Velocity-porosity relationships in smectite-rich sediments: Shikoku Basin, Japan

At the Nankai Trough, Japan, smectite‐rich sediments of the Shikoku Basin are both underthrust beneath and frontally accreted to the outer accretionary wedge. To understand the changes that occur in the physical properties of sediments as they are underthrust or incorporated into the accretionary wedge, it is necessary to characterize those physical properties outboard of the deformation front. One key relationship to constrain is between the compressional wave velocity (vp) and porosity (ϕ) in order to estimate pore fluid pressure and determine the strength of fault rocks at depth. Here we investigate how the type of clay minerals and proportions of the different clays may affect the velocity‐porosity relationship, with particular emphasis on the abundance of hydrous smectitic clays. Using data from IODP Sites C0011 and C0012 in the Shikoku Basin, we demonstrate that when the core‐based porosity is corrected for the abundance of smectite, the resultant velocity‐porosity relationship does not fit previously established transforms. Therefore, we propose a new empirical relationship for the smectite‐rich sediments of the Shikoku Basin, and caution against the indiscriminate use of global and/or pre‐existing models in smectite rich formations. Key Points Smectite‐corrected core porosity measurements significantly decrease porosity New velocity‐porosity relationship is presented using corrected data