April 13, 2005 |
Seismic Activity for March 2005
| * | On March 20, there was a M7.0 earthquake at a depth of approximately 10km off-shore west of Fukuoka prefecture. Maximum seismic intensity 6 Lower was observed in Fukuoka and Saga prefectures. This earthquake caused damage with one fatality and over 700 injured people. to supplementary reports |
| * | On March 12, there was a M5.1 earthquake at a depth of approximately 60km under mid-southern Kushiro 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 March 18, there was a shallow M5.0 earthquake off-shore northwest of Hokkaido. The aftershock activity mostly subsided within March. | |
| * | There was no remarkable activity. to supplementary reports |
| * | On March 12, there was a M4.0 earthquake at a depth of approximately 30km near the coastal area of Kujukurihama. | |
| * | There was no remarkable activity. to supplementary reports |
| * | On March 5, there was a M4.4 earthquake at a depth of approximately 50km under the southern plains region of Miyazaki prefecture. to supplementary reports |
| * | (The earthquakes and aftershock activity that occurred off-shore west of Fukuoka prefecture on March 20, are described in a separate section.) |
| * | On March 6, there was a M6.2 earthquake near Taiwan. |
| * | On April 3, there was a M4.5 earthquake at a depth of approximately 5km in the Aizu region, Fukushima prefecture. Immediately following the main shock, there was relatively strong seismicity, however it is presently subsiding. |
| * | On April 4, there was a M5.3 earthquake at a depth of approximately 45km off-shore of Fukushima prefecture. |
| * | On April 11, there was a M6.1 earthquake at a depth of approximately 50km under northeastern Chiba prefecture. Maximum seismic intensity 5 Upper was observed in Ibaraki and Chiba prefectures. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. It is thought that this event occurred at the boundary between the Philippine Sea and the Pacific plates. This seismic activity was a main shock - aftershock sequence, and the aftershock activity has been gradually declining. In this vicinity, steady seismic activity is observed. For the seismic activity since August 1923, there were eight earthquakes equal to or larger than M6.0. The last event was a M6.1 (maximum seismic intensity 5 Lower) on June 3, 2000. |
| * | On April 11, there was a M4.4 earthquake at a depth of approximately 75km under northwestern Chiba prefecture. |
April 13, 2005
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during March 2005.
| M4.0 or greater: 98 (70 events occurred in February) | |
| M5.0 or greater: 9 (12 events occurred in February) | |
| M6.0 or greater: 2 |
(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 March 2004 to the end of February 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 | ||
| - Southern Ibaraki prefecture | M5.4 on February 16, 2005(depth approximately 45km) | |
"On March 12, there was a M5.1 earthquake at a depth of approximately 60km under mid-southern Kushiro 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.":
This event is an aftershock of the earthquake off-shore of Kushiro on November 29, 2004 (M7.1), which occurred near the northwestern edge of the aftershock region. The aftershock activity of the earthquake off-shore of Kushiro 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.
- On March 24, there was a M4.1 earthquake at a depth of approximately 10km in the Hida region, Gifu prefecture.
- The aftershock activity of the earthquake that occurred off-shore southeast of the Boso Peninsula (near the triple junction of the plates) on January 19 (M6.8), mostly subsided in mid-March. On the other hand, in the area off-shore east of the Kanto region that is adjacent to the east of the aftershock region, seismic activity was observed with the largest events of M4 class, following the earthquake off-shore southeast of the Boso Peninsula. The seismicity increased from mid-March through early April (the largest event so far was a M4.6.)
- 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 March 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, March 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 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.
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 these were possibly caused by some postseismic movements of the earthquakes off-shore southeast of the Kii Peninsula on September 5."
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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