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

Evaluation of Seismic Activities for April 2012

1 Major Seismic Activities

* On April 1, there was a M5.9 earthquake offshore Fukushima prefecture. This event had a maximum seismic intensity 5 Lower in Fukushima prefecture.

* On April 29, there was a M5.8 earthquake under northeastern Chiba prefecture. This event had a maximum seismic intensity 5 Lower in Chiba prefecture.

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On April 21, there was a M4.5 earthquake at a depth of approximately 105 km under the western Hidaka region. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate.

(2) Tohoku Region

* On April 1, there was a M5.9 earthquake at a depth of approximately 55 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Pacific and the continental plates.

* On April 13, there was a M6.0 earthquake at a depth of approximately 30 km, offshore Fukushima prefecture. The focal mechanism showed a normal fault with a tension axis in a WNW-ESE direction. This event occurred within the continental plate. In addition to this event, there was a M5.6 earthquake offshore Ibaraki prefecture, and there was a sequence of seismicity offshore southern Fukushima prefecture to northern Ibaraki prefecture.

* On April 27, there was a M5.0 earthquake at a depth of approximately 65 km, offshore east of Aomori prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Pacific and the continental plates.

(3) Kanto-Chubu Region

* On April 29, there was a M5.8 earthquake at a depth of approximately 50 km under northeastern Chiba 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 Philippine Sea plates. In addition to this event, there was a sequence of seismicity such as a M5.5 earthquake on April 25.

* 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

There was no remarkable activity.

(5) Kyushu-Okinawa Region

There was no remarkable activity.

Supplements

* On May 4, there was a M4.5 earthquake at a depth of approximately 85 km under the southern Nemuro region [Near the Kunashir Island]. The focal mechanism had a tension axis in an N-S direction. This event occurred within the Pacific 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 April 2012

May 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 April 2012.

(reference)

The following events for the period from April 2011 to late March 2012 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activitie"

 
– 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  M7.1 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)
 
– Offshore Fukushima Prefecture  M5.1 on January 23, 2012
(Depth of approximately 50 km)

 
– Eastern Yamanashi Prefecture-Fujigoko  M5.4 on January 28, 2012
(Depth of approximately 20 km)
 
– Near Sado  M5.7 on February 8, 2012
(Depth of approximately 15 km)

 
– Northern Ibaraki Prefecture  M5.2 on February 19, 2012
(Depth of approximately 5 km)
  – Offshore Ibaraki Prefecture M5.3 on March 1, 2012
(Depth of approximately 55 km)
  – Northern Ibaraki Prefecture  M5.4 on March 10, 2012
(Depth of approximately 5 km)
  – Offshore Sanriku  M6.9 on March 14, 2012
  – Offshore East of Chiba Prefecture  M6.1 on March 14, 2012
(Depth of approximately 15 km)
  – Offshore Iwate Prefecture  M6.6 on March 27, 2012
(Depth of approximately 20 km)

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for the Hokkaido region.

(2) Tohoku Region

– Since mid-March, 2011, a series of seismicity has gradually been decaying in the vicinity of the prefectural border of Aizu, Fukushima prefecture and the Okitama region, Yamagata prefecture, but it has been continuing. There was a M4.4 earthquake in the Okitama region, Yamagata prefecture on April 24.

– 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, brisk seismicity has 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

"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 April 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, April 23, 2012)
"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.

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