March 7, 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Seismic Activity for February 2008

1 Major Seismic Activity

* There was no remarkable activity. to supplementary reports

• Seismic Activity for All of Japan during February 2008 (Magnitude 4.0 and greater)

2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

* There was no remarkable activity. to supplementary reports

• Hokkaido Region

(2) Tohoku Region

* There was no remarkable activity. to supplementary reports

• Tohoku Region

(3) Kanto-Chubu Region

* On February 10, there was a M5.0 earthquake at a depth of approximately 100km off-shore southeast of Chiba prefecture. The focal mechanism showed a normal fault with a tension axis in an E-W direction. This event occurred within the Pacific plate.

* On February 27, there was a M6.6 earthquake under the ocean area near Chichijima. The focal mechanism showed a reverse fault with a compression axis in an E-W direction.

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

• Kanto-Chubu Region
• Earthquake Off-shore Southeast of Chiba Prefecture on February 10
• Earthquake under the Ocean Area near Chichijima on February 27
• Continuous GPS Monitoring in the Area from Kakegawa to Omaezaki
• Recent Crustal Movements in the Tokai Region (Horizontal Movements) [Ogata is the fixed reference point] (February 2007–February 2008)
• Crustal Movements in the Tokai Region, Time series, Ogata is the fixed reference point (Effects of viscoelastic relaxation have been removed)

(4) Kinki-Chugoku-Shikoku Region

* There was no remarkable activity. to supplementary reports

• Kinki-Chugoku-Shikoku Region

(5) Kyushu-Okinawa Region

* There was no remarkable activity. to supplementary reports

• Kyushu Region
• Okinawa Region

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

March 7, 2008

1 Major Seismic Activity

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

• M4.0 or greater: 55 (67 events occurred in January)
• M5.0 or greater: 6 (2 events occurred in January)
• M6.0 or greater: 1 (There has been a total of 1 event so far in 2008.)

(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 major seismic activity during the period from February 2007 to the end of January 2008.

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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

There is no supplementary information for the Tohoku region.

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(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 February 25 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, February 25, 2008)

"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. Under central Shizuoka prefecture, the seismicity remains at a level lower than normal within the plate, and higher within the crust. In other regions, seismic activity is generally at a normal level.
There were no notable changes observed in crustal movements in the Tokai region and its neighboring areas."

* On February 10, there was a M4.2 earthquake at a depth of approximately 15km in southern Chiba prefecture [off-shore east of Chiba 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 Philippine Sea and the continental plates.

• Earthquake in Southern Chiba Prefecture [Off-shore East of Chiba Prefecture] on February 10

* Seismic activity that has been continuing intermittently in western Shizuoka prefecture since mid-November in 2007 is gradually subsiding. The largest event in this seismicity was a M4.2 earthquake on January 27.

• Seismic Activity in Western Shizuoka Prefecture

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