Dec. 14, 2005 |
Seismic Activity for November 2005
| * | On November 15, there was a M7.1 earthquake off-shore of Sanriku. Tsunamis were observed on the Pacific coast in the Tohoku region, with a tsunami height of 42cm in Ofunato city, Iwate prefecture. |
| * | There was no remarkable activity. to supplementary reports | |
| * | On November 1, there was a M4.1 earthquake at a depth of approximately 70km under the northern coast of Iwate prefecture. On November 13, there was a M4.3 earthquake at a depth of approximately 55km to the southeast of that event. |
| * | On November 1, there was a M4.6 earthquake at a depth of approximately 10km onshore in southern Iwate prefecture (near the boundary between Iwate, Miyagi and Akita prefectures). The aftershock activity subsided within two days. |
| * | On November 15, there was a M7.1 earthquake off-shore of Sanriku. Tsunamis were observed on the Pacific coast in the Tohoku region, with a tsunami height of 42cm in Ofunato city, Iwate prefecture. The focal mechanism showed a normal fault with a tension axis in a nearly E-W direction. It is thought that this event occurred in a shallow region within the Pacific plate. Seismic activity was a main shock - aftershock sequence. The aftershock activity was strong within a few days after the main shock, however it has subsequently been gradually declining. The aftershocks were distributed on the eastern side of the Japan trench off-shore of Sanriku. The largest aftershock so far was a M4.8 event on November 25. According to the GPS data, there were no remarkable changes before and after this event. To the north of the recent seismic activity, the Sanriku Earthquake (M8.1) occurred in 1933. Apart from that event, since August 1923, there have been no earthquakes equal to or larger than M7.0 on the eastern side of the Japan trench off-shore of Sanriku. to supplementary reports |
| * | The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, appear to be slightly diminishing recently. to supplementary reports | |
| * | On November 23, there was a M4.0 earthquake at a depth of approximately 30km under northern Wakayama prefecture. to supplementary reports |
| * | On November 22, there was a M6.0 earthquake at a depth of approximately 150km under the ocean near Tanegashima (off-shore south of the Satsuma Peninsula). The focal mechanism had a tension axis in the direction of the subducting plate. This event occurred within the Philippine Sea plate. In this vicinity, there is steady seismic activity observed at a depth of around 150km, related to the subduction of the Philippine Sea plate, such as the M6.4 earthquake on May 23, 1978. to supplementary reports |
| * | On December 2, there was a M6.6 earthquake at a depth of approximately 40km off-shore of 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. This event occurred approximately 10km southeast of the earthquake (M7.2) that occurred off-shore of Miyagi prefecture on August 16, and is thought to be the largest aftershock so far. The number of aftershock occurrences of the off-shore Miyagi prefecture earthquake increased temporarily immediately after the M6.6 event, however there were no remarkable changes in the aftershock activity. In addition, on December 5, there was a M5.5 earthquake approximately 30km southeast of that event. In this vicinity, there was a M6.8 earthquake on October 31, 2003. |
| * | On December 2, there was a M4.2 earthquake at a depth of approximately 50km under southern Ibaraki prefecture. |
| * | On December 4, there was a M6.1 earthquake under the ocean near Amami Oshima. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. The aftershock activity on that day was strong, however, it has been subsiding since December 5. The largest aftershock so far was a M5.3 on December 4 that occurred near the main shock. |
| * | On December 13, there was a M5.5 earthquake off-shore west of Hokkaido. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred near the northern edge of the aftershock region of the 1993 Off-shore Southwest Hokkaido Earthquake (M7.8). |
December 14, 2005
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during November 2005.
| M4.0 or greater: 109 (85 events occurred in October) | |
| M5.0 or greater: 11 (13 events occurred in October) | |
| M6.0 or greater: 3 (There has been a total of 20 events for 2005 by the end of November.) |
In addition, of all the events above, 28 earthquakes of M4.0 or greater, 1 earthquake of M5.0 or greater, and 1 earthquake of M6.0 or greater were associated with the earthquake off-shore of Sanriku on November 15 (M7.1) and its aftershocks.
(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 2004 to the end of October 2005.
| - Off-shore of Kushiro | M7.1 on November 29, 2004 (depth approximately 50km) | |
| - Southern Rumoi subprefecture | M6.1 on December 14, 2004 (depth approximately 10km) | |
| - Off-shore southeast of the Boso Peninsula (near the triple junction of the plates) | ||
| M6.8 on January 19, 2005 | ||
| - Southern Ibaraki prefecture | M5.4 on February 16, 2005 (depth approximately 45km) | |
| - Off-shore west of Fukuoka prefecture | ||
| M7.0 on March 20, 2005 (depth approximately 10km) | ||
| - Northeastern Chiba prefecture | M6.1 on April 11, 2005 (depth approximately 50km) | |
| - Amakusa Ashikita region, Kumamoto prefecture | ||
| M4.8 on June 3, 2005 (depth approximately 10km) | ||
| - Chuetsu region, Niigata prefecture | M5.0 on June 20, 2005 (depth approximately 15km) | |
| - Northwestern Chiba prefecture | M6.0 on July 23, 2005 (depth approximately 75km) | |
| - Off-shore of Miyagi prefecture | M7.2 on August 16, 2005 (depth approximately 40km) | |
| - Chuetsu region, Niigata prefecture | M5.0 on August 21, 2005 (depth approximately 15km) | |
| - Off-shore of Ibaraki prefecture | M6.3 on October 19, 2005 (depth approximately 50km) | |
- There is no supplementary information for the Tohoku region.
"The long-term changes that have been observed since 2001 in the continuous
GPS data in the Tokai region, appear to be slightly diminishing recently.":
According to the GPS data, the Pacific side of the Tokai-Chubu region maintained a steady rate with movement to the WNW, consistent with the subduction of the Philippine Sea plate toward the northwest. The results, from observations in the area centered on western Shizuoka prefecture, have been showing a trend of moderate change since about April 2001. There seems to be a slight decrease in the change recently.
(This evaluation is in agreement with the views presented at the procedural meeting on November 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, November 28, 2005)
" So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
There was no remarkable seismic activity in general. The seismicity continues at a level lower than normal directly under Hamana lake. In other regions, seismic activity is generally at a normal level.
The long-term crustal movements from the slow slip at the plate boundary in the Tokai region and its neighboring area, seem to have slightly decreased recently."
- On November 1, there was a M4.3 earthquake at a depth of approximately 45km under Kii Strait
- 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:
|