January 15, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for December 2019

1 Major Seismic Activities

* On December 19, there was a magnitude (M) 5.5 earthquake, offshore east of Aomori prefecture. The maximum seismic intensity 5 Lower was observed in Aomori prefecture.

• Seismic Activities for All of Japan in December 2019 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On December 12, there was an M4.2 earthquake at a depth of approximately 5 km in the northern Soya region. This event occurred within the crust.

• Hokkaido Region
• Earthquake in the Northern Soya Region on December 12

to supplementary reports

(2) Tohoku Region

* On December 11, there was an M5.3 earthquake at a depth of approximately 40 km, offshore Fukushima 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 December 19, there was an M5.5 earthquake at a depth of approximately 50 km, offshore east of Aomori prefecture. The focal mechanism showed a normal fault type with a tension axis in an NNE-SSW direction. This event occurred within the Pacific plate.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on December 11
• Earthquake Offshore East of Aomori Prefecture on December 19

to supplementary reports

(3) Kanto-Chubu Region

* On December 3, there was an M4.7 earthquake at a depth of approximately 50 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 December 4, there was an M4.9 earthquake at a depth of approximately 10 km in the northern Ibaraki prefecture. The focal mechanism showed a normal fault type with a tension axis in an NNE-SSW direction. This event occurred within the crust. In addition, on the 5th, there was an M4.6 earthquake at almost the same place.

* On December 4, there was an M4.8 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 an NW-SE direction. This event occurred within the crust.

* On December 11, there was an M6.1 earthquake in the ocean area near Torishima Island. The focal mechanism had a compression axis in an E-W direction.

• Kanto - Chubu Region
• Earthquake Under the Southern Ibaraki Prefecture on December 3
• Earthquakes in the Northern Ibaraki Prefecture on December 4 and 5
• Earthquake in the Northern Tochigi Prefecture on December 4
• Earthquake in the Ocean Area near Torishima Island on December 11

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 December 18, there was an M5.1 earthquake at a depth of approximately 45 km in the ocean area near Okinawa Main Island (near Okino-Erabujima Island). 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.

• Kyushu Region
• Earthquake Under the Ocean Area near Okinawa Main Island (near Okino-Erabujima Island) on December 18
• 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.

• Secular Variation of Benchmark 2595 (Omaezaki City)
• 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 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.

• Earthquake Offshore East of Chiba Prefecture on January 3

Supplementary Information to the Evaluation of Seismic Activities for December 2019

January 15, 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 December 2019.

• M4.0 or greater: 109 (63 events in November)
• M5.0 or greater: 11 (6 events in November)
• M6.0 or greater: 1 (0 event in November)

(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 December 2018 to late November 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.

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(4) Kinki-Chugoku-Shikoku Region

– The crustal deformation observed in GNSS and strain observation, in the western Shikoku beginning in autumn 2018 seems to be stagnant since June 2019. These phenomena are considered to be caused by a slow slip at the deeper part of the boundary between the Philippine Sea and the continental plates, which continues around the Bungo Channel. The long-term slow slip seems to be stagnant since June 2019.

• 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 January 10 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 10, 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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