| 7 August 2003 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in July 2003
On July 26, there was a M5.5 earthquake (foreshock) with a maximum seismic intensity 6 Lower, and a M6.2 earthquake (main shock) with a maximum seismic intensity 6 Upper at a depth of approximately 10km in northern Miyagi prefecture. The aftershock activity of the M6.2 event included a M4.8 earthquake (maximum seismic intensity 5 Lower) and a M5.3 earthquake (maximum seismic intensity 6 Lower) on July 26, and a M5.0 earthquake (maximum seismic intensity 5 Lower) on July 28. This seismic activity caused some damage and over 600 injuries. to supplementary reports
*On July 3, there was a M5.8 earthquake at a depth of approximately 30km off-shore of Kushiro. 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. to supplementary reports
*(The seismic activity (including the foreshock and the aftershock) in northern Miyagi prefecture on July 26 is described in a separate section) to supplementary reports
*On July 11, there was a M4.1 earthquake at a depth of approximately 20km in western Kanagawa prefecture.
*On July 9, there was a M4.3 earthquake at a depth of approximately 15km near the Chita Peninsula.
*The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing, although the rates seem smaller compared to 2001. to supplementary reports
There was no remarkable activity. to supplementary reports
*On July 10, there was a M5.2 earthquake under the ocean near Amami Oshima. to supplementary reports
*On July 27, there was a deep M7.1 earthquake at a depth of approximately 490km under the northern region of the Japan Sea.
| August 7, 2003 |
| Earthquake Research Committee |
77 earthquakes of M4.0 or greater occurred in Japan and the surrounding area during July (73 events occurred in June, and an average of 46 earthquakes per month have occurred over the past thirty years through the end of 2000). 10 of the events were aftershocks of the earthquake that occurred off-shore of Miyagi prefecture on May 26 (In June, 17 of the events were part of this aftershock activity). 23 of the events were part of the seismic activity in northern Miyagi prefecture on July 26. 9 of these earthquakes in July were M5.0 or greater. (8 events of M5.0 or greater occurred in June.)
Earthquakes of M6.0 or greater occurred, on average, 15 times a year during 1998 to 2002. (Over the past 30 years through the end of 2000, on average, M6.0 or greater events have occurred approximately 16 times a year) There were 2 events of magnitude equal to or over M6.0 in July 2003, and there has been a total of 4 M6.0 or greater events by the end of June this year.
The following regions have been noted as having significant seismic activity during the period from July 2002 to the end of June 2003.
| -Ocean area east of Aomori prefecture | M5.9 on Oct. 14, 2002 (depth of approximately 50km) | |
| -Off-shore of Miyagi prefecture | M6.2 on Nov. 3, 2002 (depth of approximately 45km) | |
| -Hyuganada | M5.7 on Nov. 4, 2002 (depth of approximately 35km) | |
| -Off-shore of Miyagi prefecture | M7.0 on May. 26, 2003 (depth of approximately 70km) |
There is no supplementary information for the Hokkaido region.
- The aftershock activity of the earthquake off-shore of Miyagi prefecture on May 26 (depth of approximately 70km, M7.0) 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, although the rates seem smaller compared to 2001".
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 2003, but the changes are becoming smaller compared to 2001.
(This evaluation is in agreement with the views presented at the procedural meeting on July 28 of the Regional Assessment Committee for Strengthening Countermeasures 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 28, 2003)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
Although the seismicity directly under Hamana lake had been declining, recently it has been increasing again. 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, although recently the rates seem smaller compared to 2001."
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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