May 11, 2005 |
Seismic Activity for April 2005
| * | On April 11, there was a M6.1 earthquake at a depth of approximately 50km under northeastern Chiba prefecture, with observed maximum seismic intensity 5 Upper. to supplementary reports |
| * | There was no remarkable activity. | |
| * | 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 mostly subsided by April 11. to supplementary reports |
| * | 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 subsiding. The largest aftershock so far is a M3.7 event on April 25. | |
| * | On April 11, there was a M4.4 earthquake at a depth of approximately 75km under northwestern Chiba prefecture. | |
| * | On April 17, there was a M4.4 earthquake at a depth of approximately 70km under southern Chiba prefecture. | |
| * | On April 23, there was a M4.1 earthquake at a depth of approximately 5km in northern Nagano prefecture. | |
| * | On April 19, there was a deep M6.0 earthquake at a depth of approximately 440km under the ocean near Torishima. The focal mechanism showed a compression axis that nearly corresponds to the direction of the subducting Pacific plate. This event occurred within the Pacific plate. | |
| * | The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing. | |
| * | There was no remarkable activity. to supplementary reports |
| * | Seismic activity off-shore west of Fukuoka prefecture - On April 20, there was a M5.8 earthquake at a depth of approximately 15km near Shikanoshima, which had a maximum seismic intensity 5 Upper. This event was the largest aftershock of the earthquake that occurred off-shore west of Fukuoka prefecture on March 20 (M7.0), and occurred near the southeastern edge of the aftershock region from Genkainada to Shikanoshima. The focal mechanism showed a strike-slip fault with a compression axis in an E-W direction. The secondary aftershocks associated with this event were distributed over a length of approximately 5km to the southeast of Shikanoshima. Therefore it is thought that the source is a left-lateral strike-slip fault with a nearly vertical surface in a NW-SE direction. In addition, the seismic activity temporarily increased due to this event, however the seismicity as a whole is a main shock - aftershock sequence, and the aftershock activity has been declining. - According to the GPS data, slight crustal movements associated with this largest aftershock on April 20, were observed at the Umino Nakamichi observation station (Higashi-ku, Fukuoka city, Fukuoka prefecture). - There were no remarkable changes seen in the neighboring area of the recent seismic activity. to supplementary reports |
| * | On May 2, there was a M5.0 earthquake at a depth of approximately 10km off-shore west of Fukuoka prefecture. This is an aftershock of the earthquake off-shore west of Fukuoka prefecture that occurred on March 20 (M7.0). |
| * | On May 7, there was a M4.2 earthquake at a depth of approximately 30km in eastern Tama, Tokyo. |
| * | On May 8, there was a M4.5 earthquake at a depth of approximately 10km in southern Tochigi prefecture. The seismic activity was a main shock - aftershock sequence. Immediately following the main shock, there was relatively strong seismicity, however it has been subsiding. |
May 11, 2005
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during April 2005.
| M4.0 or greater: 76 (98 events occurred in March) | |
| M5.0 or greater: 10 (9 events occurred in March) | |
| 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 April 2004 to the end of March 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) | |
| - Off-shore west of Fukuoka prefecture | M7.0 on March 20, 2005(depth approximately 10km) | |
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, 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 earthquake.
(This evaluation is in agreement with the views presented at the procedural meeting on April 25 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, April 25, 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.
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 been returning to the previous trend, with the long-term crustal movements from the slow slip at the plate boundary is thought to be continuing. "
There is no supplementary information for the Kinki - Chugoku - Shikoku region.
- The shallow seismic activity that increased temporarily off-shore west of Kagoshima prefecture in December, 2004 (the largest event was a M5.3 earthquake on December 14), had been showing a general subsiding trend with some decreases and increases. However, the number of earthquakes increased during the period from April 18 to late April. The largest earthquake in April was a M4.1 that occurred on April 22.
| 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:
|