Introduction to the special section for the 2005 West Off Fukuoka Prefecture Earthquake

Abstract

On 20 March 2005, the 2005 West Off Fukuoka Prefecture earthquake with Japan Meteorological Agency magnitude (MJMA) of 7.0 occurred in the offshore region of Fukuoka Prefecture, in northern Kyushu, Japan. This earthquake caused more than 1,000 casualties and damaged many buildings in the surrounding area, especially on Genkai Island as well as in and around Fukuoka City. Because the 2005 West Off Fukuoka Prefecture earthquake was one of the largest intraplate earthquakes in recorder history occurring ages at the junction of the Southwestern Japan Arc and the Ryukyu Arc, intensive seismological and geodetic studies were carried out in its aftermath. Ten papers from these studies were published as the special section for the 2005 West Off Fukuoka Prefecture earthquake (1) of Earth, Planets and Space, Vol. 58, No.1, 2006. In addition, eight papers are published in the special section (2) of this issue. These studies have revealed important characteristics of the earthquake as summarized in the following paragraphs. The mainshock and aftershocks were mainly aligned NW-SE in an approximately 25 km long trend, and the hypocenters of the aftershock region were distributed on a nearly vertical plane at depths of 2–16 km. The mainshock was located near the central part of the aftershock region, with a depth of 9.5 km. These findings suggest that the strike and dip of the fault plane of the earthquake are approximately N60◦W and 90◦, respectively, and the length and width of the fault are about 25 km and 14 km, respectively. The largest aftershock of MJMA 5.8 occurred near the SE edge of the main aftershock region, and the aftershock region extended about 5 km in the SE direction based on the secondary aftershock activity, which had a different strike angle from that of most of the aftershocks. Following the most intense aftershock activity, there was no distinct enlargement of the aftershock region, and the aftershock activity gradually declined (Shimizu et al., 2006). The fault structure has been discussed in detail on the basis of precise analyses of aftershock distribution based on onshore and offshore seismic observations made with densely located ocean bottom seismometers (Uehira et al., 2006). The fault plane is bent near both the NW and SE