July 12, 2006
Headquarters for Earthquake Research Promotion
Earthquake Research Committee


Seismic Activity for June 2006


1 Major Seismic Activity

* On June 12, there was a M6.2 earthquake at a depth of approximately 150km under central Oita prefecture. Maximum seismic intensity 5 Lower was recorded in Oita, Ehime and Hiroshima prefectures, and this event caused some damage. to supplementary reports

2 Nationwide seismic activity by region

(1) Hokkaido Region

* On June 13, there was a M4.7 earthquake at a depth of approximately 85km under central Tokachi subprefecture. The focal mechanism had a tension axis in the direction of the subducting Pacific plate. This event occurred within the Pacific plate. to supplementary reports

(2) Tohoku Region

There was no remarkable activity. to supplementary reports

(3) Kanto-Chubu Region

* On June 20, there was a M4.6 earthquake at a depth of approximately 65km under northwestern Chiba 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 Philippine Sea plates. In addition, on June 28, there was a M4.1 earthquake at a depth of approximately 110km to the WNW of the previous earthquake. This earthquake occurred within the Pacific plate.
* On June 3, there was a M4.1 earthquake at a depth of approximately 5km in the Reinan region, Fukui prefecture.
* The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, appear to have nearly stopped recently. to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

*There was no remarkable activity. to supplementary reports

(5) Kyushu-Okinawa Region

*On June 12, there was a M6.2 earthquake at a depth of approximately 150km under central Oita prefecture. Maximum seismic intensity 5 Lower was recorded in Oita, Ehime and Hiroshima prefectures. The focal mechanism had a tension axis in the direction of the subducting plate. This event occurred within the Philippine Sea plate. According to the GPS data, there were no anomalous changes before and after this event. This event occurred in the deepest portion of the active region within the subducting Philippine Sea plate. Since August 1923, there have been only a M6.6 earthquake in 1983 and a M6.0 earthquake in 1978, for events equal to or larger than M6.0 observed at the similar depth within a range of 50km from the recent event. to supplementary reports

Supplement

* On July 1, there was a M5.3 earthquake at a depth of approximately 40km off-shore of Miyagi prefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred at the boundary between the Pacific and the continental plates.
* On July 6, there was a M5.4 earthquake at a depth of approximately 35km off-shore of Iwate prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
* On July 6, there was a M4.3 earthquake at a depth of approximately 10km near Sendai city, Miyagi prefecture [in northern Miyagi prefecture].
* On July 6, there was a M4.0 earthquake at a depth of approximately 50km under southern Ibaraki prefecture.
* On July 11, there was a M4.0 (preliminary) earthquake at a depth of approximately 15km in western Yamaguchi prefecture.

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


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

July 12, 2006

1 Major Seismic Activity

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


M4.0 or greater: 71 (56 events occurred in May)

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

M6.0 or greater: 1 (There has been a total of 6 events for 2006 by the end of June.)

(reference) Average numbers of earthquake occurrences over the past thirty years (1971 - 2000)


M4.0 or greater: 46 / month

M5.0 or greater: 8 / month

M6.0 or greater: 1.3 / month, approximately 16 / year

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


- Amakusa Ashikita region,                       Kumamoto prefecture M4.8 on June 3, 2005 (depth approximately 10km)

- Chuetsu region, Niigata prefecture M5.0 on June 20, 2005 (depth approximately 15km)

- Northwestern Chiba prefecture M6.0 on July 23, 2005 (depth approximately 75km)

- 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

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 June 17, there were M5.7 and M5.6 events off-shore of Sanriku. Both focal mechanisms showed reverse faults with compression axes in a WNW-ESE direction.

to the text

(3) Kanto-Chubu Region

"The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, appear to have nearly stopped recently.":
According to the GPS data, the Pacific side of the Tokai-Chubu region maintained a steady rate with movement to the WNW, consistent with the subduction of the Philippine Sea plate toward the northwest. The results, from observations in the area centered on western Shizuoka prefecture, have been showing a trend of moderate change since about April 2001. This long-term changes seem to have nearly stopped recently.
(This evaluation is in agreement with the views presented at the procedural meeting on June 26 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, June 26, 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 continues 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, seem to have nearly stopped recently."

There was additional activity in the Kanto - Chubu region, as follows.
- On June 24, there was a M3.9 earthquake at a depth of approximately 10km near the border between Nagano and Gifu prefectures [in southern Nagano prefecture] (maximum seismic intensity 4).


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

There is no supplementary information for the Kyushu - Okinawa region.

to the text


Supplement

"On July 1, there was a M5.3 earthquake at a depth of approximately 40km off-shore of Miyagi prefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred at the boundary between the Pacific and the continental plates.":
This event occurred near the northern edge of the aftershock region of the earthquake off-shore Miyagi prefecture on August 16, 2005 (M7.2). In this vicinity, there was a M6.1 earthquake on December 17, 2005.

- On July 9, there was a M4.2 earthquake at a depth of approximately 25km under Tokyo Bay (maximum seismic intensity 3).
- On July 9, there was a M4.8 earthquake at a depth of approximately 10km under the ocean area near Niijima and Kozushima Islands (maximum seismic intensity 4).

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

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.