The velocity vs. porosity plots for the sediments from Sites 1173, 1174, and 808 (Fig. F10A, F10B) clearly indicate that the sediments above and below the décollement exhibit different tectonic histories. This is illustrated by the absence of a trend in increasing velocity with decreasing porosity. This is most likely due both to a more varied lithology in the sediments above the décollement and to differing tectonic histories. In addition, the abnormal compaction trend observed in the Upper Shikoku Basin sediments may also contribute to the scatter observed in the velocity vs. porosity plots. An abnormal compaction trend is indicated by the constant velocities and high porosities present in this interval, which are most likely caused by cementation related to the processes of silica diagenesis (see Shipboard Scientific Party, 2001b, 2002b).
When only the underthrust sediments are plotted (Fig. F10B), a clear trend of increasing velocity with decreasing porosity and a critical porosity transition are apparent. This is especially true for Site 808 data and, to a lesser extent, Site 1174 data. The underthrust sediments follow the normal consolidation trend proposed by Erickson and Jarrard (1998) as is expected for underthrust sediments that have been subjected only to burial compaction stresses (Davis et al., 1983; Morgan and Karig, 1995).
The results of the parameter fitting for the underthrust sediments indicate that a critical porosity transition is crossed at ~0.30. Although a single critical porosity value can be accurately fit to this data set, it appears that the critical porosity transition occurs over a range of porosities for Site 808. The median value of this range is ~0.31, which agrees relatively well with the value obtained in the parameter fitting.
The parameter fitting provided a best-fit shale fraction value of ~1.06. One possible explanation for an unphysical shale fraction being fit is the lack of lower-porosity, higher-velocity data to constrain this parameter. The average shale fraction of 0.32 calculated from the shale fraction log (Fig. F9) suggests that a lower value would more accurately describe Site 1173 data. However, the shale fraction log for Site 1173 is probably not accurate due to the lack of representative sand and shale baselines. Further analysis needs to be done to accurately determine which shale fraction best describes the Shikoku Basin underthrust sediments.
It is also important to note that the value of the shale fraction significantly affects the values determined for the critical porosity in the parameter fitting. An improved fit could probably be obtained by holding the shale fraction constant (at a value representative of the sediments at Site 1173) and allowing the critical porosity to vary.
The relationship derived by Hyndman et al. (1993) does not appear to fit either the lower- or higher-porosity sediments as well as the formulations suggested by Erickson and Jarrard (1998). The Hyndman et al. (1993) formulation does follow roughly the same trend but generally overestimates the velocity, especially for porosities near the critical porosity transition. However, this formulation was derived only for Site 808 data, and different in situ corrections have been applied to the current core data set, which could account for some of the differences.