| 14 January 2004 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity for December 2003
There was no remarkable activity. to supplementary reports
*The aftershock activity of the Off-shore Tokachi earthquake on September 26, 2003 (2003 Off-shore Tokachi Earthquake) has been declining normally. The largest aftershock in December was a M6.0 event that occurred off-shore of Kushiro on December 29. It was the first time since October 11 that an event of M6.0 or greater has occurred, which means there has been a total of 7 events of magnitude 6.0 or greater, including the recent one. Also there was a M5.6 aftershock off-shore of Kushiro on December 3, a M5.0 in southern Tokachi on December 12, and a M5.7 off-shore of Kushiro on December 22. According to the GPS data, postseismic movements that were seen following the main shock have been continuing. The largest amplitude of the deformation was 14 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
There was no remarkable activity. to supplementary reports
*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
*On December 23, there was a M4.4 earthquake at a depth of approximately 10km in northern Shiga prefecture.
*On December 13, there was a M4.6 earthquake at a depth of approximately 15km in Harimanada, which was followed by light aftershock activity. The focal mechanism showed a reverse fault with a compression axis in nearly an E-W direction. Before this earthquake, there were no earthquakes of M4.5 or greater since 1923 around this epicenter (Shodoshima region). to supplementary reports
*On December 24, there was shallow seismic activity off-shore west of Kumejima with the largest event of M6.0. The activity subsided within a day. to supplementary reports
*On December 10, there was a M 6.6 earthquake near Taiwan.
*On January 9, 2004, there was a M4.0 earthquake at a depth of approximately 15km in the Chuetsu region, Niigata prefecture.
*On January 11, 2004, there was a M4.0 earthquake at a depth of approximately 10km in central Nagano prefecture.
*On January 6, 2004, there was a M5.4 earthquake at a depth of approximately 35km in Kumanonada.
| January 14, 2004 |
| Earthquake Research Committee |
Following are the numbers of the earthquakes by magnitude that occurred in Japan and the surrounding area during December 2003.
M4.0 or greater: 114 (123 events occurred in November.)
M5.0 or greater: 20 (17 events occurred in November.)
M6.0 or greater: 3 (There has been a total of 24 events for 2003 by the end of December.)
(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 December 2002 to the end of November 2003.
| -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.
The aftershock activity of the earthquake off-shore of Fukushima prefecture on October 31, 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 December 2003 and show no signs of acceleration.
(This evaluation is in agreement with the views presented at the procedural meeting on December 22 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, December 22, 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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