Abstract
Characterising physical properties at subduction zones is essential to understanding the changes that occur as sediments are subducted and incorporated into accretionary prisms. The relationship between compressional wave velocity and porosity is key to quantifying the fluid and solid material budget in the subduction process because velocity can be remotely sensed. That relationship is intrinsically linked to the burial history and tectonic evolution of sediments. Typically, core-derived velocity and porosity data have been used to develop transform functions; however, they often require corrections for in-situ conditions and are rarely complete data sets, especially in and around fault zone settings. Borehole logging data is continuous through fault zones and measures at in-situ conditions, and therefore provides a good alternative to core data.

Recent drilling results from IODP expeditions to the Nankai Trough, SW Japan, provide an extensive data set across multiple tectonic domains. We use logging data to investigate the relationship between velocity and porosity within the Kumano basin, Shikoku Basin and underlying accretionary prism. By comparing our results with the previously established empirical velocity - porosity relationships, we demonstrate that these relationships do not apply in this region. In addition, distinctly different relationships are observed between the Kumano Basin, the underlying accretionary prism, recent slope basin sediments and the input sediments of the Shikoku Basin, consistent with their different tectonic histories.