Sediment physical and acoustic properties at ODP Sites 1150 and 1151, Japan Trench

Two sites ( Sites 1150 and 1151) located in the deep-sea terrace on the landward side of the Japan Trench were drilled during the Ocean Drilling Program ( ODP) Leg 186. Measurements of sediment physical and acoustic properties were made on whole-round core sections and on the least- disturbed portions of split- core sections. Compressional wave velocity generally increases with depth. But the data below 600 meters below seafloor ( mbsf) is more scattered due to velocity variation caused by microstructures than that of the upper section (< 600 mbsf). Physical properties at Sites 1150 and 1151 match well each other, characterized by decreasing water content and porosity and increasing wet bulk density with depth, indicating variations in hydrological and mechanical conditions. The reverse trend below 100 mbsf is probably responsible for sampling disturbance and degassing cracks by gas expansion. In addition, it is most likely due to the under consolidation caused by the rapid deposition ( 100 similar to 200 m/m.y.) and impermedable nature of the sediment ( clay and diatomaceous ooze rich) that prohibit dewatering. Correlations between physical and acoustic properties at Sites 1150 and 1151 show a nearly similar pattern, suggesting no evidence of change of sediment properties caused by frequent earthquakes. Therefore, sediment physical and acoustic properties are not likely to be related to sediment deformation caused by seismic activity occurring in the plate boundary.