| 10 December 2003 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity for November 2003
There was no remarkable activity. to supplementary reports
*The aftershock activity of the Off-shore Tokachi earthquake on September 26 (2003 Off-shore Tokachi Earthquake) has been declining normally. The largest aftershock in November was a M5.3 event that occurred near the northwestern end of the aftershock area on November 24. Also on December 3, there was a M5.6 earthquake near the northeastern end of the aftershock area. According to the GPS data, postseismic movements that were seen following the main shock have been continuing. The largest amplitude of the deformation was 13 cm in a southeastern direction at the station Erimo 1. However, there were no signs of expansion of the presumed slip region. to supplementary reports
* The aftershock activity of the earthquake off-shore of Fukushima prefecture on October 31 (M6.8) has been declining normally. There were aftershocks of M6.2 on November 1 and M5.6 on November 2, at depths of approximately 45km. to supplementary reports
*On November 15, there was a M5.8 earthquake at a depth of approximately 50km off-shore of Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in nearly E-W direction. This event occurred at the boundary between the Pacific and the continental plates.
*On November 23, there was a M5.1 earthquake at a depth of approximately 40km off-shore east of Chiba prefecture.
*On November 12, there was a M6.5 earthquake at a depth of approximately 400km off-shore of Tokaido.
*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 November 25, there was a M4.1 earthquake at a depth of approximately 10km in the region along the mountains in southern Miyazaki prefecture.
*On November 30, there was a M4.8 earthquake at a depth of approximately 10km in northwestern Kagoshima prefecture. This event occurred in the aftershock area of the earthquake (M6.6) on March 26, 1997. The focal mechanism showed a strike-slip fault with a tension axis in a NW-SE direction, which is similar to the earthquake in 1997. to supplementary reports
*On November 12, there was a deep M 6.4 earthquake under the ocean area near Chichijima.
| December 10, 2003 |
| Earthquake Research Committee |
Following are the numbers of the earthquakes by magnitude that occurred in Japan and the surrounding area during November 2003.
M4.0 or greater: 123 (167 events occurred in October.)
M5.0 or greater: 17 (21 events occurred in October.)
M6.0 or greater: 3 (There has been a total of 21 events for 2003 by the end of November.)
(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 November 2002 to the end of October 2003.
| -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) | |
| -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) |
There is no supplementary information for the Hokkaido region.
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 November 2003 and show no signs of acceleration.
(This evaluation is in agreement with the views presented at the procedural meeting on November 25 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, November 25, 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."
The fairly long term crustal movements in Bungo Strait that had been seen on data from GPS and tiltmeters since early August, has continued, however, it appears to be subsiding in general. It is thought that slow slip at the plate boundary caused the movements. The activity of low-frequency earthquakes that increased at depths of 30 - 40km in nearly the same region since late August, has also been subsiding. In addition, to the long term crustal movements mentioned above, short-term deformations on the order of a few days on data from tiltmeters, supposedly caused by slow slip, has been seen almost simultaneously with the increased activity of low-frequency earthquakes.
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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