Anisotropy of Consolidation Yield Stress, Compressive Wave Velocity and Tensile Strength in Sedimentary Rocks in the Western Part of the Boso Peninsula, Central Japan

The subduction movement of the plate is recorded in the forearc basin also, which is expected to contribute to the elucidation of the origin of the Japanese Islands located at the plate boundaries. In the forearc basin sediments of the Boso Peninsula, the maximum burial depth of the formation can be estimated by calculating the consolidation yield stress (Pc) of the sedimentary rocks through consolidation tests. In this study, considering the reverse fault type of stress state in the Boso Peninsula, consolidation tests were conducted in three directions of principal paleostresses: the maximum principal stress direction (σ1), the fold axis direction (σ2), and the direction perpendicular to the bedding plane (σ3), and the anisotropy of Pc with respect to the bedding plane was investigated. In addition, anisotropy of compressive wave velocity (Vp) and tensile strength (σt), which are fundamental physical properties of rocks, were also examined to clarify the relationship between sedimentary structure and various physical properties of the samples. The specimens were formed using 4 block samples taken from different geological formations whose sedimentary age is approximately 3 Ma.

In the consolidation test, one sample showed a significantly larger value of Pc in the σ1 direction, with the average of that direction being 19% larger than the average of the three directions as the degree of anisotropy. The remaining three samples did not exhibit high anisotropy in any direction and the degree of anisotropy was only 4-8%. Vp showed a descending order of σ1, σ2, σ3 directions in all formations. The σt was weakest in the direction where the failure plane was parallel to the bedding plane, except for one sample, and was larger in the direction parallel to the σ1 direction than in the direction perpendicular to the σ1 direction for the failure planes that were perpendicular to the bedding plane. These results indicate that the degree of consolidation progress may be influenced by past tectonic stresses, as one sample showed a remarkably large value of Pc in the σ1 direction. The anisotropy of these physical properties indicates that the mineral structure has anisotropy formed by orientation of the clay minerals (the direction of the long axis coincides with the σ2 direction) by lateral compaction.