Jan. 11, 2006 |
Seismic Activity for December 2005
| * | There was no remarkable activity. |
| * | 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. The aftershock activity is mostly subsiding. to supplementary reports | |
| * | On December 2 and 17, a M6.6 earthquake and a M6.1 earthquake occurred at depths of approximately 40km off-shore of Miyagi prefecture. Both focal mechanisms showed a reverse fault with a compression axis in a WNW-ESE direction. These events occurred at the boundary between the Pacific and the continental plates. The earthquake (M6.6) on December 2 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. According to the GPS data, small postseismic movements associated with that event were observed at the observation stations around the Oshika Peninsula. The event (M6.1) on December 17 occurred near the northern edge of the aftershock region of the earthquake on August 16. In addition, 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. |
| * | On December 5, there was a M5.5 earthquake at a depth of approximately 25km off-shore of Miyagi prefecture (approximately 30km southeast of the event on December 2). 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. In this vicinity, there was a M6.8 earthquake on October 31, 2003. |
| * | The aftershock activity of the earthquake (M7.1) that occurred off-shore of Sanriku on November 15, has been normally declining. The largest aftershocks so far were M4.8 events that occurred on November 25 and December 8. to supplementary reports |
| * | On December 2, there was a M4.2 earthquake at a depth of approximately 50km under southern Ibaraki prefecture. Also, there was a M4.8 earthquake at a depth of approximately 55km on December 28. Both focal mechanisms showed reverse faults with compression axes in a NW-SE direction. These events occurred at the boundary between the Philippine Sea and the continental plates. | |
| * | On December 24, there was a M4.8 earthquake at a depth of approximately 45km under western Aichi prefecture. The focal mechanism had a tension axis in an E-W direction. This earthquake occurred within the Philippine Sea plate. | |
| * | 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 | |
| * | There was no remarkable activity. to supplementary reports |
| * | 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 normally declining since December 5. The largest aftershock so far was a M5.3 on December 4 that occurred near the main shock. to supplementary reports |
| * | On January 1, 2006, there was a M4.1 earthquake at a depth of approximately 10km in the Reihoku region, Fukui prefecture. |
| * | On January 11, 2006, there was a M4.4 (preliminary) earthquake at a depth of approximately 120km under central Nemuro subprefecture. |
January 11, 2006
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during December 2005.
| M4.0 or greater: 98 (109 events occurred in November) | |
| M5.0 or greater: 15 (11 events occurred in November) | |
| M6.0 or greater: 3 (There has been a total of 23 events for 2005 by the end of December.) |
(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 December 2004 to the end of November 2005.
| - 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) | |
| - Off-shore of Sanriku | M7.1 on November 15, 2005 | |
- 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 December 26 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, December 26, 2005)
" 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 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."
- There is no supplementary information for the Kinki - Chugoku - Shikoku region.
- 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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