September 10, 2009
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On August 11, there was a M6.5 earthquake under Suruga Bay. This event had a maximum seismic intensity 6 Lower in Shizuoka prefecture, and caused some damage with one fatality and approximately 320 injured people. Tsunamis were observed on the Pacific coast from Suruga Bay through the Tokai region.
* On August 13, there was a M6.6 earthquake off-shore east of Hachijojima. This event had a maximum seismic intensity 5 Lower on Hachijojima.
* On August 17, there was a sequence of seismicity with M6.7 and M6.6 earthquakes under the ocean area near Ishigakijima.
There was no remarkable activity.
* On August 24, there was a M5.4 earthquake at a depth of approximately 170km, off-shore west of Aomori prefecture. The focal mechanism had a tension axis in the direction of the subducting Pacific plate. This event occurred within the Pacific plate.
* (Seismic Activity under Suruga Bay on August 11, 2009 and its aftershock activity are described in a separate section.)
* On August 9, there was a deep M6.8 earthquake at a depth of approximately 330km, off-shore south of Tokaido. The focal mechanism had a compression axis in the direction of the subducting Pacific plate. This event occurred within the Pacific plate.
* On August 13, there was a M6.6 earthquake off-shore east of Hachijojima. The focal mechanism had a compression axis in a WNW-ESE direction. It is thought that this event occurred within the Pacific plate.
* Other than the seismic activity under Suruga Bay on August 11, there have been no anomalous changes observed in the GPS data for the Tokai region.
There was no remarkable activity.
* On August 5, there was a M5.0 earthquake at a depth of approximately 35km under Hyuganada. The focal mechanism had a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate.
* On August 3, there was a M4.7 earthquake at a depth of approximately 10km in the Amakusa Ashikita region, Kumamoto prefecture. The focal mechanism showed a strike-slip fault with a tension axis in a NNW-SSE direction. This event occurred within the crust.
* On August 5, there was a M6.5 earthquake under the ocean area near Miyakojima. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. It is thought that this event occurred at the boundary between the Philippine Sea and the continental plates.
* On August 17, at 09:05 (JST) there was a M6.7 earthquake under the ocean area near Ishigakijima. Also at 19:10 on the same day, a M6.6 earthquake occurred. The focal mechanisms of these events showed strike-slip faults with tension axes in a N-S direction. After each event occurred, strong aftershock activity was seen, which has been subsiding at present.
* On September 3, there was a M6.0 earthquake at a depth of approximately 170km, off-shore west of the Satsuma Peninsula. The focal mechanism had a tension axis in the direction of the subducting Philippine Sea plate. This event occurred within the Philippine Sea plate.
* On September 4, there was a M4.5 earthquake at a depth of approximately 65km under northwestern 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 Pacific and the Philippine Sea plates.
* On September 8, there was a M4.8 earthquake at a depth of approximately 50km under eastern Hidaka subprefecture.
September 10, 2009
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during August 2009.
(reference)
The following regions have been noted as having major seismic activity during the period from August 2008 to the end of July 2009.
| – Off-shore of Tokachi | M7.1 on September 11, 2008 |
There is no supplementary information for the Hokkaido region.
There is no supplementary information for the Tohoku region.
"Other than the seismic activity under Suruga Bay on August 11, 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 August 31 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" (Seismological and Volcanological Department, Japan Meteorological Agency, August 31, 2009)
"On August 11, there was a M6.5 earthquake under Suruga Bay. The aftershock activity has been declining. There were no remarkable changes in seismic activity and crustal movements in the Tokai region. So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
1. Status of seismicity
On August 11, there was a M6.5 earthquake at a depth of 23km under Suruga Bay (hereafter the earthquake under Suruga Bay). The focal mechanism of the earthquake under Suruga Bay showed a reverse fault with a strike-slip component and a compression axis in a NNE-SSW direction, which is different from that of the so-called Tokai Earthquake. This event occurred within the Philippine Sea plate. The aftershock activity has been gradually decreasing.
There continue to be fewer events within the Philippine Sea plate under the region around Hamana lake. On the other hand, seismicity has been continuing to be slightly strong within the crust in mid-western Shizuoka prefecture. In other regions, seismic activity is generally at a normal level.
2. Status of crustal movements
Due to the occurrence of the earthquake under Suruga Bay on August 11, some step-like changes, which are commonly seen from such earthquakes, were observed on strainmeters in the Tokai region. Slow changes in strain subsequently continued. Those changes are not considered to be from slip that should take place on the expected fault plane. The slow changes gradually became smaller and returned to normal levels by 11:00 on August 11. According to the GPS data and leveling results, there were no remarkable changes observed, except for small horizontal and vertical crustal movements around Suruga Bay associated with the recent event. As for the trend of long-term subsidence at Omaezaki, it has been continuing as before."
– On August 2, there were M4.9 and 4.8 earthquakes off-shore of Kaetsu, Niigata prefecture.
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: