September. 14, 2005
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during August 2005.
(reference) Average numbers of earthquake occurrences over the past thirty years (1971 - 2000)
The following regions have been noted as having significant seismic activity during the period from August 2004 to the end of July 2005.
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- Off-shore of Iwate prefecture |
M5.8 on August 10, 2004 (depth approximately 50km) |
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- Off-shore southeast of the Kii Peninsula (Off-shore of Tokaido) |
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M7.4 on September 5, 2004 |
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- Chuetsu region, Niigata prefecture (2004 Niigata Chuetsu Earthquake) |
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M6.8 on October 23, 2004 (depth approximately 10km) |
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- Off-shore of Kushiro |
M7.1 on November 29, 2004 (depth approximately 50km) |
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- Southern Rumoi subprefecture |
M6.1 on December 14, 2004 (depth approximately 10km) |
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- Off-shore southeast of the Boso Peninsula (near the triple junction of the plates) |
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M6.8 on January 19, 2005 |
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- Southern Ibaraki prefecture |
M5.4 on February 16, 2005 (depth approximately 45km) |
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- Off-shore west of Fukuoka prefecture |
M7.0 on March 20, 2005 (depth approximately 10km) |
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- Northeastern Chiba prefecture |
M6.1 on April 11, 2005 (depth approximately 50km) |
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- Amakusa Ashikita region, Kumamoto prefecture |
M4.8 on June 3, 2005 (depth approximately 10km) |
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- Chuetsu region, Niigata prefecture |
M5.0 on June 20, 2005 (depth approximately 15km) |
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- Northwestern Chiba prefecture |
M6.0 on July 23, 2005 (depth approximately 75km) |
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2 Nationwide Seismic Activity by Region
- There is no supplementary information for the Hokkaido region.
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- On August 26, there was a M5.4 earthquake off-shore of Fukushima prefecture.
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"On August 7, there was a M4.7 earthquake at a depth of approximately 75km under northwestern Chiba prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event is related to the subduction of the Pacific plate."
This earthquake is the largest aftershock so far of the event that occurred under northwestern Chiba prefecture on July 23 (M6.0). The aftershock activity has been normally declining, and the seismic activity is returning to the level as it was before the earthquake.
"The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing.":
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, had been showing a trend of moderate change since about April 2001. Since the earthquakes off-shore southeast of the Kii Peninsula on September 5, 2004, some changes appeared in the trends. However, the deformation has recently been returning to the trend before the earthquakes.
(The Assessment Committee for Areas under Intensified Measures against Earthquake Disaster held at the Japan Meteorological Agency during the 'Comprehensive Disaster Drills' on September 1, presented the view that there had been no large changes in seismic activity and crustal deformation in the Tokai region since the same procedural meeting on July 25. The evaluation is in agreement with this view.)
(reference) "Recent seismic and crustal activity in the Tokai region and its vicinity" (Japan Meteorological Agency, Department of Seismology and Volcanology, July 25, 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.
For the crustal movements, since the M7.4 earthquake off-shore of Tokaido (off-shore southeast of the Kii Peninsula) on September 5, 2004, postseismic movements associated with these events were observed. Recently the deformations have returned to the previous trend, and the long-term crustal movements from the slow slip at the plate boundary is continuing.
In addition, during the period from July 20 to 23, there were slight changes observed on some strainmeters and tiltmeters. During this time, there were low-frequency earthquakes under eastern Aichi prefecture."
There was additional activity in the Kanto-Chubu region, as follows.
- Seismic activity increased off-shore east of Hachijojima (approximately 250km east of Hachijojima) since July 26. The seismic activity has been continuing with temporary increases and decreases in the number of the earthquake occurrences before and after the M5.0 event on August 10.
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- There is no supplementary information for the Kinki - Chugoku - Shikoku
region.
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- There is no supplementary information for the Kyushu - Okinawa region.
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Supplement
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On September 4, there was a M4.3 earthquake at the depth of approximately 55km under Iyonada. |
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:
| 1) | Reference information related to seismic activity described in the "Evaluation of Seismic Activity" | | 2) | Recent earthquake activity over the past year related to the events described in "Major Seismic Activity" in the "Evaluation of Seismic Activity" | | 3) | Seismic activity that has been evaluated, but not included in the "Evaluation of Seismic Activity" because it is within normal bounds and not prominent. |
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