The 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) occurred on March 20, 2005 in the northern part of Kyushu, Japan. To study the aftershock activity, we deployed eleven pop-up type ocean bottom seismometers (OBSs), sixteen locally recorded temporary stations, and eight telemetered temporary stations in and around the epicenter region. We combined data from these stations and permanent stations located around the aftershock area, and determined the hypocenter of the mainshock and aftershocks. The mainshock was in the northwestern central part of the aftershock region, at a depth of 9.5 km. The mainshock was on a left-lateral strike-slip fault. Aftershocks were located in a depth range of 1-16 km and laterally extend for about 25 km in a NW-SE direction. We found that the aftershocks fell into four groups. This might be due to the heterogeneous structure in the source region. In the group that includes the mainshock, we estimated two fault planes bordering on the depth of the mainshock. There are 10-degree differences in both strike and dip angles between the lower and upper planes. From the aftershock distribution and the focal mechanisms, the rupture first propagated downward, and then propagated upward.
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