Variation of physical properties beneath a fault observed by a two-ship seismic survey off southwest Japan

We conducted a two-ship seismic reflection survey at Kumano-nada off the southwest Japan arc, in 2002, in order to clarify physical properties along a splay fault that branches from the plate boundary of the Nankai Trough subduction zone. During the survey, seismic reflection data were collected out to maximum offset of 20 km using multiple passes of a shooting and a receiving vessel separated by a different constant offset on each pass. We conducted analyses on reflection polarity and amplitude variation with offset (AVO) effects to examine physical properties of layers immediately beneath the splay fault. Reflections from the splay fault showed varying polarities along the fault. In the present study, we assumed that the Poisson's ratio of the layer above the fault was 0.30-0.40. Where polarity is reversed, the Poisson's ratio of the layer immediately below the fault was estimated to be 0.33-0.445 (11% increase on average). Where polarity was normal, the ratio was estimated to be 0.263-0.365 (11% decrease on average). If the reverse polarity indicates the existence of fluid as many authors suggest, the polarity analysis demonstrates inhomogeneous distribution of fluid in the splay fault shear zone. The results of AVO analysis also support the possibility of the spatial variation of fluid distribution under some assumptions because the Poisson's ratio changes with content of fluid in rocks. The variation in reflection polarity and corresponding AVO effects may signify the possibility of localized fluid migration in the splay fault shear zone.