April 10, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for February 2020

1 Major Seismic Activities

There was no remarkable activity.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On February 13, there was a magnitude (M) 7.2 earthquake at a depth of approximately 160 km, offshore southeast of Etorofu Island. The focal mechanism had a tension axis in an NNW-SSE direction. This event occurred within the Pacific plate.

* On February 20, there was an M4.6 earthquake at a depth of approximately 5 km, in the central and southern Kushiro regions. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction. This event occurred within the crust.

• Hokkaido Region
• Earthquake Offshore Southeast of Etorofu Island on February 13
• Earthquake in the Central and Southern Kushiro Regions on February 20
• Earthquake in the Central and Southern Kushiro Regions on February 20, 2020 (The Focal Mechanism Solution by F-net)

to supplementary reports

(2) Tohoku Region

* On February 12, there was an M5.4 earthquake at a depth of approximately 85 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 Pacific plate.

• Tohoku Region
• Earthquake offshore Fukushima Prefecture on February 12

to supplementary reports

(3) Kanto-Chubu Region

* On February 1, there was an M5.0 earthquake at a depth of approximately 50 km, offshore east of Chiba 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 Philippine Sea plates.

* On February 1, there was an M5.3 earthquake at a depth of approximately 65 km under the southern Ibaraki 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 Philippine Sea plates.

* On February 20, there was an M4.3 earthquake at a depth of approximately 30 km under the northeastern Chiba 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
• Earthquake Offshore East of Chiba Prefecture on February 1
• Earthquake Under the Southern Ibaraki Prefecture on February 1
• Earthquake in the Northeastern Chiba Prefecture on February 20

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

# : [The northeastern Chiba prefecture] was the regional name of the epicenter used in the JMA announcement.

Supplementary Information to the Evaluation of Seismic Activities for February 2020

April 10, 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 February 2020.

• M4.0 or greater: 78 (71 events in January)
• M5.0 or greater: 10 (6 events in January)
• M6.0 or greater: 1 (0 event in January)

(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 February 2019 to late January 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.

to the text

(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.

to the text

(4) Kinki-Chugoku-Shikoku Region

– In the area of the western and central Shikoku, deep low-frequency earthquakes (tremors) were observed since February 10, 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 synchronized with the deep low-frequency earthquakes (tremors). It is presumed that these are due to the short-term slow slips at the boundary between the Philippine Sea and the continental plates.

– According to the GNSS observation in the western Kii Peninsula and the eastern Shikoku, the crustal deformation different from the ordinary trend has been observed from spring 2019. This phenomenon is considered to be caused by a 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 Slow Slip in Shikoku
• Observed Result of Strain in Shikoku Region (2020/02/01 00:00-2020/03/03 00:00 (JST))
• Observed Result of Tilt in Shikoku Region (2020/02/01 00:00-2020/03/03 00:00 (JST))(1)
• Observed Result of Tilt in Shikoku Region (2020/02/01 00:00-2020/03/03 00:00 (JST))(2)
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[A]) 2020/02/08-11AM
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[B]) 2020/02/11PM-14
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[C]) 2020/02/15-18
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[D]) 2020/02/19-20AM
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[E]) 2020/02/20PM-21AM
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[F]) 2020/02/21PM-22
• Slow Slip Synchronous with Deep Low-Frequency Tremor in the Western Shikoku Estimated from GNSS Data (provisional)
• Non-Stationary Horizontal Crustal Deformation of the Western Kii Peninsula and Eastern Shikoku (Linear trend, annual component, semi-annual component removed)
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and Eastern Shikoku
• Long-Term Slow Slip in the Kii Channel Estimated from GNSS Data (provisional)

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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 March 6 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, March 6, 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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