March 9, 2017
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for February 2017

1 Major Seismic Activities

* On February 28, there was a magnitude (M) 5.7 earthquake offshore Fukushima prefecture. Maximum seismic intensity 5 Lower was observed in Miyagi and Fukushima prefectures.

• Seismic Activities for All of Japan in February 2017 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On February 27, there was a M4.7 earthquake at a depth of approximately 50 km under the eastern Hidaka region [the southern Tokachi region]. 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 Pacific and the continental plates.

• Hokkaido Region
• Earthquake under the eastern Hidaka region on February 27

to supplementary reports

(2) Tohoku Region

* On February 11, there was a M5.4 earthquake at a depth of approximately 20 km (due to CMT solution), 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 crust of the continental plate.

* On February 27, there was a M4.9 earthquake at a depth of approximately 50 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 February 28, there was a M5.7 earthquake at a depth of approximately 50 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.

• Tohoku Region
• Earthquakes Offshore Fukushima Prefecture on February 11
• Earthquakes Offshore Fukushima Prefecture on February 27
• Earthquakes Offshore Fukushima Prefecture on February 28

to supplementary reports

(3) Kanto-Chubu Region

* On February 19, there was a M5.4 earthquake at a depth of approximately 50 km under the northeastern Chiba prefecture [offshore east of Chiba 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.

* In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.

• Kanto - Chubu Region
• Earthquakes under the Northeastern Chiba Prefecture on February 19
• Continuous GNSS Monitoring in the Area from Kakegawa to Omaezaki
• Secular Variation of Benchmark 2595 (Omaezaki City)

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

Supplement

* On March 2, there was a M5.3 earthquake at a depth of approximately 35 km under Hyuganada. The focal mechanism had a tension axis in an NW-SE direction. This event occurred within the Philippine Sea plate.

* On March 5, there was a M4.7 earthquake at a depth of approximately 50 km under the northeastern Chiba prefecture.

• Earthquake under Hyuganada on March 2

Note: [ ] is the regional name of the epicenter used in the Japan Meteorological Agency announcement.
    GNSS (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for February 2017

March 9, 2017
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 2017.

• M4.0 or greater: 82 (76 events in January)
• M5.0 or greater: 7 (10 events in January)
• M6.0 or greater: 0 (0 event in January)

(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 February 2016 to late January 2017 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

– In the northern Ibaraki prefecture, where a M6.3 earthquake occurred on December 28, 2016, 16 earthquakes with seismic intensity 1 or over occurred in February (13 in January). Of these the maximum was the M4.4 one on February 7. The seismic activity still continues, decaying as a whole.

• Seismic Activity in the Northern Ibaraki Prefecture beginning on December 28, 2016

–"In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.":
(This evaluation is in agreement with the views presented at the regular meeting on February 27 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster (reference below).)

(Reference)

Recent seismic and crustal activity in the Tokai region and its vicinity (Seismological and Volcanological Department, Japan Meteorological Agency, February 27, 2017)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"

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(4) Kinki-Chugoku-Shikoku Region

There is no supplementary information for Kinki-Chugoku-Shikoku region.

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(5) Kyushu-Okinawa Region

– In February, no earthquake with maximum seismic intensity 4 or over (1 in January) and 1 earthquake with maximum seismic intensity 3 (1 in January) occurred in the Kumamoto and Aso regions, Kumamoto prefecture. A series of seismic activity of the 2016 Kumamoto Earthquakes in the Kumamoto and Aso regions still continues, decaying as a whole. According to the GNSS observation, postseismic deformation is still observed in the area centered on the series of the seismic activity.

• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (1)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (2)

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