region

to supplementary reports

(2) Tohoku Region

* On November 6, there was an M5.7 earthquake at a depth of approximately 15 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.

• Tohoku Region
• Earthquake Offshore East of Aomori Prefecture on November 6

to supplementary reports

(3) Kanto-Chubu Region

* On November 11, there was an M6.2 shallow earthquake in the ocean area near Iwotou Island.

* On November 22, there was an M5.7 earthquake at a depth of approximately 45 km, offshore 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. According to the GNSS observation, no significant crustal deformation associated with this event was observed.

• Kanto - Chubu Region
• Earthquake in the Ocean Area near Iwotou Island on November 11
• Earthquake in the Ocean Area near Iwotou Island on November 11 (Comparison of Seismic Wave Forms)
• Earthquake Offshore Ibaraki Prefecture on November 22
• Observed Data Before and After the Earthquake Offshore Ibaraki Prefecture (November 22, M5.7) (provisional)

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

There was no remarkable activity.

• Kyushu Region
• Okinawa Region

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
• Vertical Movements of GNSS-Based Control Stations - Kii Peninsula and Around Muroto Cape

to supplementary reports

Supplement

* 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).

• Earthquake Offshore West of Sakhalin on December 1
• Distribution Map of Seismic intensity of the earthquake offshore west of Sakhalin occurring on December 1
• Examples of Distribution Map of Seismic Intensity of the Past Earthquakes Accompanied with the Abnormal Seismic Intensity Area

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

Supplementary Information to the Evaluation of Seismic Activities for November 2020

December 9, 2020
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 November 2020.

• M4.0 or greater: 69 (64 events in October)
• M5.0 or greater: 14 (4 events in October)
• M6.0 or greater: 1 (0 event in October)

(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 November 2019 to late October 2020 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.

to the text

(3) Kanto-Chubu Region

There is no supplementary information for Kanto-Chubu region.

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(4) Kinki-Chugoku-Shikoku 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.

– From November 20 till 27 in the western Shikoku, near the boundary between the Philippine Sea and the continental plates, deep low-frequency earthquakes (tremors) were observed. According to the strain and tilt observation data, slight crustal deformations in the surrounding area, almost synchronous with the deep low-frequency earthquakes (tremors), were observed. These phenomena are considered to be caused by the short-term slow slip at the deep part of the boundary between the Philippine Sea and the continental plates.

• 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)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Western Shikoku
• Fig.6 Observed results of Strain, Tilt and Underground Water in the Western Shikoku (2020/11/06 00:00 – 2020/12/02 00:00 (JST))
• Fig.7 Fault Model Explaining the Variation of Strain, Tilt and Underground Water (Fig.6[A]) during 2020/11/20 – 22
• Fig.8 Fault Model Explaining the Variation of Strain, Tilt and Underground Water (Fig.6[B]) during 2020/11/23 – 25

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(5) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

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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 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, 2020)
"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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