March 10, 2015
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for February 2015

1 Major Seismic Activities

* On February 6, there was a magnitude (M) 5.1 earthquake in southern Tokushima prefecture. Maximum seismic intensity 5 Upper was observed in Tokushima prefecture.

* On February 17, there was a M6.9 earthquake offshore Sanriku. In addition to tsunami with height of 27 cm at Kuji Port, Iwate prefecture, slight tsunami was observed along the Pacific coast from Hokkaido to Iwate prefecture.

* On February 17, there was a M5.7 earthquake offshore Iwate prefecture. Maximum seismic intensity 5 Upper was observed in Aomori prefecture.

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On February 21, there was a M5.1 earthquake at a depth of approximately 55 km, offshore Urakawa. The focal mechanism of this event showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and continental plates.

(2) Tohoku Region

* On February 17, there was a M6.9 earthquake offshore Sanriku. The focal mechanism of this event showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and continental plates. In addition to tsunami with height of 27 cm at Kuji Port, Iwate prefecture, slight tsunami was observed along the Pacific coast from Hokkaido to Iwate prefecture. GNSS observation showed a small crustal deformation in horizontal components at several points along the coast of Iwate prefecture. Afterwards in the vicinity of this event, there was an active seismicity, such as a M6.5 earthquake on 20th and a M6.4 on 21th.

* On February 17, there was a M5.7 earthquake at a depth of approximately 50 km, offshore Iwate prefecture. The focal mechanism of this event showed a reverse fault type with a compression axis in an NW-SE direction.

* On February 26, there was a M5.0 earthquake at a depth of approximately 50 km, offshore Miyagi prefecture. The focal mechanism of this event had a compression axis in an NW-SE direction.

(3) Kanto-Chubu Region

* On February 1, there was a M4.4 earthquake at a depth of approximately 15 km in Toyama bay [Noto Region, Ishikawa Prefecture]. The focal mechanism had a compression axis in an NNE-SSW direction. This event occurred within the crust.

* On February 23, there was a M4.5 earthquake at a depth of approximately 70 km under northeastern Chiba prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction.

* On February 25, there was a M6.1 earthquake under the ocean near Torishima. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction.

* In the GNSS data for the Tokai region, there have been no changes of activity which are likely to be associated with the Tokai earthquake.

(4) Kinki-Chugoku-Shikoku Region

* On February 6, there was a M5.1 earthquake at a depth of approximately 10 km in southern Tokushima prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an E-W direction. This event occurred within the crust.

(5) Kyushu-Okinawa Region

There was no remarkable activity.

(6) Other Region

* On February 14, there was a M6.2 earthquake near Taiwan. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction.

Supplement

* On March 4, there was a M4.6 earthquake at a depth of approximately 40 km under western Aichi prefecture [Middle Western Mino Region, Gifu Prefecture]. The focal mechanism of this event showed a strike-slip fault type with a tension axis in an NE-SW direction. This event occurred within the Philippine Sea plate.


Note:  [ ] was 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 2015

March 10, 2015
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during February 2015.

(reference)

The following events for the period from February 2014 to late January, 2015 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities".

  – Iyonada  M6.2 on March 14, 2014
(Depth of approximately 80 km)
  – Along the Northern Coast of Chile  Mw8.1 on April 2, 2014
  – Ocean Area Near the Izu-Oshima  M6.0 on May 5, 2014
(Depth of approximately 160 km)
  – Rat Islands, Aleutian Islands  Mw7.9 on June 24, 2014
  – Offshore IwatePrefecture  M5.9 on July 5, 2014
(Depth of approximately 50 km)
  – Middle Eastern Iburi Region  M5.6 on July 8, 2014
(Depth of approximately 5 km)
  – Offshore Fukushima Prefecture  M7.0 on July 12, 2014
  – Offshore East of Aomori Prefecture  M6.1 on August 10, 2014
(Depth of approximately 50 km)
  – Northern Tochigi Prefecture  M 5.1 on September 3, 2014
(Depth of approximately 5 km)
  – Southern Ibaraki Prefecture  M 5.6 on September 16, 2014
(Depth of approximately 45 km)
  – Northern Nagano Prefecture  M 6.7 on November 22, 2014
(Depth of approximately 5 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) Seismic activities and crustal deformations associated with the 2011 off the Pacific Coast of Tohoku Earthquake

(Refer to the Evaluation of the seismic activity after the 2011 off the Pacific Coast of Tohoku Earthquake)

(4) Kanto-Chubu Region

"In the GNSS data for the Tokai region, there have been no changes of activity which are likely to be associated with the Tokai earthquake":
(This evaluation is in agreement with the views presented at the regular meeting on February 23 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 23, 2015)
"No changes of activity which are likely to be associated with the Tokai earthquake have been observed until now. (Excerpted)"

(5) Kinki-Chugoku-Shikoku Region

"On February 6, there was a M5.1 earthquake at a depth of approximately 10 km in southern Tokushima prefecture. (Excerpted) "
The aftershock activity is low: any aftershock with seismic intensity 1 or over has not occurred.

– Non-stationary crustal deformation around the Bungo Channel that was observed in the summer of 2014 is stagnant now. This deformation is considered to be caused by the gradual slip (slow-slip) at the boundary between the Philippine Sea and the continental plates.

(6) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

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