September 8, 2004 |
Seismic Activity for August 2004
There was no remarkable activity. to supplementary reports
There was no remarkable activity. to supplementary reports
*On August 10, there was a M5.8 earthquake at a depth of approximately 50km off-shore of Iwate prefecture. This event had a maximum seismic intensity 5 Lower. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred at the boundary of the Pacific and the continental plates. So far, there have been no aftershocks of M3.0 or greater, and the seismic activity has been returning to normal levels.
*On August 19, there was a M5.0 earthquake at a depth of approximately 75km off-shore of Fukushima prefecture. The focal mechanism showed a normal fault with a tension axis in a NNE-SSW direction. This event occurred within the Pacific plate. to supplementary reports
*On August 6, there was a M4.6 earthquake at a depth of approximately 75km under northwestern Chiba prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the Philippine Sea plates.
*On August 25, there was a M4.4 earthquake at a depth of approximately 50km under Tokyo Bay.
*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 August 21, there was a shallow M5.6 earthquake under the ocean near Yonaguni Island. The focal mechanism had a compression axis in a NNW-SSE direction. Aftershock activity mostly subsided within a day. to supplementary reports
Supplement
*On September 1, there was a M5.6 earthquake at a depth of approximately 30km off-shore of Fukushima prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
*On September 1, there was a M5.2 earthquake under the ocean near Amami Oshima.
*(The earthquakes and aftershock activity that occurred off-shore southeast of the Kii Peninsula on September 5, are described in a separate section.)
*On September 7, there was a M4.3 (preliminary) earthquake at a depth of approximately 15 km under the Chuetsu Region, Niigata prefecture.
| September 8, 2004 |
| Earthquake Research Committee |
Following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during August 2004.
| M4.0 or greater: 91 (132 events occurred in July) | |
| M5.0 or greater: 8 (17 events occurred in July) | |
| M6.0 or greater: 0 (4 events in total by the end of August 2004) |
Numbers of earthquakes near Taiwan
| M4.0 or greater: 21 (67 events occurred in July) | |
| M5.0 or greater: 1 (6 events occurred in July) |
Numbers of earthquakes, excluding the Taiwan region
| M4.0 or greater: 70 (65 events occurred in July) | |
| M5.0 or greater: 7 (11 events occurred in July) |
(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 |
| -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 August 17, there was a M5.1 earthquake at a depth of approximately 45km off-shore of Tokachi. Within the following two hours, there were M4.6, M4.9 and M4.7 earthquakes at nearly the same location. These earthquakes are located at the southwestern edge of the aftershock region 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.
There is no supplementary information for the Tohoku 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 August 2004 and show no signs of acceleration.
(The Assessment Committee for Areas under Intensified Measures against Earthquake Disaster held at the Japan Meteorological Agency during the eComprehensive Disaster Drillsf on September 1, presented the view that there had been no large changes in seismic activity and crustal deformation in the Tokai region since the same procedural meeting on July 26. The evaluation is in agreement with this view.)
(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 nearly 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."
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
There is no supplementary information for the Kyushu-Okinawa region.
| 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:
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