Model Implementation
This modeling study focused on Sites 1173, 1174, and 808 to examine the
development of pore pressures along the Muroto Transect during early subduction.
Modeling is one-dimensional, and thus, lateral fluid flow cannot be included. However,
as discussed above, evidence for lateral fluid flow at this Transect is inconclusive. A
method previously described by Screaton and Ge (2000) and Kemerer and Screaton
(2001) was modified for this investigation to add the permeability-porosity relationship
(Eq. 2). The modeling method combines a loading program to simulate pore pressure
increases due to sedimentation and initial subduction with an existing fluid flow and
transport code, SUTRA (Voss, 1984).
Based on the rate of sedimentation or thickening of the overriding prism for each of
the segments, the loading program calculates the additional thickness of each added
sediment layer. As new sediment layers are added to the top of the model, the layers
beneath were moved down one row, and their pore pressures were incremented with the
additional pore fluid pressure due to the load of the new layers multiplied by y (Eq. 6).
The additional load is calculated from the thickness and bulk density of the new layer,
with the hydrostatic pressure subtracted.
The updated pore pressures were used as inputs to SUTRA as initial conditions to
perform transient fluid flow simulations. Once the pore pressures (P) at the end of each
loading step were calculated by SUTRA, they were transferred back into the loading
program, in which effective stress was calculated using Equation 7. If effective stress
increased relative to the previous loading step, the porosity decrease was determined
using an iterative method to solve Equation 10 for change in porosity. Vertical spacing