Velocity, porosity, and pore-fluid loss from the Nankai subduction zone accretionary prism

Abstract

Seismic velocity-depth profiles are available from four types of measurements in the area of Ocean Drilling Program Leg 131. Site 808; 1) laboratory measurements on core samples, 2) downhole velocity logs, 3) a downhole vertical seismic profile (VSP), and 4) split-spread (SSP) surface ship seismic reflection. The main deviations at the site from the general increase in velocity with depth are higher velocities in the upper 200m associated with coarse-grained trench-fill sediments, and abrupt downward velocity decreases associated with an intersected thrust and the decollement. A velocity-porosity relation has been used to convert the velocity data to porosity-depth sections. The estimated amount of pore-fluid expulsion with distance across the seaward poriton of the accretionary prism is estimated to be at least a factor of five less than for the DSDP Leg 87 profile to the west, because of the much shallower seafloor slope angle in the area of the drill site and lower wedge taper. -from Authors