January 15, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for December 2023

1 Major Seismic Activities

* On December 2, there was a moment magnitude (Mw) 7.5 earthquake in Mindanao, the Philippine Islands, which caused tsunami observed in the Pacific coasts from Miyagi to Kagoshima Prefectures, Okinawa Prefecture, the Izu Islands, and the Ogasawara Islands.

• Seismic Activities for All of Japan in December 2023 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On December 28, there was a magnitude (M) 6.6 earthquake at a depth of approximately 40 km (due to CMT solution), offshore southeast of Etorofu Island. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction.

• Hokkaido Region
• Earthquake Offshore Southeast of Etorofu Island on December 28

to supplementary reports

(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* With regard to the 2024 Noto Peninsula Earthquake (#1), refer to another report "Evaluation of the 2024 Noto Peninsula Earthquake."�

• Kanto - Chubu Region

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 a great earthquake 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 Cape Muroto

to supplementary reports

(7) Other Regions

* On December 2, there was an Mw7.5 earthquake in Mindanao, the Philippine Islands. 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 Philippine Sea and the Eurasian Plates. This event caused tsunami observed in the Pacific coasts from Chiba to Kagoshima Prefectures, Okinawa Prefecture, the Izu Islands, and the Ogasawara Islands such as height of 0.4 m at Yaene, Hachijojima Island, the Izu Islands and so on.

• Earthquake in Mindanao, the Philippine Islands on December 2, 2023 - Summary
• Earthquake in Mindanao, the Philippine Islands on December 2, 2023 - Tsunami
• Earthquake in Mindanao, the Philippine Islands on December 2, 2023 – Tsunami (continued)
• Earthquake in Mindanao, the Philippine Islands on December 2, 2023 – Tsunami (continued)

to supplementary reports

Supplement (Seismic Activity Since December 1)

* With regard to the 2024 Noto Peninsula Earthquake, refer to another report "Evaluation of the 2024 Noto Peninsula Earthquake."�

   #1: "The 2024 Noto Peninsula Earthquake" (named by JMA) denotes the series of earthquakes after January 1, 2024 and the series of seismic activity in the Noto region since December 2020.

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

Supplementary Information to the Evaluation of Seismic Activities for December 2023

January 15, 2024
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 2023.

• M4.0 or greater: 90 (69 events in November)
• M5.0 or greater: 6 (7 events in November)
• M6.0 or greater: 1 (0 event in November)

(Reference)

• M4.0 or greater: 81 / month (69-104)
• (Median monthly number from 1998 to 2017, half of numbers 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 numbers 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 numbers 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 numbers are included in the interval shown in parentheses)

The following events for the period from December 2022 to late November 2023 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

– According to the GNSS observation, the crustal deformation different from the ordinary trend in the area extending from the western Shizuoka to the eastern Aichi Prefectures has been observed from early 2022. This phenomenon is considered to be caused by a long-term slow slip around the Atsumi Peninsula at the deeper part of the boundary between the Philippine Sea and the continental plates.

– In the period from December 22 till January 6, in the area extending from Tokai to the central Kii Peninsula, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain and tilt data, almost synchronized with the deep low-frequency seismic (tremor) activity, slight crustal deformations were observed in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at the boundary between the Philippine Sea and the continental plates.

• Coordinate Time Series (Black Circles) and Calculated Values (Red Lines) of Observing Stations in Tokai Region
• Long-Term Slow Slip in the Tokai Region Estimated from GNSS Data (provisional)
• Deep Low-Frequency Seismic (Tremor) Activity and Short-Term Slow-Slip in the Area Extending from Tokai to the Central Kii Peninsula
• Fig.2 Time Variation of Strain and Tilt (1) (2023/12/16 00:00 – 2024/01/06 00:00 (JST))
• Fig.2 Time Variation of Strain and Tilt (2) (2023/12/16 00:00 – 2024/01/06 00:00 (JST))
• Fig.2 Time Variation of Strain and Tilt (3) (2023/12/16 00:00 – 2024/01/06 00:00 (JST))
• Fig.2 Time Variation of Strain and Tilt (4) (2023/12/16 00:00 – 2024/01/06 00:00 (JST))

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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 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. Recently this crustal deformation seems to be decaying.

– In the western Shikoku, in the period from November 30 till December 22, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain, tilt and GNSS data, almost synchronized with the deep low-frequency seismic (tremor) activity, slight crustal deformations were observed in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at the boundary between the Philippine Sea and the continental plates.

– In the central Shikoku, in the period from December 23 till January 2, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain and tilt data, almost synchronized with the deep low-frequency seismic (tremor) activity, slight crustal deformations were observed in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at the boundary between the Philippine Sea and the continental plates.

• Time Series (Black Points) Representing the Coordinates of the Observatories in the Central Shikoku and Calculated Values (Red Lines) – Time-Dependent Inversion –
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (Provisional)
• Deep Low-Frequency Seismic (Tremor) Activity and Short-Term Slow-Slip in the Western Shikoku
• Fig.6 Time Variation of Strain and Tilt (1) (2023/11/23 00:00 – 2023/12/20 00:00 (JST))
• Fig.6 Time Variation of Strain and Tilt (2) (2023/11/23 00:00 – 2023/12/20 00:00 (JST))
• Fig.6 Time Variation of Strain and Tilt (3) (2023/11/23 00:00 – 2023/12/20 00:00 (JST))
• Slow-Slip in the Western Shikoku Synchronized with Deep Low-Frequency Seismic (Tremor) Activity, Estimated from GNSS Data (Provisional)
• Deep Low-Frequency Seismic (Tremor) Activity and Short-Term Slow-Slip in the Central Shikoku
• Fig.13 Time Variation of Strain and Tilt (1) (2023/12/17 00:00 – 2024/01/06 00:00 (JST))
• Fig.13 Time Variation of Strain and Tilt (2) (2023/12/17 00:00 – 2024/01/06 00:00 (JST))

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(5) Kyushu-Okinawa Region

– Within the continental plate of the ocean area near Tokara Islands (near Takarajima Island), in December there were 15 earthquakes with seismic intensity 1 or over observed. The largest is the M3.6 earthquake on December 27.

– According to the GNSS observation, the crustal deformation different from the ordinary trend in the southern Kyushu has been observed from early 2023. This phenomenon is considered to be caused by a long-term slow slip around the southern Hyuganada Sea at the deeper part of the boundary between the Philippine Sea and the continental plates. This crustal deformation seems to have already stopped.

• Seismic Activity in the Ocean Area near the Tokara Islands (near Takarajima Island)
• Seismic Activity in the Ocean Area near the Tokara Islands (near Takarajima Island) (continued)
• Seismic Activity in the Ocean Area near the Tokara Islands (near Akusekijima, Kodakarajima, and Takarajima Islands) Comparison with Past Situation of Activities M2.5 or over, 120 Days Period
• Time Series (Black Points) Representing the Coordinates of the Observatories in Kyushu Region and Calculated Values (Red Lines) – Time-Dependent Inversion –
• Long-Term Slow Slip Around the Southern Hyuganada Sea 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 a great earthquake 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, 2024)
"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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