| 12 November 2003 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity for October 2003
On October 31, there was a M6.8 earthquake at a depth of approximately 30km off-shore of Fukushima prefecture. This event caused a tsunami with a height of 30 cm in Oshika, Miyagi prefecture. to supplementary reports
*The aftershock activity of the Off-shore Tokachi earthquake on September 26 (2003 Off-shore Tokachi Earthquake) has been declining normally. In October, there were 14 aftershocks of M5.0 or greater, and 2 events of M6.0 or greater. The largest aftershock in October was a M6.4 event with a maximum seismic intensity 4, that occurred near the northeastern end of the aftershock area on October 8. According to the GPS data, postseismic movements that were seen following the main shock have been continuing. The amplitude of the deformation has reached 11 cm in a southeastern direction at the station Erimo 1, and 10 cm in the same direction at the station Erimo 2.
*On October18, there was a M4.5 earthquake at a depth of approximately 10km in northern Tokachi. After the Off-shore Tokachi earthquake, low-level shallow seismic activity has been occurring in some regions of eastern Hokkaido. This event also occurred in this active region.
*On October 29, there was a M6.0 earthquake off-shore of eastern Hokkaido prefecture. to supplementary reports
*On October 23, there was a M4.4 earthquake at a depth of approximately 10km in northern Miyagi prefecture. This event is an aftershock of the earthquake (M6.4) that occurred in northern Miyagi prefecture on July 26.
*On October 31, there was a M6.8 earthquake at a depth of approximately 30km off-shore of Fukushima prefecture. This event had a maximum seismic intensity 4. This event caused a tsunami with a height of 30 cm in Oshika, Miyagi prefecture. 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. The activity had a main shock - aftershock sequence, and it has been declining. The largest aftershock so far was the M6.2 earthquake that occurred on November 1. According to the GPS data, slight crustal movements associated with this event were observed. The Earthquake Research Committee has expected the repeat of the 1978 earthquake to occur in the near future, and the recent earthquake was located closely east of the expected event. to supplementary reports
*On October 15, there was a M5.1 earthquake at a depth of approximately 75km under northwestern Chiba prefecture. The focal mechanism had a compression axis in a NE-SW direction. This event occurred at the boundary between the Pacific and the Philippine Sea plates.
*On October 5, there was a M4.5 earthquake at a depth of approximately 15km in the Hida Region, Gifu prefecture.
*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
There was no remarkable activity. to supplementary reports
| November 12, 2003 |
| Earthquake Research Committee |
Followings are numbers of the earthquakes by magnitudes that occurred in Japan and the surrounding area during October, 2003.
M4.0 or greater: 167 (168 events occurred in September.)
M5.0 or greater: 21 (41 events occurred in September.)
M6.0 or greater: 4 (There has been a total of 18 events for 2003 by the end of October.)
Of all the events above, 76 earthquakes of M4.0 or greater, 14 earthquakes of M5.0 or greater, and 2 earthquakes of M6.0 or greater were part of the Off-shore Tokachi Earthquake on September 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 |
The following regions have been noted as having significant seismic activity during the period from October 2002 to the end of September 2003.
| -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) | |
| -Off-shore of Tokachi (2003 Off-shore Tokachi Earthquake) | M8.0 on Sep. 26, 2003 (depth of approximately 40km) |
There is no supplementary information for the Hokkaido region.
"On October 23, there was a M4.4 earthquake at a depth of approximately 10km in northern Miyagi prefecture. This event is an aftershock of the earthquake (M6.4) that occurred in northern Miyagi prefecture on July 26.":
The aftershock activity of the northern Miyagi prefecture earthquake (M6.4) has been declining normally.
There was additional activity in the Tohoku region, as follows.
- On October 6, there was a M3.5 earthquake in the Murayama Region, Yamagata prefecture.
"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 October 2003 and show no signs of acceleration.
(This evaluation is in agreement with the views presented at the procedural meeting on October 27 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, October 27, 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."
- According to the GPS data, crustal deformations were observed around Bungo Strait. It is thought that slow slip at the boundary between the Philippine Sea and the continental plates caused the movements. Movements started around early August, and station Misaki in Ehime prefecture has moved about 1 cm in a southeastern direction. In addition, activity of low-frequency earthquakes increased at depths of 30 - 40km in nearly the same region. A long term slow slip was also observed in this vicinity during 1996 - 1997.
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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