| 9 October 2003 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in September 2003
On September 26, there was a M8.0 earthquake at a depth of approximately 40km off-shore of Tokachi. This event had a maximum seismic intensity 6 Lower and caused a tsunami on the Pacific coast from Hokkaido to the Tohoku region. On the same day, there was a M7.1 aftershock (maximum seismic intensity 6 Lower), which was the largest aftershock so far. The seismic activity caused 2 missing people and over 800 injuries. to supplementary reports
*(The earthquake that occurred off-shore of Tokachi on September 26 and its aftershock activity are described in a separate section) to supplementary reports
There was no remarkable activity. to supplementary reports
*On September 20, there was a M5.8 earthquake at a depth of approximately 70km under southern Chiba prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred near the boundary between the Pacific and the Philippine Sea plates.
*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 September 28, there was shallow seismic activity under the ocean near Amami Oshima with the largest event a M6.0. The active seismicity subsided within a day. to supplementary reports
*On September 1, there was a deep M6.2 earthquake at a depth of approximately 540km near Vladivostok.
*On October 5, there was a M4.5 earthquake at a depth of approximately 15km in the Hida Region, Gifu prefecture.
| October 9, 2003 |
| Earthquake Research Committee |
Followings are numbers of the earthquakes by magnitudes that occurred in Japan and the surrounding area during September, 2003.
M4.0 or greater: 168 (57 events occurred in August.)
M5.0 or greater: 41 (4 events occurred in August)
M6.0 or greater: 7 (There has been a total of 13 events for 2003 by the end of September.)
Of all the events above, 97 earthquakes of M4.0 or greater, 30 earthquakes of M5.0 or greater, and 5 earthquakes of M6.0 or greater were
part of the Off-shore Tokachi Earthquake on July 26 and its aftershock activity.
(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 |
| -Ocean area east of Aomori prefecture | M6.1 on Oct. 14, 2002 (depth of approximately 50km) | |
| -Off-shore of Miyagi prefecture | M6.3 on Nov. 3, 2002 (depth of approximately 45km) | |
| -Hyuganada | M5.9 on Nov. 4, 2002 (depth of approximately 35km) | |
| -Off-shore of Miyagi prefecture | M7.1 on May. 26, 2003 (depth of approximately 70km) | |
| -Northern Miyagi prefecture | M6.4 on July 26, 2003 (depth of approximately 10km) |
There is no supplementary information for the Hokkaido region.
- The aftershock activity of the earthquake that occurred in northern Miyagi prefecture on July 26 (M6.4), has been declining normally.
"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 September 2003 and show no signs of acceleration.
(This evaluation is in agreement with the views presented at the procedural meeting on September 29 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, September 29, 2003)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
Although light seismicity had been seen directly under Hamana lake in May and June, the seismic activity has been at a low level since the end of 2002. In the other regions, no remarkable changes of the seismic activity can be seen.
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."
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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