July 9, 2010
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Seismic Activity for June 2010

1 Major Seismic Activity

* On June 13, there was a M6.2 earthquake off-shore of Fukushima prefecture. This event had a maximum seismic intensity 5 Lower in Fukushima prefecture.

2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

* On June 5, there was a M5.5 earthquake off-shore east of Hokkaido. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction.

* On June 18, there was a M6.5 earthquake off-shore southeast of Etorofu Island. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction

* On June 28, there was a M5.1 earthquake at a depth of approximately 55km, off-shore of Tomakomai. The focal mechanism showed a strike-slip fault with a tension axis in a WNW-ESE direction.

(2) Tohoku Region

* On June 13, there was a M6.2 earthquake at a depth of approximately 40km, off-shore of Fukushima prefecture. The focal mechanism showed a reverse fault with a compression axis in a NNW-SSE direction. According to the GPS data, there have been no remarkable changes before and after the event.

(3) Kanto-Chubu Region

There was no remarkable activity.

There have been no anomalous changes observed in the GPS data for the Tokai region

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

(5) Kyushu-Okinawa Region

* On June 13, there was a M5.2 earthquake off-shore northwest of Amami Oshima. The focal mechanism showed a normal fault with a tension axis in a NNW-SSE direction.

Supplements

* On July 4, there was a M5.2 earthquake at a depth of approximately 5km in the southern inland area of Iwate prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred within the crust.

* On July 4, there was a M4.5 earthquake at a depth of approximately 90km, under southern Ibaraki prefecture. This event occurred within the Pacific plate. The focal mechanism had a tension axis in a WNW-ESE direction.

* On July 5, there was a M6.4 earthquake at a depth of approximately 35km, off-shore of Iwate prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the continental plates.




Supplementary Information to the Evaluation of Seismic Activity for June 2010 by the Earthquake Research Committee

July 9, 2010

1 Major Seismic Activity

The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during June 2010.

(reference)

The following events for the period from June 2009 to the end of May 2010 have been listed as "Major Seismic Activity" in monthly "Evaluation of Seismic Activity"


– Under Suruga Bay M6.5 on August 11, 2009 (depth of approximately 25km)

– Off-shore East of Hachijojima M6.6 on August 13, 2009

– Under the Ocean Area near Ishigakijima M6.7, M6.6 on August 17, 2009

– Off-shore East of the Izu Peninsula Seismic activity with
M5.0 on December 17,
M5.1 on December 18, 2009

 
– Under the Ocean Area near Ishigakijima M6.5 on February 7, 2010

 
– Under the Ocean Area near the Okinawa Main Island    M7.2 on February 27, 2010

 
– Along the Central Coast of Chile Mw8.8 on February 27, 2010
 
– Off-shore of Fukushima Prefecture M6.7 on March 14, 2010

2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

There is no supplementary information for the Hokkaido region.

(2) Tohoku Region

There is no supplementary information for the Tohoku region.

(3) Kanto-Chubu Region

"There have been no anomalous changes observed in the GPS data for the Tokai region":
(This evaluation is in agreement with the views presented at the procedural meeting on June 28 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster. (see reference below))

(reference) Recent seismic and crustal activity in the Tokai region and its vicinity (Seismological and Volcanological Department, Japan Meteorological Agency, June 28, 2010)

"So far, there have been no changes of activity which are likely to be associated with the Tokai earthquake.

1. Status of seismicity
General seismicity has been slightly strong since mid-2005 within the crust under mid-western Shizuoka prefecture.
There continue to be fewer events within the Philippine Sea plate under the region around Hamanako. In other regions, seismic activity is generally at a normal level.

2. Status of crustal movements
There have been no remarkable changes observed over the entire region.
According to the GPS data and leveling results, the trend of long-term subsidence at Omaezaki has been continuing as before."

  –In the area off-shore east of Hachijojima (on the east side of the trench axis), there has been slightly strong seismic activity with the largest M4.8 since around May 29.

(4) Kinki-Chugoku-Shikoku Region

  –Non-stationary crustal movements are still observed near Bungo Strait, which have been seen since around autumn 2009. It is thought that they have been caused by slow slip at the boundary between the Philippine Sea and the continental plates.

(5) Kyushu-Okinawa Region

  "On June 13, there was a M5.2 earthquake off-shore northwest of Amami-Oshima. (excerpted)":
Since around May 16, there has been slightly strong seismic activity off-shore northwest of Amami Oshima. The recent event occurred while the seismic activity was subsiding.

Supplements

"On July 4, there was a M5.2 earthquake at a depth of approximately 5km in the southern inland area of Iwate prefecture. (excerpted)":
This event occurred in the aftershock area of the Iwate-Miyagi Nairiku Earthquake in 2008 (M7.2). It is the first time since the earthquake of M5.3 on June 16, 2008 (maximum seismic intensity 4) that an earthquake of M5.0 or over occurred in the same aftershock area.


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