June 9, 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On May 8, there was a M7.0 earthquake off-shore of Ibaraki prefecture, which had a maximum seismic intensity 5 Lower and caused small tsunamis. Related to this event, some damage was observed. to supplementary reports
*On May 11, there was a M5.1 earthquake at a depth of approximately 90km near the Nemuro Peninsula [near Kunashiri Island]. The focal mechanism showed a normal fault with a tension axis in a NNW-SSE direction. This event occurred within the Pacific plate. to supplementary reports
*On May 31, there was a M5.0 earthquake at a depth of approximately 30km off-shore of Iwate prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the continental plates.
*On May 29, there was a shallow M4.8 earthquake under the southern inland area of Akita prefecture. The focal mechanism showed a strike-slip fault with a compression axis in a WNW-ESE direction. This event occurred within the crust.
*On May 2, there was a M5.1 earthquake at a depth of approximately 45km off-shore of Fukushima prefecture. The focal mechanism showed a normal fault with a tension axis in a NW-SE direction. It is thought that this event occurred within the Pacific plate. to supplementary reports
*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 WNW-ESE 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 during 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 a M5.8 event on May 9. 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 a large area of the Kanto region.
The source fault model of this M7.0 event, determined from the GPS and seismic waveform data, is a reverse fault striking in a NNE-SSW direction and dipping toward the west. From the seismic waveform data, 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, and 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 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 9, there was a M4.6 earthquake at a depth of approximately 75km under northwestern Chiba prefecture. This event occurred at the boundary between the Philippine Sea and the Pacific plates.
*In the southern source region of the Niigataken Chuetsu-oki Earthquake in 2007, a detailed tsunami analysis showed that there was approximately 20cm uplift of the ocean floor along a sedimentary anticline structure located far off-shore. The displacement there possibly extended to the fairly shallow part of the fault dipping to the southeast.
*There have been no anomalous changes observed in the GPS data for the Tokai region. to supplementary reports
There was no remarkable activity. to supplementary reports
*On May 11, there was a M4.1 earthquake at a depth of approximately 10km under the mountain area in southern Miyazaki prefecture. The focal mechanism showed a normal fault with a tension axis in a N-S direction. This event occurred within the crust. to supplementary reports
*On June 1, there was a M6.2 earthquake under the northern Philippines [near the Philippines].
[ ] were the regional names of the epicenters used in the Japan Meteorological Agency announcement.
June 9, 2008
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during May 2008.
(reference)
The following regions have been noted as having major seismic activity during the period from May 2007 to the end of April 2008.
| - 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.
There is no supplementary information for the Tohoku region.
"On May 8, there was a M7.0 earthquake off-shore of Ibaraki prefecture. (text omitted)":
In 1943, a M6.7 earthquake occurred about one month after a M6.3 event. In other cases of 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.
"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 June 3 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, June 3, 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."
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) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: