February 9, 2005 |
Seismic Activity for January 2005
| * | On January 19, there was a M6.8 earthquake off-shore southeast of the Boso Peninsula (near the triple junction of the plates). It caused tsunamis with heights of less than or equal to 30cm at Miyakejima, Oshima and Hachijojima of the Izu Islands. to supplementary reports |
| * | On January 18, there was a M6.4 earthquake at a depth of approximately 50km off-shore of Kushiro. This event had a maximum seismic intensity 5 Upper. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. It is thought that this event is an aftershock of the earthquake (M7.1) that occurred off-shore of Kushiro on November 29, 2004. | |
| * | On January 31, there was a M5.4 earthquake at a depth of approximately 50km off-shore of Tokachi. 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. to supplementary reports | |
| * | On January 6, there was a M5.3 earthquake at a depth of approximately 60km off-shore east of Aomori prefecture. to supplementary reports |
| * | The aftershock activity of the Niigata Chuetsu Earthquake on October 23 (the 2004 Niigata Chuetsu Earthquake) has been continuing to decline. The largest aftershock in January was a M4.7 event on January 18 that occurred in the northern aftershock region. In addition, on January 9, there was a M4.2 aftershock to the east of the main shock. | |
| * | On January 1, there was a M5.0 earthquake at a depth of approximately 90km off-shore of Ibaraki prefecture. | |
| * | On January 2, there was a M4.2 earthquake at a depth of approximately 5km in southern Nagano prefecture. | |
| * | On January 9, there was a M4.7 earthquake at a depth of approximately 15km in western Aichi prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. | |
| * | On January 19, there was a M6.8 earthquake off-shore, southeast of the Boso Peninsula (near the triple junction of the plates). It caused tsunamis with heights of less than or equal to 30cm at Miyakejima, Oshima and Hachijojima of the Izu Islands. The focal mechanism showed a reverse fault with a compression axis in an ENE-WSW direction. The largest aftershock so far is the M5.8 event on January 21. The aftershock activity has been gradually declining since January 22. In this vicinity, there was a M6.7 earthquake on May 30, 2004 that caused tsunamis with heights of less than 10cm in the Izu islands. to supplementary reports | |
| * | There was no remarkable activity. to supplementary reports |
| * | On January 15, there was a M4.1 earthquake at a depth of approximately 10km under the Kumamoto region, Kumamoto prefecture. to supplementary reports |
| * | On February 8, there was a M4.8 (preliminary) earthquake at a depth of approximately 65km under southern Ibaraki prefecture. |
February 9, 2005
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during January 2005.
| M4.0 or greater: 100 (103 events occurred in December) | |
| M5.0 or greater: 15 (11 events occurred in December) | |
| M6.0 or greater: 2 |
In addition, of the events above, 29 earthquakes of M4.0 or greater, 7 earthquakes of M5.0 or greater, and 1 earthquake of M6.0 or greater, occurred in the region off-shore, southeast of the Boso Peninsula (near the triple junction of the plates).
(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 January 2004 to the end of December 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 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) | ||
| - 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) | |
"On January 18, there was a M6.4 earthquake at a depth of approximately 50km off-shore of Kushiro. This event had a maximum seismic intensity 5 Upper. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. It is thought that this event is an aftershock of the earthquake (M7.1) that occurred off-shore of Kushiro on November 29, 2004.":
The aftershock activity of the earthquake (M7.1) off-shore of Kushiro has
been continuing to decline. Associated with the event on January 18, seismicity
temporarily increased to the southwestern of the aftershock region, however
it subsided by January 19. According to the GPS data, postseismic movements
that were seen following the earthquake off-shore of Kushiro, have been
continuing at a low level.
"On January 31, there was a M5.4 earthquake at a depth of approximately 50km off-shore of Tokachi. 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 within the aftershock region of the 2003 Off-shore
Tokachi Earthquake (between the main shock and the largest aftershock).
There was additional activity in the Hokkaido region, as follows.
- The aftershock activity of the earthquake (M6.1) that occurred under southern Rumoi subprefecture on December 14, 2004, has been continuing to decline normally.
There is no supplementary information for the Tohoku region.
- There were no remarkable changes, before and after the earthquake off-shore, southeast of the Kii Peninsula on September 5, 2003, in the trends of the long-term crustal movements that have been observed since 2001 in the continuous GPS data for the Tokai region.
(This evaluation is in agreement with the views presented at the procedural meeting on February 2 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, February 2, 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 is 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.
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 before and after the earthquake in the trends of the crustal movements, that have been observed since the beginning of 2001,."
- The micro seismic activity that started under southwestern Hyogo prefecture on December 14, increased temporarily with a M2.9 earthquake and associated seismicity on January 13, however it mostly subsided by late January.
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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