October 10, 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Seismic Activity for September 2008

1 Major Seismic Activity

* On September 11, there was a M7.1 earthquake off-shore of Tokachi. This event had maximum seismic intensity 5 Lower and caused tsunamis observed on the Pacific coast in Hokkaido and in the Tohoku region, with a tsunami height of 0.5m at Erimo and 0.2m at Urakawa. to supplementary reports

2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

* On September 11, at 09:20 (JST) there was a M7.1 earthquake off-shore of Tokachi. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
This event caused tsunamis observed on the Pacific coast in Hokkaido and in the Tohoku region, with a tsunami height of 0.5m at Erimo and 0.2m at Urakawa.
Associated with this event, small crustal movements were observed in the area centered on the southern Tokachi region and the Hidaka region, Hokkaido.
The seismic activity was a main shock-aftershock sequence. The largest aftershock, until October 9, was a M5.7 event at 09:32 on September 11. The aftershock activity has been generally declining.
The source fault model of this event, determined from GPS data, is a low-angle reverse fault striking in a NNE-SSW direction and dipping toward the WNW.
This event occurred in the aftershock region of the 2003 Off-shore Tokachi Earthquake.

* On September 12, there was a M5.2 earthquake at a depth of approximately 40km off-shore of Kushiro. to supplementary reports

(2) Tohoku Region

* On September 22, there was a M5.6 earthquake at a depth of approximately 150km under Tsugaru Strait. The focal mechanism had a tension axis in a NW-SE direction. This event occurred within the subducting Pacific plate. to supplementary reports

(3) Kanto-Chubu Region

* On September 21, there was a M4.8 earthquake at a depth of approximately 70km under Tokyo Bay [under northwestern Chiba prefecture]. This event occurred near the boundary between the subducting Pacific plate and the Philippine Sea plate. The focal mechanism had a compression axis in a NNE-SSW direction.

* There have been no anomalous changes observed in the GPS data for the Tokai region. to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity. to supplementary reports

(5) Kyushu-Okinawa Region

*On September 14, there was a M5.1 earthquake off-shore northeast of Amami Oshima. to supplementary reports

Supplement

*On October 4, there was a M3.5 earthquake at a depth of approximately 10km in mid-northern Rumoi subprefecture.

*On October 8, there was a M4.7 earthquake at a depth of approximately 65km under northwestern Chiba prefecture. to supplementary reports


[ ] were the regional names of the epicenters used in the Japan Meteorological Agency announcement.




Supplementary Information to the Evaluation of Seismic Activity for September 2008 by the Earthquake Research Committee

October 10, 2008

1 Major Seismic Activity

The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during September 2008.

(reference)

The following regions have been noted as having major seismic activity during the period from September 2007 to the end of August 2008.


- Western Kanagawa prefecture M4.9 on October 1, 2007 (depth approximately 15km)

- Noto region, Ishikawa prefecture M4.8 on January 26, 2008 (depth approximately 10km)

- Off-shore of Ibaraki prefecture M7.0 on May 8, 2008

- Iwate-Miyagi Nairiku Earthquake in 2008 M7.2 on June 14, 2008 (depth approximately 10km)

- Off-shore of Ibaraki prefecture M5.2 on July 5, 2008 (depth approximately 50km)

- Near Okinoerabujima M6.1 on July 8, 2008 (depth approximately 45km)

- Off-shore of Fukushima prefecture M6.9 on July 19, 2008

- Central Iwate prefecture M6.8 on July 24, 2008 (depth approximately 110km)

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 September 29 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" (Japan Meteorological Agency, Department of Seismology and Volcanology, September 29, 2008)

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

1. Status of seismicity
There was no remarkable seismic activity in general.
There are continued less frequent occurrences of events equal to or larger than M3.5 within the Philippine Sea plate under mid-western Shizuoka prefecture, and there is slightly strong seismicity within the crust in Aichi prefecture. In other regions, seismic activity is generally at a normal level.
In addition, there were deep low-frequency earthquakes observed near the plate boundary under the area from southern Nagano prefecture to Aichi prefecture during the periods from August 25 through September 4 and from September 7 through 8.

2. Status of crustal movements
There were no notable changes observed in general.
According to the GPS data and leveling results, the trend of subsidence at Omaezaki has been continuing as before.
In addition, there were crustal movements observed on the strainmeters in the neighboring region from August 25 through September 4, which were thought to be caused by the short-term slow slip in the area around the plate boundary under southern Nagano prefecture through Aichi prefecture. This deformation is still continuing. A similar phenomenon was seen in this vicinity in January this year."

(4) Kinki-Chugoku-Shikoku Region

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

(5) Kyushu-Okinawa Region

- On September 10, there was a M4.5 earthquake at a depth of approximately 50km under the ocean area near Miyakojima.

Supplement

- On October 1, there was a M4.4 earthquake at a depth of approximately 15km in mid-western Mino, Gifu prefecture.



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