March 21, 2005 |
Earthquake Off-shore West of Fukuoka Prefecture on March 20, 2005
On March 20, at 10:53 (JST) there was a M7.0 (preliminary) earthquake at a depth of approximately 10km off-shore west of Fukuoka prefecture. Maximum seismic intensity 6 Lower was observed in Fukuoka and Saga prefectures. Judging from the sequence of occurrence, it is thought that the seismic activity was a main shock - aftershock sequence, with the M7.0 earthquake as the main shock. These earthquakes were distributed in a NW-SE direction over a length of approximately 25km. The focal mechanism of the main shock shows a strike-slip fault with a compression axis in an E-W direction. The source fault, which was estimated from the aftershock distribution and the focal mechanism of the main shock, is a left-lateral strike-slip fault that has a nearly vertical fault surface in a NW-SE direction. The largest aftershock until 12:00 on March 21, was the M4.7 (preliminary) earthquake (maximum seismic intensity 2) that occurred at 19:52 on March 20 near the northwestern edge of the aftershock region.
According to the GPS data, crustal movements associated with this event were observed centered on Fukuoka prefecture. There was a displacement of approximately 17cm to the southwest at the Fukuoka observation station (Higashi-ku, Fukuoka city, Fukuoka prefecture), and a displacement of approximately 8cm to the south at the Maebaru observation station (Maebaru city, Fukuoka prefecture). These are consistent with the focal mechanism of the main shock.
Around the region of activity, the earthquake (M7) that occurred near Iki and Tsushima in 1700, was the only known event equal to or larger than M7. For other past activity, there were M6.0 and M5.8 earthquakes in Itoshima in 1898, and there was a M5.1 event in 1929 and a M5.0 in 1930 in western Fukuoka prefecture. Since then, there had been no earthquakes over M5.
To the northeast of the aftershock region, there are two active faults extending several kilometers in the same direction as the aftershock distribution. Also in northern Fukuoka prefecture, there are many active faults located in a NW-SE direction, of which the Kego fault, running from Fukuoka city through Chikushino city, is located near the southeast extension of the aftershock region.
It is estimated that there is a 10 percent probability of occurrence of M5.5 or greater aftershocks (seismic intensity 5 Lower - 5 Upper in some areas) within the 3 days from 16:00 on March 21.