September 13, 2006
Headquarters for Earthquake Research Promotion
Earthquake Research Committee


Seismic Activity for August 2006


1 Major Seismic Activity

* There was no remarkable activity. to supplementary reports

2 Nationwide seismic activity by region

(1) Hokkaido Region

* There was no remarkable activity. to supplementary reports

(2) Tohoku Region

* On August 17, there was a M4.3 earthquake at a depth of approximately 75km off-shore of Miyagi prefecture. to supplementary reports

(3) Kanto-Chubu Region

* On August 31, there was a M4.8 earthquake at a depth of approximately 75km under Tokyo Bay. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred near the boundary between the Pacific and the Philippine Sea plates.
* 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 August 13, there was a M5.3 earthquake under the ocean near Amami Oshima. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. to supplementary reports

(6) Other Region

* On August 7, there was a M6.2 earthquake under the ocean near Chichijima.
* On August 18, there was a M5.9 earthquake under southern Sakhalin [under the ocean near Sakhalin]. The focal mechanism showed a reverse fault with a compression axis in a WSW-ENE direction.

Supplement

* On September 1, there was a M5.4 earthquake under the ocean near Amami Oshima.
* On September 7, there was a M5.1 earthquake at a depth of approximately 40km off-shore east of Chiba prefecture.
* On September 7, there was a M4.6 earthquake at a depth of approximately 70m under northwestern Chiba prefecture.

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



Supplementary Information to the Evaluation of Seismic Activity for August 2006 by the Earthquake Research Committee

September 13, 2006

1 Major Seismic Activity

The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during August 2006.


M4.0 or greater: 68 (44 events occurred in July)

M5.0 or greater: 11 (4 events occurred in July)

M6.0 or greater: 1 (There has been a total of 8 events for 2006 by the end of August.)

(reference)


M4.0 or greater: 73 / month (Median number of earthquakes over the past 10 years from 1996 through 2005)

M5.0 or greater: 9 / month (Median number of earthquakes over the past 30 years from 1976 through 2005)

M6.0 or greater: 1.4 / month, approximately 17 / year (Average number of earthquakes over the past 80 years from 1926 through 2005)

The following regions have been noted as having significant seismic activity during the period from August 2005 to the end of July 2006.


- Off-shore of Miyagi prefecture
M7.2 on August 16, 2005 (depth approximately 40km)

- Chuetsu region, Niigata prefecture M5.0 on August 21, 2005 (depth approximately 15km)

- Off-shore of Ibaraki prefecture M6.3 on October 19, 2005 (depth approximately 50km)

- Off-shore of Sanriku M7.2 on November 15, 2005

- Hyuganada M5.5 on March 27, 2006 (depth approximately 35km)

- Off-shore east of the Izu Peninsula seismic activity with M5.8 on April 21 and M4.5 on April 30, 2006

- Central Oita M6.2 on June 12, 2006 (depth approximately 150km)

to the text

2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

There is no supplementary information for the Hokkaido region.

to the text

(2) Tohoku Region

"On August 17, there was a M4.3 earthquake at a depth of approximately 75km off-shore of Miyagi prefecture.":
This event occurred within the aftershock region of the earthquake off-shore of Miyagi prefecture on May 26, 2003 (M7.1, maximum seismic intensity 6 Lower).

- Although it has been a year since the earthquake off-shore of Miyagi prefecture that occurred on August 16, 2005 (M7.2, maximum seismic intensity 6 Lower), the postseismic movements that were observed following the earthquake at the GPS observation stations around the Oshika Peninsula, have been continuing at a low level.

to the text

(3) Kanto-Chubu Region

"On August 31, there was a M4.8 earthquake at a depth of approximately 75km under Tokyo Bay. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred near the boundary between the Pacific and the Philippine Sea plates.":
In this vicinity, there was a M6.0 earthquake (maximum seismic intensity 5 Upper) on July 23, 2005.

"There have been no remarkable changes observed in GPS data in the Tokai region.":
(According to the reviews presented at the (drill) meeting of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster, at the Japan Meteorological Agency on August 31, there have been no notable changes in seismic activity or crustal movements in the Tokai region since the procedural meeting on July 31 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster. This evaluation is in agreement with those views.)

(reference) "Recent seismic and crustal activity in the Tokai region and its vicinity" (Japan Meteorological Agency, Department of Seismology and Volcanology, July 31, 2006)

"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
There was no remarkable seismic activity in general. The seismicity remains at a level lower than normal directly under the area from eastern Hamana Lake through central Shizuoka prefecture. In other regions, seismic activity is generally at a normal level.
The long-term crustal movements from the slow slip at the plate boundary in the Tokai region and its neighboring area, are thought to have already stopped."

- During the period from August 27 through September 1, there were low-frequency earthquakes (tremor) observed with strain and tilt changes in the area from southern Nagano prefecture through eastern Aichi prefecture. Recently similar movements were observed in eastern Aichi prefecture in July 2005, and in the area from the border of Mie and Nara prefectures through eastern Aichi prefecture in January 2006.

to the text

(4) Kinki-Chugoku-Shikoku Region

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

to the text

(5) Kyushu-Okinawa Region

- On August 28, there was a M5.3 earthquake at a depth of approximately 120km under the ocean near Yonaguni Island.

to the text

Supplement

- On September 9, there was a M4.9 earthquake at a depth of approximately 65km off-shore of Miyagi prefecture.

Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity"(Monthly Report of Seismic Activity in Japan) are events of M6.0 and greater, or events of M4.0 and greater (M5.0 or greater off-shore) that have maximum intensity of 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.