Seismic evidence for the influence of fluids on the 2005 west off Fukuoka prefecture earthquake in southwest Japan

High-resolution 3-D seismic velocity (Vp and Vs) and Poissonʹs ratio (σ) structures in the epicentral area of the 2005 west off Fukuoka prefecture earthquake were determined by inverting a large number of high-quality P- and S-wave arrival times and sP depth phase data to better understand the genesis of the main-shock and its aftershock sequence. The hypocenters of the earthquakes used in this study were relocated with a combined method which enables to accurately relocate the earthquakes occurred outside the seismic network by using sP depth phase data together with double-difference travel time data. This new approach of earthquake location can be applied to other offshore regions of subduction zones where few seismic stations exist while numerous suboceanic earthquakes occur. Strong crustal velocity heterogeneities were revealed in the hypocentral area, which might reflect the existence of fluids, geometrical irregularities of the fault zone, and geothermal anomalies in the region. The main-shock hypocenter was located near a distinct zone characterized by low-velocity, high-Poissonʹs ratio and high-conductivity anomalies, being considered to be associated with the existence of fluids in and around the source area. Such fluids might have reduced the seismic velocity of the crustal materials around the main-shock hypocenter and weakened the mechanic strength of the fractured rock matrix in the source area, and thus triggered the 2005 west off Fukuoka prefecture earthquake.