was reduced to maintain constant solid volume and to ensure mass balance (Eq. 11). The
new porosity values were then used to calculate the matrix compressibility for the next
sedimentation step (Eq. 12). SUTRA was modified to calculate specific storage for each
node based on the compressibility and porosity calculated by the loading program.
Model Dimensions, Boundary Conditions, and Initial Conditions
The geometry and hydrogeology of the Nankai accretionary complex were
simplified into three hydrogeologic units including the turbidites, upper Shikoku and
lower Shikoku Basin hemipelagic sequences. The model domain was discretized into
200 rows. Because the model is one-dimensional, lateral flow along the decollement
could not be simulated and thus, special physical properties such as higher permeabilities
were not assigned to the decollement.
The upper and lower Shikoku Basin layers were assigned similar porosity and
permeability parameters as the two layers were mainly composed of hemipelagic muds.
The physical properties for hemipelagic units were assigned based on geological
observations and laboratory measurements. The lithology of the turbidite unit at Nankai
is composed of a variety of thick to thin bedded sand and silt turbidites with some
hemipelagic muds (Shipboard Scientific Party, 2001). Values of no (0.77) and b (1.1 x
10-3 m-1) for hemipelagic sediments were obtained from Screaton et al. (2002) while for
turbidites (no=0.65, b = 7 x 10-4 m-1), the porosity-depth relationship of Bekins and Dreiss
(1992) was used. To test the sensitivity of the b coefficient for hemipelagic sediments, I
used the porosity data from Site 1173 to estimate the standard deviation for the b values
as 3 x 10-4 m-1. Using the standard deviation, I checked the sensitivity ofb values at the
minimum possible value of 1.4 x 10-3 m-1 and a maximum possible value of 8 x 10-4 m1.
For the turbidites, I used b values of 1.0 x 10-3 m-1 and 4 x 10-4 m-1 as the maximum and