February 9, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for January 2021

1 Major Seismic Activities

There was no remarkable activity.

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

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On January 12, there was a magnitude (M) 6.0 earthquake at a depth of approximately 240 km under the central and eastern Iburi regions, Hokkaido. The focal mechanism had a tension axis in an NNE-SSW direction. This event occurred within the Pacific plate.

* On January 27, there was an M5.4 earthquake at a depth of approximately 130 km, offshore west of Hokkaido. The focal mechanism had a tension axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquake Offshore West of Hokkaido on January 12
• Earthquake Under the Central and Eastern Iburi Regions on January 27

to supplementary reports

(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* On January 1, there was an M4.7 earthquake at a depth of approximately 10 km in the ocean area near Niijima and Kozushima Islands. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the crust of the Philippine Sea plate. In the vicinity of the recent event, on December 18, 2020 at 18:09 (JST), there was an M5.0 earthquake and on the same day at 20:36 (JST), there was an M4.6 one. In the period from December 15 to January 31, there were 40 earthquakes with maximum seismic intensity 1 or over.

• Kanto - Chubu Region
• Earthquake in the Ocean Area near Niijima and Kozushima on January1

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 February 2, there was an M4.0 earthquake at a depth of approximately 5 km in the northern Tochigi prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust.

• Earthquake in the Northern Tochigi Prefecture on February 2

Supplementary Information to the Evaluation of Seismic Activities for January 2021

February 9, 2021
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 January 2021.

• M4.0 or greater: 81 (74 events in December)
• M5.0 or greater: 9 (15 events in December)
• M6.0 or greater: 1 (3 events in December)

(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 January 2020 to late December 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.

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(3) Kanto-Chubu Region

– Offshore of southeast of the Kii Peninsula, in the period from December 6 to January 21 and from January 26 to 29, shallow low-frequency earthquakes (tremors) and shallow ultra-low-frequency earthquakes were observed near the boundary between the Philippine Sea and the continental plates. Synchronously to the shallow low-frequency earthquakes (tremors) and shallow ultra-low-frequency earthquakes, crustal deformations were observed on the several pore water pressure gauges in the hole settled in the surrounding ocean area. Such crustal deformations have several times been observed on the in-hole observation which began in 2010, the recent being the largest. Also slight crustal deformations were observed on the several strain-meters settled in the Kii Peninsula. Note that all crustal deformations have already converged. These phenomena are possibly caused by the slow slip at the boundary between the Philippine Sea and the continental plates.

• Tremor Activity Offshore Southeast of the Kii Peninsula (December 2020 – January 2021)
• Shallow Ultra-Low-Frequency Earthquake Activity Around Japan (December 2020 – January 2021)
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (1)
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (2) Comparison with the Similar Activity During March to April 2009 (Takemura, Noda et al., 2019)
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (3) Monitoring of SSE - Low-Frequency Tremor During the Recent 2 Months (December 1, 2020 – January 31, 2021)
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (4) Variation of the Pore Pressure at the Time of SSE, Observed at the In-Hole Observatories
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (5) Slow Variations of Pore Pressure in the Drilled Holes, of Records of In-Hole Tilt-Meters and Ocean-Bottom Tilt-Meters Settled in Kumanonada Sea
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (6) Fault Model Explaining the Variations of Pore Pressure and Tilt in the Drilled Holes, C0002, C0010, and C0006, and of Ocean-Bottom Tilt of Station BMS1 During the Period from December 16 to 21, 2020.
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (7) Observed Results of Strain in the Kii Peninsula (2020/11/20 00:00 - 2020/12/01 00:00 (JST)) / Fault Model Explaining the Variation of Strain (Fig.8[A]) During 2020/12/11 - 2021/01/19
• Shallow Ultra-Low-Frequency Earthquake Activity Offshore Southeast of the Kii Peninsula During December 2020 to January 2021 (8) Fault Model Explaining the Variation of Strain (Fig.8[A]) During 2020/12/11 - 2021/01/19

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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.

– In the western and central Shikoku, from January 10 till 30, 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, slight crustal deformations were observed in the surrounding area, almost synchronously with the deep low-frequency earthquakes (tremors). These phenomena are considered to be caused by the short-term slow slip at 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 Shikoku (1) Deep Low-Frequency Earthquake (Tremor) Activity
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in Shikoku (2) Time Variation of Strain and Tilt (2021/01/07 00:00-2021/02/01 00:00 (JST))
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in Shikoku (3) Time Variation of Tilt (2021/01/07 00:00-2021/02/01 00:00 (JST))
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in Shikoku (4) Fault Model Explaining the Variation of Strain, and Tilt During 2021/01/11-18 (Fig.2-3[A])
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in Shikoku (5) Fault Model Explaining the Variation of Strain, and Tilt During 2021/01/19-21AM (Fig.2-3[B])
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in Shikoku (6) Fault Model Explaining the Variation of Strain, and Tilt During 2021/01/21PM-23 (Fig.2-3[C])
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in Shikoku (7) Fault Model Explaining the Variation of Strain, and Tilt During 2021/01/24-28AM (Fig.2-3[D])
• Slow Slip Synchronous with Deep Low-Frequency Tremor in the Western Shikoku 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 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.

• Non-Stationary Horizontal Crustal Deformation of the Northern and Southern Kyushu (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Northern and Southern Kyushu (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Northern and Southern Kyushu (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Northern and 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 February 5 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, Febuary 5, 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)"

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