Seismic Activity Off-shore West of Fukuoka Prefecture

April 13, 2005
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Seismic Activity Off-shore West of Fukuoka Prefecture

* On March 20, at 10:53 (JST) there was a M7.0 earthquake at a depth of approximately 10km off-shore west of Fukuoka prefecture (maximum seismic intensity 6 Lower). (see the 140th Evaluation of the Earthquake Research Committee "Earthquake Off-shore West of Fukuoka Prefecture on March 20, 2005") The seismic activity was a main shock - aftershock sequence. The aftershock activity has been declining. These earthquakes were linearly distributed in a NW-SE direction over a length of approximately 30km from Genkainada to Shikanoshima. In addition, the shallow seismic activity that started near Hakata Bay a day after the main shock, was mostly distributed in a NNW-SSE direction over a length of approximately 10km. The largest aftershock up to 12:00 on April 13, was a M5.4 event at 15:55 on March 22 (maximum seismic intensity 4), which occurred near the main shock.

* According to the GPS data, crustal movements associated with this event were observed centered on Fukuoka prefecture. There was a displacement of approximately 18cm to the southwest at the Fukuoka observation station (Higashi-ku, Fukuoka city, Fukuoka prefecture), and a displacement of approximately 9cm to the south at the Maebaru observation station (Maebaru city, Fukuoka prefecture). Also, small postseismic movements were observed following the main shock at the Fukuoka observation station.

* Around the region of activity, no remarkable changes of seismicity can be seen. Also, there was no significant activity observed near the Kego fault.

* From the analysis of the focal process of the main shock, it is estimated that the rupture initiated on the deeper part of the fault plane, and an area of large slip was located in the shallower region southeast of the main shock hypocenter.

* Aftershocks equal to or larger than M3.0 (perceptible to humans within the small areas near the events) currently occur on average of once a day, and it is estimated that it will be once every two days in late April.

*	On March 20, at 10:53 (JST) there was a M7.0 earthquake at a depth of approximately 10km off-shore west of Fukuoka prefecture (maximum seismic intensity 6 Lower). (see the 140th Evaluation of the Earthquake Research Committee "Earthquake Off-shore West of Fukuoka Prefecture on March 20, 2005") The seismic activity was a main shock - aftershock sequence. The aftershock activity has been declining. These earthquakes were linearly distributed in a NW-SE direction over a length of approximately 30km from Genkainada to Shikanoshima. In addition, the shallow seismic activity that started near Hakata Bay a day after the main shock, was mostly distributed in a NNW-SSE direction over a length of approximately 10km. The largest aftershock up to 12:00 on April 13, was a M5.4 event at 15:55 on March 22 (maximum seismic intensity 4), which occurred near the main shock.
*	According to the GPS data, crustal movements associated with this event were observed centered on Fukuoka prefecture. There was a displacement of approximately 18cm to the southwest at the Fukuoka observation station (Higashi-ku, Fukuoka city, Fukuoka prefecture), and a displacement of approximately 9cm to the south at the Maebaru observation station (Maebaru city, Fukuoka prefecture). Also, small postseismic movements were observed following the main shock at the Fukuoka observation station.
*	Around the region of activity, no remarkable changes of seismicity can be seen. Also, there was no significant activity observed near the Kego fault.
*	From the analysis of the focal process of the main shock, it is estimated that the rupture initiated on the deeper part of the fault plane, and an area of large slip was located in the shallower region southeast of the main shock hypocenter.
*	Aftershocks equal to or larger than M3.0 (perceptible to humans within the small areas near the events) currently occur on average of once a day, and it is estimated that it will be once every two days in late April.