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.

• Seismic Activities for All of Japan in February 2015 (Magnitude 4.0 or over)

to supplementary reports

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.

• Hokkaido Region
• Earthquake Offshore Urakawa on February 21

to supplementary reports

(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.

• Tohoku Region
• Aftershock Activities of the 2011 off the Pacific Coast of Tohoku Earthquake
• Crustal Movements After the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) (Horizontal) -One Month-
• Crustal Movements After the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) (Vertical) -One Month-
• Crustal Movements After the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) (Time Series)
• Crustal Movements Before and After the 2011 off the Pacific Coast of Tohoku Earthquake
• Seismic Activity offshore Sanriku beginning on February 17 (1)
• Seismic Activity offshore Sanriku beginning on February 17 (2)
• Seismic Activity offshore Sanriku beginning on February 17 (3)
• Crustal Deformation Associated with the Sanriku-oki Earthquake (February 17, M6.9)
• Earthquake Offshore Iwate Prefecture on February 17
• Earthquake Offshore Miyagi Prefecture on February 26

to supplementary reports

(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.

• Kanto - Chubu Region
• Earthquake in Toyama Bay on February 1
• Earthquake under Northwestern Chiba Prefecture on February 23
• Earthquake under the Ocean near Torishima on February 25
• 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

* 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.

• Kinki-Chugoku-Shikoku Region
• Earthquake in Southern Tokushima Prefecture on February 6

to supplementary reports

(5) Kyushu-Okinawa Region

There was no remarkable activity.

• Kyushu Region
• Okinawa Region

to supplementary reports

(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.

• Earthquake near Taiwan on February 14

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.

• Earthquake under Western Aichi Prefecture on March 4

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.

• M4.0 or greater: 108 (92 events in January, 2015)
• M5.0 or greater: 15 (10 events in January, 2015)
• M6.0 or greater: 5 (0 event in January, 2015)

(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 2014 to late January, 2015 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.

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(2) Tohoku Region

There is no supplementary information for Tohoku region.

to the text

(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)"

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(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.

• Non-Stationary Crustal Deformation around the Bungo Channel

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(6) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

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