In the Nankai Trough, the decollement megathrust initiates within the incoming sedimentary sequence along an interface within the lower Shikoku Basin facies. Scientists on Ocean Drilling Program Legs 190 and 196 penetrated the stratigraphic horizon that develops into the decollement at a reference site 12 km seaward of the deformation front, Site 1173, and in the decollement zone, at Site 1174. The core and logging-while-drilling data collected at these sites examine the lateral variation in physical properties prior to and just after initiation of decollement thrusting. We used a prestack time migrated three-dimensional (3-D) seismic reflection data volume that overlaps with the Leg 190 and 196 drill sites, along with Site 1173 and Site 1174 physical property data, to invert a seismic reflection transect (Line 215) for seismic impedance across the trench from Site 1173 to the deformation front. We used porosity estimated from seismic inversion to infer compaction and dewatering activity within the upper and lower Shikoku Basin facies as the trench wedge overburden rapidly accumulates. A compacted layer (with porosity = approximately 40%) develops at the upper/lower Shikoku Basin facies boundary and the stratigraphic equivalent of the decollement zone in the trench landward of Site 1173. The compacted layer may develop because of localized dewatering, diagenetic, or cementation effects within the compacted layer or the over- and underlying layers. We speculate that the compacted layer serves as an aquitard to delay consolidation beneath the horizon that becomes the decollement zone and contributes to the decollement development.