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

Seismic Activity for October 2008

1 Major Seismic Activity

There was no remarkable activity. to supplementary reports

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

2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

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

• Hokkaido Region
• Earthquake in Mid-Northern Rumoi Subprefecture on October 4

(2) Tohoku Region

* On October 30, there was a M5.1 earthquake at a depth of approximately 85km off-shore of Miyagi prefecture. The focal mechanism had a tension axis in the direction of the subducting Pacific plate. This event occurred within the subducting Pacific plate. to supplementary reports

• Tohoku Region
• Earthquake Off-shore of Miyagi Prefecture on October 30

(3) Kanto-Chubu Region

* On October 8, there was a M4.7 earthquake at a depth of approximately 65 km 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 tension axis in a NW-SE direction.

* On October 14, there was a M4.3 earthquake at a depth of approximately 25km in northeastern Chiba prefecture [off-shore east of Chiba prefecture]. The focal mechanism showed a reverse fault with a compression axis in a N-S direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

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

• Kanto-Chubu Region
• Earthquake under Northwestern Chiba Prefecture on October 8
• Earthquake in Northeastern Chiba Prefecture [Off-shore East of Chiba Prefecture] on October 14
• 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] (October 2007–October 2008)
• Crustal Movements in the Tokai Region, Time series, Ogata is the fixed reference point (Postseismic movements 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

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

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

November 10, 2008

1 Major Seismic Activity

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

• M4.0 or greater: 70 (79 events occurred in September)
• M5.0 or greater: 11 (15 events occurred in September)
• M6.0 or greater: 0 (There has been a total of 17 events by the end of October in 2008.)

(reference)

• M4.0 or greater: 73 / month (Median number of earthquakes over the past 10 years from 1998 through 2007)
• M5.0 or greater: 9 / month (Median number of earthquakes over the past 35 years from 1973 through 2007)
• M6.0 or greater: 1.4 / month, approximately 17 / year (Average number of earthquakes over the past 84 years from 1924 through 2007)

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

"On October 14, there was a M4.3 earthquake at a depth of approximately 25km in northeastern Chiba prefecture [off-shore east of Chiba prefecture]. (excerpted)":
At nearly the same location as this event, there were M4.2 earthquakes on October 12 and 16.

"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 October 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, October 27, 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 southern Nagano prefecture during the period from October 9 through 18. In this vicinity, deep low-frequency earthquakes were also observed during the period 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."

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

• Earthquake in Mid-western Mino, Gifu Prefecture on October 1

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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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