Empirical Green’s function simulation of broadband ground motions on Genkai Island during the 2005 West Off Fukuoka Prefecture earthquake

The 2005 West Off Fukuoka Prefecture earthquake caused serious damage to and on Genkai Island as well as to downtown Fukuoka City. There were no strong motion instruments on the island, therefore no one knows how the strong ground motion occurred during the mainshock. The ground motion simulation on Genkai Island is very important to our understanding of earthquake damage at the near-source region. We have conducted an aftershock observation on the island in order to verify site amplification due to steep topography and to record aftershocks for reproducing ground motion during the mainshock by the empirical Green’s function method. The observed records of aftershocks show small variations in the input motions in the island, indicating that the amplification due to the topography seems to be small below 2 Hz. We first estimated the strong motion generation area for the mainshock using the observation records at stations surrounding the source region. We then carried out broadband ground motion simulation on Genkai Island by using the aftershock records as empirical Green’s functions. The simulated ground velocities exceed 1 m/s with a dominant period of 1–2 s due to the forward rupture directivity, and the instrumental seismic intensity reaches 6.6.