Abstract:
Two sites (Sites 1150 and 1151) located in the landward deep-sea terrace of the Japan Trench were drilled during the Ocean Drilling Program (ODP) Leg 186. Site 1150 is seismically active, whereas Site 1151, aseismic. Measurements of sediment physical properties were made on both whole-round core and split-core sections. Bulk density is relatively uniform with similar variation with depth, although its absolute values are slightly different. It is most likely attributable to coring disturbance. Magnetic susceptibility is characterized by the highest value at the lithologic unit I. Depth variation of water content and porosity shows gradual decreasing pattern, except for lithologic unit I, similar to that of natural Gamma radiation. The abnormal characteristics of unit I are likely related to the under-consolidation caused by rapid deposition (100-200 m/myr) as well as impermeable nature of the sediment preventing dewatering. Grain density is higher in the unit I, but below which is relatively constant with depth. P-wave velocity gradually increases from 1500 to 2000 m/s with depth, caused by sediment compaction and/or consolidation. Sediment physical properties of both sites are very similar, although p-wave velocity is slightly higher ( approximately 20 m/s on the average) at Site 1151. This indicates that sediment source, mineral composition and sediment diagenesis are not significantly different at both sites, implying that seismic activities such as earthquake do not likely cause marked changes of sediment physical properties.