October 13, 2004 |
Seismic Activity for September 2004
On September 5, there was a M7.4 earthquake off-shore southeast of the Kii Peninsula (off-shore of Tokaido). This event had a maximum seismic intensity 5 Lower in Mie, Nara and Wakayama prefectures. This earthquake caused a tsunami on the Pacific coast from the Izu islands to the Shikoku region. Also, about five hours before that event, there was a M6.9 foreshock. The foreshock had a maximum seismic intensity 5 Lower in Nara and Wakayama prefectures and caused a tsunami on the Pacific coast from the Izu islands to the Shikoku region. These events caused damage with injured persons. The M6.5 event on September 8 was the largest aftershock so far. to supplementary reports
* On September 10, there was a M5.1 earthquake at a depth of approximately 50km under the southern Tokachi subprefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. to supplementary reports
*On September 24, there was a M4.2 earthquake at a depth of approximately 90km under the northern coast of Iwate prefecture.
*On September 1, there was a M5.6 earthquake at a depth of approximately 30km off-shore of Fukushima prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary of the Pacific and the continental plates. to supplementary reports
*On September 7, there was a M4.3 earthquake under the Chuetsu Region, Niigata prefecture.
*(The earthquakes and aftershock activity that occurred off-shore southeast of the Kii Peninsula on September 5, are described in a separate section.) to supplementary reports
*On September 21, there was a M4.2 earthquake at a depth of approximately 50km under southeastern Hiroshima prefecture. to supplementary reports
*On September 1, there was a M5.2 earthquake under the ocean near Amami Oshima. to supplementary reports
Supplement
*On October 1, there was a M4.2 earthquake at a depth of approximately 75km under the Hamadori Region, Fukushima prefecture.
*On October 5, there was a M4.8 earthquake at a depth of approximately 10km under 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 mostly subsided within a day.
*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. In this region steady seismic activity has been observed over the last few decades with earthquakes equal to or larger than M5.0 occurring at a rate of about once a year, and no earthquakes equal to or greater than M6.0 have occurred.
*On October 7, there was a M4.4 earthquake at a depth of approximately 60km in Kashimanada.
*On October 8, there was a M5.1 earthquake at a depth of approximately 50km under the southern Tokachi subprefecture.
| October 13, 2004 |
| Earthquake Research Committee |
Following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during September 2004.
| M4.0 or greater: 159 (90 events occurred in August) | |
| M5.0 or greater: 24 (8 events occurred in August) | |
| M6.0 or greater: 4 (8 events in total by the end of September 2004) |
Of all the events above, 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 |
| -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) | |
| -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) |
- 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 a low level.
There is no supplementary information for the Tohoku region.
"On September 7, there was a M4.3 earthquake under the Chuetsu Region, Niigata prefecture.":
There was additional activity in the Kanto-Chubu region, as follows.
- Within the subducting Philippine Sea plate in the Tokai region, micro-seismicity slightly increased temporarily after the earthquakes off-shore the Kii Peninsula on September 5. Also, in the GPS data of the Tokai region, step changes mainly associated with earthquakes were observed over a wide area.
(This evaluation is in agreement with the views presented at the procedural meeting on September 28 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, September 28, 2004)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
On September 5, there was a M7.4 earthquake off-shore of Tokaido (off-shore southeast of the Kii Peninsula). This earthquake and the M6.9 foreshock that occurred about five hours before that event, showed normal step changes associated with such earthquakes, on strainmeters in the Tokai region. Following, slow changes continued at some observation points, however, these movements subsided within several days. It appears that these slow changes are due to local movements near the observation points.
In the Tokai region, micro-seismicity temporarily increased slightly immediately after the earthquake on September 5. On the other hand, the seismicity continues at a level lower than normal directly under Hamana lake.
It is estimated that the effect from the (off-shore southeast of the Kii Peninsula) earthquakes in the assumed source region (of the Tokai earthquake) is about the same level as the peak tidal changes. Judging from this fact and the observation results mentioned above, it appears that there was no direct effect on the occurrence of the so-called Tokai earthquake from the seismic activity off-shore of the Kii Peninsula / off-shore of Tokaido (off-shore southeast of the Kii Peninsula)
In addition, crustal movements were widely observed in the Tokai region associated with the earthquakes off-shore of the Kii Peninsula / off-shore of Tokaido (off-shore southeast of the Kii Peninsula). It seems that these are mainly step changes from the earthquakes. "
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
There is no supplementary information for the Kyushu-Okinawa region.
- There has been shallow seismic activity off-shore west of Amami Oshima (under the ocean near Amami Oshima) since October 1. As of October 12, the largest events were the M5.3 events on October 3 and 6. The focal mechanisms showed strike-slip faults with tension axes in a NNW-SSE direction. The activity has been gradually subsiding since October 7. In this vicinity, there was a M6.1 earthquake on June 16, 1990.
| 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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