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.

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.

(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.

(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.

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

(5) Kyushu-Okinawa Region

There was no remarkable activity.

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.



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.

(reference)

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."

  – Kumamoto Earthquakes  M6.5 on April 14, 2016 and M7.3 on April 16, 2016
(Depth of approximately 10 km)
  – Southern Ibaraki Prefecture  M5.5 on May 16, 2016
(Depth of approximately 40 km)
  – Uchiura Bay  M5.3 on June 16, 2016
(Depth of approximately 10 km)
  – Northern Ibaraki Prefecture  M5.4 on July 27, 2016
(Depth of approximately 55 km)
  – Kumamoto Region, Kumamoto Prefecture  M5.2 on August 31, 2016
(Depth of approximately 15 km)
  – Ocean Area near Okinawa Main Island  M5.6 on September 26
(Depth of approximately 45 km)
  – Central Tottori Prefecture  M6.6 on October 21, 2016
(Depth of approximately 10 km)
  – Offshore Fukushima Prefecture  M7.4 on November 22, 2016
(Depth of approximately 10 km)
  – Northern Ibaraki Prefecture  M6.3 on December 28, 2016
(Depth of approximately 10 km)

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

(2) Tohoku Region

There is no supplementary information for Tohoku region.

(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.

–"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)"

(4) Kinki-Chugoku-Shikoku Region

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

(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.


Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.

  1. Events of M6.0 and greater, or events that have maximum seismic intensity 4 or greater
  2. Onshore events of M4.5 and greater that have maximum seismic intensity 3 or greater
  3. Events under the ocean areas of M5.0 or greater that have maximum seismic intensity 3 or greater

Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes:

  1. Reference information related to seismic activity described in the "Evaluation of Seismic Activity"
  2. Recent earthquake activity over the past year related to the events described in "Major Seismic Activity" in the "Evaluation of Seismic Activity"
  3. Seismic activity that has been evaluated, but not included in the "Evaluation of Seismic Activity" because it is within normal bounds and not prominent