| 10 September 2003 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in August 2003
There was no remarkable activity. to supplementary reports
*On August 30, there was a M5.2 earthquake at a depth of approximately 55km off-shore of Urakawa. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred at the boundary between the Pacific and the continental plates. to supplementary reports
*The aftershock activity of the earthquake that occurred in northern Miyagi prefecture on July 26, has been declining normally. There were aftershocks of M4.0 or greater on August 8 (M4.4), August 9 (M4.0), August 12 (M4.4), August 27 (M4.1), and September 5 (M4.0). According to the GPS data, no significant postseismic movement has been seen since the main shock (M6.2) on July 26. to supplementary reports
*On August 18, there was a M4.6 earthquake at a depth of approximately 70km 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.
*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
*On August 6, there was a M4.0 earthquake at a depth of approximately 5km in northern Wakayama prefecture.
*On August 14, there was a M4.6 earthquake at a depth of approximately 35km in eastern Kochi prefecture. The focal mechanism showed a strike-slip fault with a tension axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate. to supplementary reports
There was no remarkable activity. to supplementary reports
* On September 1, there was a deep M6.2 earthquake at a depth of approximately 540km near Vladivostok.
| September 10, 2003 |
| Earthquake Research Committee |
57 earthquakes of M4.0 or greater occurred in Japan and the surrounding area during August (77 events occurred in July, and an average of 46 earthquakes per month have occurred over the past thirty years through the end of 2000). 4 of these earthquakes in August were M5.0 or greater. (9 events of M5.0 or greater occurred in July.)
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 was no event of magnitude equal to or over M6.0 in August 2003, and there has been a total of 6 M6.0 or greater events by the end of July this year.
The following regions have been noted as having significant seismic activity during the period from August 2002 to the end of July 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) | |
| -Northern Miyagi prefecture | M6.2 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, has been declining normally. There were aftershocks of M4.0 or greater on August 8 (M4.4), August 9 (M4.0), August 12 (M4.4), August 27 (M4.1), and September 5 (M4.0). According to the GPS data, no significant postseismic movement has been seen since the main shock (M6.2) on July 26.":
From a high-precision aftershock distribution and focal mechanism analyses using data from temporary seismic stations, a N-S striking fault surface (dipping toward the west) was inferred in the north and a NE-SW striking fault surface (dipping toward the northwest), was inferred in the south. According to the crustal movement data from GPS monitoring and leveling, a reverse fault model was estimated that has two fault surfaces, as described above.
There was additional activity in the Tohoku region, as follows.
- On August 22, there was a M4.6 aftershock at a depth of approximately 70km that occurred near the northern end of the aftershock area of the off-shore of Miyagi prefecture earthquake on May 26 (M7.0). The aftershock activity 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.
(The Assessment Meeting for Areas under Intensified Measures against Earthquake Disaster held at the Japan Meteorological Agency during the 'Comprehensive Disaster Drills' on September 1, presented the view that there had been no large changes in seismic activity in the Tokai region since the same procedural meeting on July 28. 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 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.
- On August 19, there was a shallow M4.9 earthquake under the ocean near Amami Oshima.
| 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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