November 10, 2004 |
Seismic Activity for October 2004
*On October 23, there was a M6.8 earthquake* at a depth of approximately 10km in the Chuetsu region, Niigata prefecture. This event had a maximum seismic intensity 7 in Niigata prefecture. There were 19 M5.0 or greater aftershocks (4 were equal to or larger than M6.0) with the largest aftershock of M6.5 on the same day. Two aftershocks had a maximum seismic intensity 6 Upper, two aftershocks had a maximum of 6 Lower, 6 aftershocks had a maximum of 5 Upper, and 4 aftershocks had a maximum of 5 Lower. This seismic activity caused considerable damage and fatalities.
*: The Japan Meteorological Agency named the event the "Mid Niigata Prefecture Earthquake in 2004".
*On October 6, there was a M5.7 earthquake at a depth of approximately 65km under southern Ibaraki prefecture. This event had a maximum seismic intensity 5 Lower and caused some damage.
*On October 15, there was a M6.6 earthquake under the ocean near Yonaguni Island. This event had a maximum seismic intensity 5 Lower.
to supplementary reports
*On October 8, there was a M5.1 earthquake at a depth of approximately
50km under southern Tokachi subprefecture. 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
*On October 1, there was a M4.2 earthquake at a depth of approximately
75km under the Hamadori region, Fukushima prefecture. to supplementary reports
*(The Niigata Chuetsu Earthquake on October 23 and its aftershock activity are described in a separate section.)
*On October 7, there was a M4.4 earthquake at a depth of approximately 60km in Kashimanada.
*On October 6, there was a M5.7 earthquake at a depth of approximately 65km under southern Ibaraki prefecture. This event had a maximum seismic intensity 5 Lower and caused some damage. 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 Philippine Sea plates. The subsequent aftershock activity has been at a low level.
*On October 17, there was a M5.7 earthquake at a depth of approximately 50km off-shore of Ibaraki prefecture. Also about an hour and a half before this event, there was a M5.5 earthquake at nearly the same location. Both focal mechanisms showed reverse faults with compression axes in a WNW-ESE direction. These events are associated with the subduction of the Pacific plate.
*On October 5, there was a M4.8 earthquake at a depth of approximately
10km in the Reihoku region, Fukui prefecture. The focal mechanism showed
a strike-slip fault with a compression axis in an E-W direction. The aftershock
activity subsided by October 6. to supplementary reports
*On October 27, there was a M4.4 earthquake at a depth of approximately
40km in the Kii Strait. to supplementary reports
*On October 15, there was a M6.6 earthquake at a depth of 80km under the
ocean near Yonaguni Island. This event had a maximum seismic intensity
5 Lower. The focal mechanism had a tension axis in the direction of the
subducting plate. It is thought that this event occurred within the Philippine
Sea plate. to supplementary reports
Supplement
*On November 4, there was a M4.2 earthquake at a depth of approximately 15km in the Chikugo region, Fukuoka prefecture.
*On November 4, there was a M5.8 earthquake at a depth of approximately 60km under the ocean near Kunashiri Island. The focal mechanism showed a reverse fault with a compression axis in a NNW-SSE direction.
*On November 5, there was a M4.0 earthquake at a depth of 50km off-shore of Iwate prefecture.
*On November 7, there was a deep M6.0 earthquake at a depth of 510km under the southern Sea of Okhotsk.
*On November 9, there was a M5.7 earthquake off-shore of Tokaido. The focal mechanism showed a reverse fault with a compression axis in a N-S direction. It is thought that this event occurred within the Philippine Sea plate.
*On November 9, there was a M6.4 earthquake near Taiwan.
| November 10, 2004 |
| Earthquake Research Committee |
Following are the numbers of earthquakes by magnitude that occurred in
Japan and the surrounding area during October 2004.
| M4.0 or greater: 165 (159 events occurred in September) | |
| M5.0 or greater: 29 (24 events occurred in September) | |
| M6.0 or greater: 6 (14 events in total by the end of October 2004) |
Of all the events above, 87 earthquakes of M4.0 or greater, 18 earthquakes of M5.0 or greater, and 5 earthquakes of M6.0 or greater were part of the Niigata Chuetsu Earthquake on October 23 and its aftershock activity. (In September, 80 earthquakes of M4.0 or greater, 11 earthquakes of M5.0 or greater, and 4 earthquakes of M6.0 or greater were part of the seismic activity off-shore southeast of the Kii Peninsula.)
(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 2003 to the end of September 2004.
| -Off-shore of Fukushima prefecture | M6.8 on Oct. 31, 2003 (depth of approximately 30km) | |
| -Off-shore southeast of the Boso Peninsula (near the triple junction of the plates) |
M6.7 on May 30, 2004 | |
| -Off-shore of Iwate prefecture | M5.8 on August 10, 2004 (depth of approximately 50km) | |
| - Off-shore southeast of the Kii Peninsula (Off-shore of Tokaido) | ||
| M7.4 on September 5, 2004 | ||
- The aftershock activity of the 2003 Off-shore Tokachi Earthquake is continuing to decline. According to the GPS data, postseismic movements that were seen following the main shock have been continuing at an extremely low level.
There is no supplementary information for the Tohoku region.
- The aftershock activity of the M7.4 earthquake off-shore southeast of the Kii Peninsula on September 5 has been continuing to decline. There are no particular changes observed in the active region.
- Associated with the earthquake off-shore southeast of the Kii Peninsula on September 5, step-like changes were observed in the GPS data over a wide area of the Tokai region. There were no remarkable changes in the trends of the crustal movements before and after the earthquake.
(This evaluation is in agreement with the views presented at the procedural meeting on November 1 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 1, 2004)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
In the Tokai region, micro-seismicity temporarily increased slightly immediately after the M7.4 earthquake off-shore of Tokaido (off-shore southeast of the Kii Peninsula) on September 5, however, it subsided within a short period of time. Generally there was no remarkable seismic activity. Directly under the Hamana lake, the seismicity continues at a level lower than normal.
Crustal movements were widely observed in the Tokai region associated with the earthquakes off-shore of Tokaido (off-shore southeast of the Kii Peninsula). It seems that these are mainly step-like changes from the earthquakes. There were no remarkable changes in the trends of the crustal movements before and after the earthquake."
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
- There was shallow seismic activity off-shore west of Amami Oshima (near Amami Oshima) which started on October 1, and mostly subsided by mid-October. The largest events were the M5.3 events on October 3 and October 6. The focal mechanisms showed strike-slip faults with tension axes in a NNW-SSE direction.
| 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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