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

Seismic Activity for March 2006

1 Major Seismic Activity

* On March 27, there was a M5.5 earthquake at a depth of approximately 35km under Hyuganada. This event had a maximum seismic intensity 5 Lower in Oita prefecture. to supplementary reports

2 Nationwide seismic activity by region

(1) Hokkaido Region

* On March 25, there was a M4.8 earthquake at a depth of approximately 50km under eastern Hidaka subprefecture. 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 continental plates. to supplementary reports

(2) Tohoku Region

* On March 12, there was a M5.0 earthquake at a depth of approximately 35km off-shore of Iwate prefecture. to supplementary reports

(3) Kanto-Chubu Region

* On March 13, there was a M4.1 earthquake at a depth of approximately 55km under southern Ibaraki prefecture.
* On March 28, there was a deep M6.0 earthquake at a depth of approximately 440km off-shore of Tokaido. The focal mechanism had a compression axis that nearly corresponds to the direction of the subducting plate. This event occurred within the Pacific plate.
* 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

* On March 2, there was a M4.1 earthquake at a depth of approximately 5km in northern Wakayama prefecture. to supplementary reports

(5) Kyushu-Okinawa Region

* On March 27, there was a M5.5 earthquake at a depth of approximately 35km under Hyuganada. This event had a maximum seismic intensity 5 Lower in Oita prefecture. The focal mechanism had a tension axis in a WNW-ESE direction. This event is related to the subduction of the Philippine Sea plate. Seismic activity was a main shock - aftershock sequence. The aftershock activity has been subsiding. According to the GPS data, there were no remarkable changes before and after this event. to supplementary reports

Supplement

* On April 1, there was a M6.4 earthquake near Taiwan.
*
 On April 2, there was a M4.3 earthquake at a depth of approximately 60km under southern Ibaraki prefecture.
*
 On April 10, there was a M5.1 earthquake at a depth of approximately 120km under northern Nemuro subprefecture.
*
 On April 11, there was a M5.0 (preliminary) earthquake at a depth of approximately 65km off-shore southeast of the Boso Peninsula.

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

April 12, 2006

1 Major Seismic Activity

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

M4.0 or greater: 76 (74 events occurred in February)

M5.0 or greater: 10 (8 events occurred in February)

M6.0 or greater: 1  (There has been a total of 3 events for 2006 by the end of March.)

(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, 16 / year

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


 - Off-shore west of Fukuoka prefecture


M7.0 on March 20, 2005 (depth approximately 10km)

- Northeastern Chiba prefecture M6.1 on April 11, 2005 (depth approximately 50km)

- 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

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2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

- There is no supplementary information for the Hokkaido region.

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(2) Tohoku Region

- On March 19, there was a M4.1 earthquake at a depth of approximately 65km off-shore of Miyagi prefecture. Also on March 24, there was a M4.0 earthquake at almost the same location.

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(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 change seems to have nearly stopped recently.
 (This evaluation is in agreement with the views presented at the procedural meeting on March 27 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, March 27, 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 Hamana lake. 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 March 13, there were two M5.1 earthquakes off-shore of Ibaraki prefecture.
- Light seismicity which started off-shore east of the Izu Peninsula on February 19, was continuing at a low level through March. However the activity increased again on March 30. The main active region is to the southeast of the activity in February. Subsequent seismicity has been on a decreasing trend. The largest event so far was a M3.1 earthquake on March 31 (maximum seismic intensity 1). Crustal movements associated with the recent events were observed on GPS, tiltmeters and strainmeters in the surrounding area.

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(4) Kinki-Chugoku-Shikoku Region

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

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(5) Kyushu-Okinawa Region

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

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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.