January 11, 2012
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for December 2011

1 Major Seismic Activities

There was no remarkable activity.

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On December 10, there was a M5.7 earthquake at a depth of approximately 430 km under the southern Okhotsk Sea. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate.

(2) Tohoku Region

* On December 20, there was a M5.0 earthquake at a depth of approximately 30 km, offshore Iwate prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred on the boundary between the Pacific and the continental plates.

(3) Kanto-Chubu Region

* On December 1, there was a M4.6 earthquake at a very shallow depth in northern Nagano prefecture. The focal mechanism had a compression axis in an NW-SE direction. This event occurred within the crust.

* On December 3, there was a M5.2 earthquake at a depth of approximately 20 km in northeastern Chiba prefecture [southern Chiba prefecture]. The focal mechanism showed a reverse fault with a compression axis in an NNW-SSE direction. This event occurred on the boundary between the Philippine Sea and the continental plates.

* On December 14, there was a M5.1 earthquake at a depth of approximately 50 km under eastern Mino, Gifu prefecture. The focal mechanism showed a normal fault with a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

* On December 31, there was a M4.5 earthquake at a depth of approximately 10 km in northern Ibaraki prefecture. The focal mechanism showed a normal fault with a tension axis in a WNW-ESE direction. This event occurred within the crust

* In the GPS 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

There was no remarkable activity.

(5) Kyushu-Okinawa Region

* On December 11, there was an M5.5 earthquake in the sea near Amami Oshima. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction.

Supplements

* On January 1, there was a M4.2 earthquake at a depth of approximately 5 km in the vicinity of the border of Nagano and Niigata prefectures. The focal mechanism showed a strike-slip fault with a compression axis in an NNW-SSE direction. This event occurred within the crust.

* On January 1, there was a M7.0 earthquake at a depth of approximately 400 km under the sea near Torishima. The focal mechanism had a compression axis in a direction of the subducting Pacific plate. This event occurred within the Pacific plate.


Note:[ ] was the regional name of the epicenter used in the Japan Meteorological Agency announcement.





Supplementary Information to the Evaluation of Seismic Activities for December 2011

January 11, 2012
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 December 2011.

(reference)

The following events for the period from December 2010 to the end of November 2011 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities

 
– Under the Ocean near Chichijima

 M7.8 on December 22, 2010
 
– Offshore Sanriku

 M7.3 on March 9, 2011
 
– The 2011 off the Pacific Coast of Tohoku Earthquake  M9.0 on March 11, 2011
(Depth of approximately 25 km)
 
– Izu Region, Shizuoka Prefecture  M4.6 on March 11, 2011
(Depth of approximately 5 km)
 
– Near the Border between Nagano and Niigata Prefectures    M6.7 on March 12, 2011
(Depth of approximately 10 km)

 
– Eastern Shizuoka Prefecture  M6.4 on March 15, 2011
(Depth of approximately 15 km)

 
– Northern Ibaraki Prefecture  M6.1 on March 19, 2011
(Depth of approximately 5 km)
 
– Hamadori, Fukushima Prefecture  M6.0 on March 23, 2011
(Depth of approximately 10 km)
 
– Southern Ibaraki Prefecture  M4.8 on March 24, 2011
(Depth of approximately 50 km)
 
– Northern Inland, Akita Prefecture  M5.0 on April 1, 2011
(Depth of approximately 10 km)
 
– Southern Ibaraki Prefecture  M5.0 on April 2, 2011
(Depth of approximately 55 km)
 
– Offshore Miyagi Prefecture  M7.2 on April 7, 2011
(Depth of approximately 65 km)

 
– Hamadori, Fukushima Prefecture  M7.0 on April 11, 2011
(Depth of approximately 5 km)

 
– Northern Nagano Prefecture  M5.6 on April 12, 2011
(Very shallow depth)

 
– Offshore East of Chiba Prefecture  M6.4 on April 12, 2011
(Depth of approximately 25 km)

 
– Southern Ibaraki Prefecture  M5.9 on April 16, 2011
(Depth of approximately 80 km)

 
– Near the Border between Nagano and Niigata Prefectures  M4.9 on April 17, 2011
(Depth of approximately 10 km)

 
– Southern Inland, Akita Prefecture  M4.9 on April 19, 2011
(Depth of approximately 5 km)

 
– Offshore East of Chiba Prefecture  M6.0 on April 21, 2011
(Depth of approximately 45 km)
 
– Hamadori, Fukushima Prefecture  M5.2 on May 6, 2011
(Depth of approximately 5 km)
 
– Chuetsu Region, Niigata Prefecture  M4.7 on June 2, 2011
(Depth of approximately 5 km)
 
– Offshore Iwate Prefecture  M6.9 on June 23, 2011
(Depth of approximately 35 km)
 
– Central Nagano Prefecture  M5.4 on June 30, 2011
(Depth of approximately 5 km)
 
– Northern Wakayama Prefecture  M5.5 on July 5, 2011
(Depth of approximately 5 km)
 
– Offshore Sanriku  M7.3 on July 10, 2011
 
– Southern Ibaraki Prefecture  M5.4 on July 15, 2011
(Depth of approximately 65 km)
 
– Suruga Bay  M6.2 on August 1, 2011
(Depth of approximately 25 km)

 
– Offshore Fukushima Prefecture  M6.5 on August 19, 2011
(Depth of approximately 50 km)
 
– Central Hidaka Region  M5.1 on September 7, 2011
(Depth of approximately 10 km)

 
– Northern Ibaraki Prefecture  M5.2 on September 21, 2011
(Depth of approximately 10 km)

 
– Hamadori, Fukushima Prefecture  M5.4 on September 29, 2011
(Depth of approximately 10 km)

 
– Kumamoto Region, Kumamoto Prefecture  M4.5 on October 5, 2011
(Depth of approximately 10 km)
 
– Northern Ibaraki Prefecture  M5.3 on November 20, 2011
(Depth of approximately 10 km)

 
– Northern Hiroshima Prefecture  M5.4 on November 21, 2011
(Depth of approximately 10 km

 
– Offshore Urakawa  M6.2 on November 24, 2011
(Depth of approximately 45 km)

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

"On December 10, there was a M5.7 earthquake at a depth of approximately 430 km under the southern Okhotsk Sea. (Excerpted)":
Due to an efficient transmission of seismic waves within the subducting Pacific plate, the area along the Pacific coast from the Hokkaido region to the northern Tohoku region had stronger shaking (Zone of abnormal seismic intensity).

(2) Tohoku Region

– Since mid-March, a series of seismicity has been continuing in the vicinity of the border of Aizu, Fukushima prefecture and the Okitama region, Yamagata prefecture.

"On December 31, there was a M4.5 earthquake at a depth of approximately 10 km in northern Ibaraki prefecture. (Excerpted)":
In the region from Hamadori, Fukushima prefecture through northern Ibaraki prefecture, brisk seismicity has been continuing since the 2011 off the Pacific Coast of Tohoku Earthquake.

(3) Seismic activities and crustal deformations associated with the 2011 off the Pacific Coast of Tohoku Earthquake

– In the aftershock area of the 2011 off the Pacific Coast of Tohoku Earthquake that occurred on March 11, brisk seismic activities have been seen. There is fear that large aftershocks will occur from now on, and there is a possibility that it will be hit by strong shaking and high tsunami. Crustal movements to the east which are thought to be the postseismic deformation have still been observed in a wide area from the Tohoku through the Kanto-Chubu regions, but they have gradually been decaying.

(4) Kanto-Chubu Region

"On December 3, there was a M5.2 earthquake at a depth of approximately 20 km in northeastern Chiba prefecture [southern Chiba prefecture]. (Excerpted)"
Within the area from offshore east of Chiba prefecture to the eastern Boso Peninsula in the vicinity of the recent event, there was a series of seismic activities from around October 25. While these activities were decaying, the recent event occurred.

"In the GPS 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 December 19 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, December 19, 2011)

"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

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

(6) Kyushu-Okinawa Region

There is no supplementary information for the Kyushu-Okinawa region.

Supplements

"On January 1, there was a M7.0 earthquake at a depth of approximately 400 km under the sea near Torishima. (Excerpted)":
Due to an efficient transmission of seismic waves within the subducting Pacific plate, the area along the Pacific coast from the Tohoku region to the Kanto region had stronger shaking (Zone of abnormal seismic intensity).

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