11 August 2004 |
Seismic Activity for July 2004
There was no remarkable activity. to supplementary reports
*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
*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
*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
There was no remarkable activity. to supplementary reports
*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.
| August 11, 2004 |
| Earthquake Research Committee |
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 |
| -Northern Miyagi prefecture | M6.4 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 |
"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.
- 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.
"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."
- On July 12 a M3.9 earthquake occurred at a depth of about 10 km in southwest Hyogo prefecture.
"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.
| 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:
|