| 14 May 2004 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity for April 2004
There was no remarkable activity. to supplementary reports
*On April 12, there was a M5.8 earthquake at a depth of approximately 50km off-shore of Kushiro. 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 2003 Off-shore Tokachi Earthquake. This event occurred near the northeastern edge of the aftershock area. to supplementary reports
There was no remarkable activity. to supplementary reports
*On April 4, there was a M5.8 earthquake at a depth of approximately 50km off-shore of Ibaraki prefecture. 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.
*On April 28, there was a M4.0 earthquake at a depth of 10km in northern Tochigi prefecture.
*The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing. to supplementary reports
*On April 6, there was a M4.0 earthquake at a depth of approximately 5km in southern Tokushima prefecture.
*On April 20, there was a M4.6 earthquake at a depth of approximately 50km under Iyonada. The focal mechanism had a tension axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate. to supplementary reports
*On April 21, there was a M5.0 earthquake at a depth of approximately 25km under Hyuganada. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred at the boundary of the Philippine Sea and the continental plates. to supplementary reports
*On May 6, there was a M5.7 earthquake at a depth of approximately 40km off-shore of Kushiro.
| May 14, 2004 |
| Earthquake Research Committee |
Following are the numbers of the earthquakes by magnitude that occurred in Japan and the surrounding area during April 2004.
| M4.0 or greater: 82 (73 events occurred in March.) | |
| M5.0 or greater: 8 (7 events occurred in March.) | |
| M6.0 or greater: 0 |
(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 2003 to the end of February 2004.
| -Off-shore of Miyagi prefecture | M7.1 on May. 26, 2003 (depth of approximately 70km) | |
| -Northern Miyagi prefecture | M6.4 on July 26, 2003 (depth of approximately 10km) | |
| -Off-shore of Tokachi (2003 Off-shore Tokachi Earthquake) | M8.0 on Sep. 26, 2003 (depth of approximately 40km) | |
| -Off-shore of Fukushima prefecture | M6.8 on Oct. 31, 2003 (depth of approximately 30km) |
"On April 12, there was a M5.8 earthquake at a depth of approximately 50km off-shore of Kushiro. 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 2003 Off-shore Tokachi Earthquake. This event occurred near the northeastern edge of the aftershock area.":
The aftershock activity of the off-shore Tokachi earthquake on September 26, 2003 (2003 Off-shore Tokachi Earthquake) is continuing to decline. Following the earthquake on April 12, seismic activity temporarily increased near the northeastern edge of the aftershock area, however, it had mostly subsided by April 12. According to the GPS data, postseismic movements that were seen following the main shock have been continuing at a low level.
There was additional activity in the Hokkaido region, as follows.
- Since April 27, there has been light seismic activity near the boundary of Abashiri and Nemuro (Abashiri region, Abashiri subprefecture) that has been continuing into May. The largest event as of May 14 was a M3.4 event that occurred on May 5.
- On April 23, there was a M4.9 earthquake at a depth of approximately 70km off-shore east of Aomori prefecture.
"The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing.":
The Pacific side of the Tokai-Chubu region maintains a steady rate with movement to the WNW, consistent with the subduction of the Philippine Sea plate toward the northwest. The results from GPS observations in the area centered on western Shizuoka prefecture, have been showing a trend of moderate change since about April 2001. The trends have continued through April 2004 and show no signs of acceleration.
(This evaluation is in agreement with the views presented at the procedural meeting on April 26 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 26, 2004)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
There is no remarkable seismic activity in general. The seismicity continues at a level lower than normal directly under Hamana lake. In the other regions, seismic activity is generally at a normal level.
The long-term changes in crustal movements in the Tokai region and its vicinity caused by the slow slip on the plate boundary, have been continuing."
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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