below the decollement at both sites were less than observed values. The simulated
porosities above the decollement were higher at Site 1174 than observed, while at Site
808 they reasonably matched the observed porosities.
I also examined the sensitivity of porosity and X* to a permeability relationship (k=-
20+5.25n) in the range used by Saffer and Bekins (1998) (Table 3-4, Figure 3-6). The
simulated X* values using this relationship predicts greater X* values than those predicted
from the base run suggesting that simulated X* values are very sensitive to even slight
changes in the log-linear permeability-porosity relationship (Eq. 2). The predicted X*
using this relationship were greater by 0.06 (Site 1174) and 0.11 (Site 808) compared to
the X* values predicted from the base run (Table 3-4).
In comparison to the k* profile of Saffer (2003), the values of k* predicted for log
(ko) = -20.82 is in the same range as the k* values predicted by Saffer (2003). The
general trend of the k* profile matches values predicted by Saffer (2003) best in the upper
(Site 808) and mid (Site 1174) portions of the underthrust and with depth the k* profile
gradually deviates to lower values than predicted by Saffer (2003).
Sensitivity to Lateral Stress
In reality, the prism sediments are structurally deformed by lateral stresses caused
by tectonic compression. To examine the effects of tectonic compression on excess pore
pressures and porosities, I included lateral stress within the prism sediments as an
additional fraction of the vertical loading (Jaeger and Cook, 1969). Following Domenico
and Schwartz (1998) the ratio of horizontal to vertical stress becomes greater than one in
areas of tectonic compression. I varied the lateral stress factor from 0.1 to 0.9 (i.e., the