March 9, 2005 |
Seismic Activity for February 2005
| * | On February 16, there was a M5.4 earthquake at a depth of approximately 45km under southern Ibaraki prefecture. This event had a maximum seismic intensity 5 Lower, and caused damages in Ibaraki prefecture. to supplementary reports |
| * | There was no remarkable activity. | |
| * | On February 26, there was a M5.7 earthquake at a depth of approximately 45km off-shore east of Aomori prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred within the Pacific plate. The aftershock activity was at a low level, and subsided within several days. to supplementary reports |
| * | On February 8, there was a M4.8 earthquake at a depth of approximately 65km under southern Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred at the boundary between the Philippine Sea and the Pacific plates. | |
| * | On February 16, there was a M5.4 earthquake at a depth of approximately 45km under southern Ibaraki prefecture. This event had a maximum seismic intensity 5 Lower in Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred at the boundary between the Philippine Sea and continental plates. Associated with this event, seismicity increased with some events which seemed to be aftershocks. Later, the activity has been returning to normal levels. In addition, on February 23, to the northwest of this event, there was a M4.4 earthquake at a depth of approximately 50km. From its focal mechanism, it is thought that this event occurred within the Philippine Sea plate. | |
| * | In the Chuetsu Region, Niigata Prefecture, there was a M4.1 earthquake on February 20, and a M4.0 earthquake on February 26. These events are aftershocks of the 2004 Niigata Chuetsu Earthquake. | |
| * | On February 14, there was a M4.1 earthquake at a depth of approximately 15km under southeastern Hyogo prefecture. to supplementary reports |
| * | There was no remarkable activity. to supplementary reports |
| * | On February 10, there was a M6.5 earthquake under the ocean near Chichijima. |
| * | On March 5, there was a M4.4 earthquake at a depth of approximately 50km in the southern plain region of Miyazaki prefecture. |
| * | On March 6, there was a 6.2 earthquake near Taiwan. |
March 9, 2005
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during February 2005.
| M4.0 or greater: 70 (100 events occurred in January) | |
| M5.0 or greater: 12 (15 events occurred in January) | |
| M6.0 or greater: 1 |
(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 February 2004 to the end of January 2005.
| - 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) | |
| - Off-shore southeast of the Boso Peninsula (near the triple junction of the plates) | ||
| M6.8 on January 19, 2005 | ||
- The aftershock activity of the earthquake (M7.1) that occurred off-shore of Kushiro on November 29, 2003 has been continuing to decline. According to the GPS data, postseismic movements that were seen following the earthquake off-shore of Kushiro have been continuing at a low level.
There is no supplementary information for the Tohoku region.
"In the Chuetsu Region, Niigata Prefecture, there was a M4.1 earthquake on February 20, and a M4.0 earthquake on February 26. These events are aftershocks of the 2004 Niigata Chuetsu Earthquake.":
The aftershock activity of the 2004 Niigata Chuetsu Earthquake has been continuing to decline.
There was additional activity in the Kanto - Chubu region, as follows.
- The aftershock activity of the earthquake (M6.8) that occurred off-shore southeast of the Boso Peninsula (near the triple junction of the plates) on January 19, has been continuing to decline, though seismicity slightly increased in late February. The largest aftershock in February was a M5.1 event that occurred near the northern edge of the aftershock region on February 25.
- Since the earthquakes off-shore, southeast of the Kii Peninsula on September 5, 2004, there were some changes in the trends of the long-term crustal movements that have been observed since 2001 in the continuous GPS data for the Tokai region. It is thought that this was possibly caused by some postseismic movements of these off-shore earthquakes.
(This evaluation is in agreement with the views presented at the procedural meeting on February 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, February 28, 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, 2004. Some other changes appeared in the movements of E-W components in the data, apart from the step-like changes associated with the earthquakes. It is thought that this was possibly caused by some postseismic movements of the earthquakes off-shore southeast of the Kii Peninsula on September 5."
"On February 14, there was a M4.1 earthquake at a depth of approximately 15km under southeastern Hyogo prefecture.":
The event occurred within the aftershock region of the 1995 Southern Hyogo Prefecture Earthquake.
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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