May 12, 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* There was no remarkable activity. to supplementary reports
There was no remarkable activity. to supplementary reports
*On April 29, there was a M5.7 earthquake at a depth of approximately 60km off-shore east of Aomori 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 17, there was a M5.8 earthquake at a depth of approximately 170km under the border between Akita and Yamagata prefectures [in the Mogami region, Yamagata prefecture]. The focal mechanism had a tension axis in the direction of the subducting Pacific plate. This event occurred within the Pacific plate. to supplementary reports
*On April 4, there was a M5.0 earthquake at a depth of approximately 55km under southern Ibaraki prefecture. The focal mechanism had a tension axis in an E-W direction. It is thought that this event occurred within the Philippine Sea plate.
*On April 25, there was a M4.8 earthquake at a depth of approximately 50km off-shore east of Chiba 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 Philippine Sea and the Pacific plates.
*There have been no anomalous changes observed in the GPS data for the Tokai region. to supplementary reports
*On April 17, there was a M4.1 earthquake at a depth of approximately 10km under Akashi Strait [under Osaka Bay]. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at shallow depth within the crust. This event is an aftershock of the 1995 Southern Hyogo Prefecture Earthquake. to supplementary reports
*On April 28, there was a M5.2 earthquake under the ocean area near Miyakojima. The focal mechanism showed a normal fault with a tension axis in a NE-SW direction. It is thought that this event occurred within the crust. to supplementary reports
*On April 24, there was a M6.3 earthquake near Taiwan.
[ ] were the regional names of the epicenters used in the Japan Meteorological Agency announcement.
*On May 1, there was a M4.6 earthquake at a depth of approximately 35km off-shore east of Chiba prefecture. The focal mechanism showed a strike-slip fault with a tension axis in a NW-SE direction. This event occurred within the Philippine Sea plate.
*On May 2, there was a M5.1 earthquake at a depth of approximately 45km off-shore of Fukushima prefecture.
*On May 8, there was a M7.0 earthquake off-shore of 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 Pacific and the continental plates. Small tsunamis associated with this event were observed on the coast of Miyagi and Fukushima prefectures.
M4–5 class seismicity started in the evening on May 7. Prior to the M7.0 event, there was a M6.4 earthquake approximately 40 minutes before the mainshock, and a M6.3 event approximately 30 minutes before the mainshock. Immediately after the M7.0 event, the aftershock activity temporarily increased, including some M5 class events. At present, the aftershock activity has generally been declining although some M4 class aftershocks sometimes occur.
According to the GPS data, small crustal movements associated with this event were observed in the large area of the Kanto region.
From the analysis of the focal process, the ruptured area of the recent M7.0 event corresponds to that of the M7.0 earthquake in 1982.
There is frequent seismic activity in this region. Since 1940, there have been four M6.7 to M7.0 interplate earthquakes in 1943, 1961, 1965, 1982. And the recent event occurred at nearly the same location. Judging from the source region, the focal mechanism and the magnitude of the earthquake, the recent event is thought to be the interplate earthquake off-shore of Ibaraki prefecture (estimated to be approximately M6.8) that the Earthquake Research Committee had expected to occur.
In addition, the long-term evaluation announced by the Earthquake Research Committee on July 31, 2002, stated that earthquakes occur at an average frequency of once every 15.5 years, and that there was a 50 percent probability of occurrence of an approximately M6.8 earthquake within ten years, a 70 percent probability within 20 years, and a 90 percent probability within 30 years (Poisson process).
*On May 9, there was a M4.6 earthquake at a depth of approximately 75km under northwestern Chiba prefecture.
May 12, 2008
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during April 2008.
(reference)
The following regions have been noted as having major seismic activity during the period from April 2007 to the end of March 2008.
| - Central Mie prefecture | M5.4 on April 15, 2007 (depth approximately 15km) | |
| - Off-shore northwest of Miyakojima | seismic activity with M6.3, M6.7 and M6.1 on April 20, 2007 | |
| - Niigataken Chuetsu-oki Earthquake in 2007 | M6.8 on July 16, 2007 (depth approximately 10km) | |
| - Off-shore west of Sakhalin | M6.4 on August 2, 2007 | |
| - Near the coastal area of Kujukurihama | seismic activity with M5.3 on August 16, and M4.8 on August 18, 2007 | |
| - Coast of Peru | M8.0 on August 16, 2007 | |
| - Western Kanagawa prefecture | M4.9 on October 1, 2007 (depth approximately 15km) | |
| - Noto region, Ishikawa prefecture | M4.8 on January 26, 2008 (depth approximately 10km) |
There is no supplementary information for the Hokkaido region
- Seismic activity of earthquakes equal to or larger than M5.0 on the Pacific side of the Tohoku region was declining to a level lower than usual since the beginning of last year. However, in the southern part of the region, seismicity has returned to the normal level, while it still has been continuing at a lower level than usual in the northern part.
"On April 25, there was a M4.8 earthquake at a depth of approximately 50km off-shore east of Chiba 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 Philippine Sea and the Pacific plates.":
In this vicinity, there was a M6.1 earthquake (maximum seismic intensity 5 Upper) at nearly the same location on April 11, 2005.
"There have been no anomalous changes observed in the GPS data for the Tokai region.":
(This evaluation is in agreement with the views presented at the procedural meeting on April 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, April 28, 2008)
"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. Under central Shizuoka prefecture, the seismicity remains at a level lower than normal within the plate, and higher within the crust. In other regions, seismic activity is generally at a normal level.
There were no notable changes observed in crustal movements in the Tokai region and its neighboring areas."
- On April 20, there was a M4.3 earthquake at a depth of approximately 35km under western Aichi prefecture. The focal mechanism showed a strike-slip fault with a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate.
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
There is no supplementary information for the Kyushu-Okinawa region.
"On May 8, there was a M7.0 earthquake off-shore of 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 Pacific and the continental plates. (text omitted)":
In 1943, a M6.7 earthquake occurred about one month after a M6.3 event. In other cases for a M6.8 event in 1961, a M6.7 event in 1965 and a M7.0 in 1982, their aftershock activity subsided within a few to ten days.
Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: