December 8, 2004 |
Seismic Activity for November 2004
| * | On November 29, there was a M7.1 earthquake at a depth of approximately 50km off-shore of Kushiro. (see the 135th Evaluation of the Earthquake Research Committee "Seismic Activity Off-shore of Kushiro") This event had a maximum seismic intensity 5 Upper in Hokkaido, and caused small tsunamis on the eastern Pacific coast of Hokkaido. The largest aftershock so far is the M6.9 event (maximum seismic intensity 5 Upper) on December 6. This seismic activity caused damage with injured people. to supplementary reports |
| * | On November 7, there was a deep M6.0 earthquake at a depth of approximately 510km under the southern Sea of Okhotsk. | |
| * | 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. | |
| * | Seismic Activity Off-shore of Kushiro | |
| - | On November 29, there was a M7.1 earthquake at a depth of approximately 50km off-shore of Kushiro. This event had a maximum seismic intensity 5 Upper. This was a main shock - aftershock sequence. The aftershock activity is declining. The largest aftershock so far is the M6.9 event (maximum seismic intensity 5 Upper) on December 6 that occurred to the south of the main shock. | |
| - | There was a M7.2 earthquake on August 12, 1961 near the recent main shock. It is thought that this earthquake occurred on the plate boundary, similar to the recent event. In addition, on November 15, 1961, there was a M6.9 earthquake within the active region of the recent earthquake. | |
| - | According to the GPS data, crustal movements associated with the main shock were observed with a displacement of a few centimeters in eastern Hokkaido, and followed by small postseismic movements. Also, small movements were observed in eastern Hokkaido associated with the largest aftershock on December 6. These crustal movements are consistent with the focal mechanisms of the main shock and the largest aftershock. | |
| * | On November 27, there was a M5.6 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. This event occurred at the boundary between the Pacific and the continental plates. | |
| * | On November 11, there was a M6.3 earthquake at a depth of approximately 40km off-shore of Kushiro. 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 November 5, there was a M4.0 earthquake at a depth of approximately 50km off-shore of Iwate prefecture. |
| * | On November 19, there was a M4.5 earthquake at a depth of approximately 75km off-shore of Miyagi prefecture. to supplementary reports |
| * | Seismic Activity in the Chuetsu Region, Niigata Prefecture | |
| - | The aftershock activity of the 2004 Niigata Chuetsu Earthquake on October 23 has been continuing to decline. There were aftershocks, M5.2 on November 4 and a M5.9 on November 8, that had maximum seismic intensities 5 Upper. There has been no earthquake equal to or greater than M5.0 since a M5.3 (maximum seismic intensity 5 Lower) event on November 10. | |
| - | According to the GPS data, there have been no remarkable postseismic movements observed so far. In addition, there was a small observed displacement associated with the M5.9 earthquake on November 8, at the Sumon observation station (Sumon, Uonuma city / old Sumon-mura, Minamiuonuma-gun, Niigata prefecture) located near the event. | |
| - | Aftershocks equal to or larger than M3.0 (perceptible to human senses within the small areas near the events) currently occur on average once a day, and it is thought that the activity will continue to decline. In addition, it is estimated that there is approximately a 10 percent probability of occurrence of a M5.0 (seismic intensity 5 Lower in some areas) earthquake within 3 days from 18:00 on December 8. | |
| - | Associated with this seismic activity, there were surface displacements along a total length of over 1km observed in Obirowo, Hirokami, Uonuma-city (old Hirokami-village). The surface displacements had a nearly N-S strike with uplift of approximately 10 to 15cm on the western side. It is thought that this surface fault is a part of an extension of the mainshock fault that from the deeper regions of the recent activity. | |
| * | 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. to supplementary reports | |
There was no remarkable activity. to supplementary reports
| * | On November 4, there was a M4.2 earthquake at a depth of approximately 15km in the Chikugo region, Fukuoka prefecture. |
| * | On November 21, there was a M4.1 earthquake at a depth of approximately 10km in the Kumamoto region, Kumamoto prefecture. to supplementary reports |
| * | On November 9, there was a M6.4 earthquake near Taiwan. |
Supplement
| * | On December 1, there was a M4.2 earthquake at a depth of approximately 75km under northern Miyagi prefecture. |
| * | On December 1, there was a M4.0 earthquake at a depth of approximately 15km under southern Kyoto prefecture. |
December 8, 2004
The following are the numbers of earthquakes by magnitude that occurred in
Japan and the surrounding area during November 2004.
| M4.0 or greater: 126 (165 events occurred in October) | |
| M5.0 or greater: 27 (29 events occurred in October) | |
| M6.0 or greater: 5 (19 events in total by the end of November 2004) |
Of all the events above, 20 earthquakes of M4.0 or greater, 8 earthquakes of M5.0 or greater, and no earthquake of M6.0 or greater were part of the Niigata Chuetsu Earthquake on October 23 and its aftershock activity. (In October, there were 87 earthquakes of M4.0 or greater, 18 earthquakes of M5.0 or greater, and 5 earthquakes of M6.0 or greater, that were part of the Niigata Chuetsu earthquake sequence.)
In addition, 16 earthquakes of M4.0 or greater, 2 earthquakes of M5.0 or greater, and 2 earthquakes of M6.0 or greater were part of the earthquake off-shore of Kushiro on November 29 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 November 2003 to the end of October 2004.
| -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 | ||
| - Chuetsu region, Niigata prefecture (2004 Niigata Chuetsu Earthquake) | ||
| M6.8 on Oct. 23, 2004 (depth approximately 10km) | ||
"On November 29, there was a M7.1 earthquake at a depth of approximately
50km off-shore of Kushiro. This event had a maximum seismic intensity 5
Upper. ... (snip)... Also with the largest aftershock on December 6, small
movements were observed in eastern Hokkaido. These crustal movements are
consistent with the focal mechanisms of the main shock and the largest
aftershock.":
Aftershocks were distributed in several streaks extending over approximately 40km in a WNW-ESE direction, with the main shock nearly centered in the region of active aftershocks. Also, the largest aftershock occurred to the south of the main shock.
"On November 11, there was a M6.3 earthquake at a depth of approximately
40km off-shore of Kushiro. 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.":
This event occurred within the aftershock region of the 2003 Off-shore Tokachi Earthquake. Also, on November 15, there was a M5.7 earthquake near the epicenter of the Off-shore Tokachi Earthquake. According to the GPS data, the postseismic movements observed after the Off-shore Tokachi Earthquake are continuing at a very low level.
- On November 2, there was a M5.4 earthquake at a depth of approximately
20km off-shore of Miyagi prefecture.
"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.":
The seismic activity was a foreshock - main shock - aftershock sequence. It subsided within about 10 days.
There was additional activity in the Kanto-Chubu region, as follows.
- The aftershock activity of the earthquake off-shore southeast of the Kii Peninsula on September 5 (M7.4) has been declining normally. There are no particular changes observed in the active region. On November 9, there was a M5.7 earthquake off-shore of Tokaido, to the northeast of that event, however there are no changes observed on the seismicity before and after the recent event.
- There were no remarkable changes, before and after the earthquake off-shore southeast of the Kii Peninsula on September 5, in the trends of the long-term crustal movements that have been observed since 2001 in the continuous GPS data in the Tokai region.
(This evaluation is in agreement with the views presented at the procedural meeting on November 29 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 29, 2004)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.to the text
There was no remarkable seismic activity within the expected focal region. Directly under the Hamana lake, the seismicity continues at a level lower than normal. In addition, although there was a M5.7 earthquake off-shore of Tokaido on November 9, it is thought that this seismic activity does not have a direct affect on the expected Tonankai earthquake, since it was located away from the expected focal region.
Crustal movements were widely observed in the Tokai region associated with the M7.4 earthquakes off-shore of Tokaido (off-shore southeast of the Kii Peninsula) on September 5. 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, that have been observed since the beginning of 2001. "
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
There is no supplementary information for the Kyushu-Okinawa region.
- Micro-seismic activity started at a depth of approximately 10km under northern Hyogo prefecture (near the border with Tottori prefecture) on December 2. It increased with a M3.4 earthquake (maximum seismic intensity 2) on December 3, however it has been mostly subsiding since December 5. In this vicinity, in the period from December 2000 and into 2001, strong seismic activity was observed with a M5.6 earthquake (maximum seismic intensity 4) on January 12, 2001.
| 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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