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

Evaluation of Seismic Activities for August 2012

1 Major Seismic Activities

* On August 25, there was a 6.1 earthquake under the southern Tokachi region. This event had a maximum seismic intensity 5 Lower in the Tokachi and Hidaka regions, Hokkaido.

* On August 30, there was a M5.6 earthquake offshore Miyagi prefecture. This event had a maximum seismic intensity 5 Upper in Miyagi prefecture and caused damage with seriously injured persons

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On August 25, there was a 6.1 earthquake at a depth of approximately 50 km under the southern Tokachi region. The focal mechanism showed a reverse fault with a compression axis in an N-S direction. This event occurred near the boundary between the Pacific and the continental plates. In addition, in the vicinity of this event, there was a 5.2 earthquake on August 22.

(2) Tohoku Region

* On August 12, there was a M3.8 earthquake at a depth of approximately 5 km in Nakadori, Fukushima prefecture. The focal mechanism showed a normal fault with a tension axis in an NE-SW direction. This event occurred within the crust.

* On August 30, there was a M5.6 earthquake at a depth of approximately 60 km, offshore Miyagi prefecture. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction. This event occurred within the Pacific plate.

(3) Kanto-Chubu Region

* On August 3, there was a M4.6 earthquake at a depth of approximately 45 km under southern Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction. This event occurred on the boundary between the Philippine Sea and the continental plates.

* On August 20, there was a M5.2 earthquake at a depth of approximately 55 km under southern Ibaraki 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 Philippine Sea plates.

* 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

* On August 17, there was a M4.9 earthquake at a depth of approximately 10 km in Amakusanada. The focal mechanism showed a normal fault with a tension axis in an N-S direction. This event occurred within the crust.

(6) Other region

* On August 14, there was a M7.3 earthquake at a depth of approximately 650 km under the southern Okhotsk Sea. The focal mechanism had a compression axis in an inclined 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.
     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 August 2012

September 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 July 2012.

(reference)

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

 
– 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)
  – Offshore Fukushima Prefecture  M5.9 on April 1, 2012
(Depth of approximately 55 km)
  – Northeastern Chiba Prefecture  M5.8 on April 29, 2012
(Depth of approximately 50 km)
  – Offshore East of Aomori Prefecture  M6.1 on May 24, 2012
(Depth of approximately 60 km)
  – Northern Nagano Prefecture  M5.2 on July 10, 2012
(Depth of approximately 10 km)

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for the Hokkaido region.

(2) Tohoku Region

"On August 12, there was a M3.8 earthquake at a depth of approximately 5 km in Nakadori, Fukushima prefecture":
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 August 30 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, August 30, 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

– Under the sea near Tokara Islands, after August 16, there was a sequence of seismicity with the largest M3.9, and there were 15 earthquakes with a maximum seismic intensity 1.

(7) Other region

"On August 14, there was a M7.3 earthquake at a depth of approximately 650 km under the southern Okhotsk Sea":
This event gave stronger shaking in the Pacific coast side from the Hokkaido to the northern Tohoku regions, due to a transmission of seismic waves within the subducting Pacific plate (anomalous 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