February 12, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for January 2020

1 Major Seismic Activities

There was no remarkable activity.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On January 28, there was a magnitude (M) 5.5 earthquake at a depth of approximately 95 km, offshore southeast of the Nemuro Peninsula. The focal mechanism had a tension axis in an N-S direction. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquake Offshore Southeast of the Nemuro Peninsula on January 28

to supplementary reports

(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* On January 3, there was an M5.8 earthquake at a depth of approximately 35 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 continental plates.

* On January 14, there was an M4.8 earthquake at a depth of approximately 45 km under the southern Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

* On January 21, there was an M4.2 earthquake at a depth of approximately 55 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.

• Kanto - Chubu Region
• Earthquake Offshore East of Chiba Prefecture on January 3
• Earthquake Under the Southern Ibaraki Prefecture on January 14
• Earthquakes Offshore Ibaraki Prefecture on January 21

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* On January 16, there was an M4.2 earthquake at a depth of approximately 15 km in the ocean area near Okinawa Main Island. This event occurred within the crust of the continental plate.

• Kyushu Region
• Okinawa Region
• Earthquake in the Ocean Area near Okinawa Main Island on January 16

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 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.

• Earthquake Offshore East of Chiba Prefecture on February 1
• Earthquake Under the Southern Ibaraki Prefecture on February 1

# : [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 January 2020

February 12, 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 January 2020.

• M4.0 or greater: 71 (109 events in December)
• M5.0 or greater: 6 (11 events in December)
• M6.0 or greater: 0 (1 event in December)

(reference)

• M4.0 or greater: 73 / month (Median number of earthquakes over the past 10 years from 1998 through 2007)
• M5.0 or greater: 9 / month (Median number of earthquakes over the past 35 years from 1973 through 2007)
• M6.0 or greater: 1.4 / month, approximately 17 / year (Average number of earthquakes over the past 84 years from 1924 through 2007)

The following events for the period from January 2019 to late December 2019 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

– 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.

– According to the GNSS and strain observation in the western Shikoku, the crustal deformation different from the ordinary trend beginning in autumn 2018 seems to have ended. This phenomenon is considered to be caused by a slow slip around the Bungo Channel at the deeper part of the boundary between the Philippine Sea and the continental plates. The long-term slow slip seems to have already stopped.

• 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) - Distribution of Estimated Slip
• Non-Stationary Horizontal Crustal Deformation of Western Shikoku (Linear trend, annual component, semi-annual component removed)
• Time Series of GNSS Continuous Observation: Western Shikoku
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (1) - Distribution of Estimated Slip -
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (2) - Comparison Between Observed Values (black) and Calculated Values (white) -

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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 February 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, February 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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