11 August 2004
Headquarters for Earthquake Research Promotion
Earthquake Research Committee


Seismic Activity for July 2004


1 Major Seismic Activity

There was no remarkable activity. to supplementary reports

2 Nationwide seismic activity by region

(1) Hokkaido Region

*On July 20 there was a M5.0 earthquake at a depth of about 100 km under southern Tokachi subprefecture. The focal mechanism showed a tension axis in the direction of the subducting Pacific plate and this event occurred within the Pacific plate.
*On July 29 there was a M4.7 earthquake at a depth of about 50 km under southern Tokachi subprefecture. This earthquake is considered to be an aftershock of the 2003 Off-shore Tokachi earthquake. to supplementary reports

(2) Tohoku Region

*On July 4 there was a M4.5 earthquake at a depth of about 110km under the Sampachi-Kamikita district, Aomori prefecture.
*On July 9 there was a M4.4 earthquake at a depth of about 10 km onshore in northern Iwate prefecture. to supplementary reports

(3) Kanto-Chubu Region

*On July 20 there was a M4.1 earthquake at a depth of about 55 km under northern Ibaraki prefecture.
*On July 10 there was a M4.7 earthquake at a depth of about 50 km under southern Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. The earthquake occurred at the boundary between the Philippine Sea and continental plates.
*On July 17 there was a M5.5 earthquake at a depth of 70 km offshore southeast of the Boso Peninsula. The sequence shows a main shock-aftershock type of activity and seismicity is gradually declining.
*On July 27 there was a M4.5 earthquake at a depth of about 10 km in west central Mino, Gifu prefecture. Aftershock activity subsided within a few days. The M6.6 earthquake on September 9, 1969 occurred near the epicenter of this recent event.
*The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing. to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity. to supplementary reports

(5) Kyushu-Okinawa Region

*On July 22 there was a M6.1 earthquake near the Okinawa main island. The focal mechanism showed a tension axis in a NW-SE direction. Aftershock activity is gradually declining. to supplementary reports

(6) Other Region

*On July 8, a M6.3 earthquake occurred in the Chishima Islands (Kuril Islands).

Supplement

*On August 6 there was a M4.6 earthquake at a depth of about 75 km under northwest Chiba prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. The earthquake occurred at the boundary of the Pacific and Philippine Sea plates.
*On August 10 there was a M5.8 earthquake at a depth of about 50km offshore of Iwate prefecture with a maximum seismic intensity of 5 lower. The focal mechanism showed a reverse fault with a compression axis in a E-W direction. The earthquake occurred at the boundary between the Pacific and continental plates.


Supplementary Information to the Evaluation of Seismic Activity for July 2004 by the Earthquake Research Committee

August 11, 2004
Earthquake Research Committee

1 Major Seismic Activity

Following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during July 2004.

M4.0 or greater: 132 (87 events occurred in May.)
M5.0 or greater: 17 (10 events occurred in May.)
M6.0 or greater: 2 (4 events occurred in 2004 through July)

Numbers of earthquakes, excluding the Taiwan region.

M4.0 or greater: 65
M5.0 or greater: 11

(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 July 2003 to the end of June 2004.

-Northern Miyagi prefectureM6.4 on July 26, 2003 (depth of approximately 10km)
-Off-shore of Tokachi
     (2003 Off-shore Tokachi Earthquake)
M8.0 on Sep. 26, 2003 (depth of approximately 40km)
-Off-shore of Fukushima prefecture M6.8 on Oct. 31, 2003 (depth of approximately 30km)
-Off-shore southeast of the Boso Peninsula     
     (near the triple junction of the plates)
M6.7 on May 30, 2004

to the text

2 Nationwide Seismic Activity by Region

(1) Hokkaido Region

"On July 29 there was a M4.7 earthquake at a depth of about 50 km under southern Tokachi subprefecture. This earthquake is considered to be an aftershock of the 2003 Off-shore Tokachi Earthquake":
The aftershock activity of the 2003 Off-shore Tokachi Earthquake is continuing to decline. According to the GPS data, postseismic movements that were seen following the main shock have been continuing at a low level.

to the text

(2) Tohoku Region

- On July 21 there were M5.5 and M5.4 earthquakes off-shore of Sanriku. Focal mechanisms of both events showed reverse faults with compression axes in a WNW-ESE direction. A M5.2 earthquake occurred in the almost same location on July 3.

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, still appear to be continuing.":
The Pacific side of the Tokai-Chubu region maintains a steady rate with movement to the WNW, consistent with the subduction of the Philippine Sea plate toward the northwest. The results from GPS observations in the area centered on western Shizuoka prefecture, have been showing a trend of moderate change since about April 2001. The trends have continued through July 2004 and show no signs of acceleration.
(This evaluation is in agreement with the views presented at the procedural meeting on July 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, July 26, 2004)

"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
There is no remarkable seismic activity in general. The seismicity continues at a level lower than normal directly under Hamana lake. In the other regions, seismic activity is generally at a normal level.
The long-term changes in crustal movements in the Tokai region and its vicinity caused by the slow slip on the plate boundary, have been continuing. The deformation in 2003 was larger than in 2002 and about the same as in 2001. Also, the center of slip was in the northeast region of Hamana lake and does not appear to have moved."

to the text

(4) Kinki-Chugoku-Shikoku Region

- On July 12 a M3.9 earthquake occurred at a depth of about 10 km in southwest Hyogo prefecture.

to the text

(5) Kyushu-Okinawa Region

"On July 22 there was a M6.1 earthquake near the Okinawa main island. The focal mechanism showed a tension axis in a NW-SE direction. Aftershock activity is gradually declining.":
The largest aftershock was a M5.9 event on July 25.

to the text

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.