2015 年 124 巻 3 号 p. 321-332
Recent active-source seismic studies on the old Pacific Plate (125-121 Myr old) off the Kuril Trench provide new insights into formation/alternation processes of the crust and uppermost mantle structures. These studies show high P-wave velocities of 8.5-8.6 km/s and strong azimuthal anisotropies of 8.5-9.8% in the uppermost mantle, immediately below the clearly imaged Moho and the lower crustal ridge-ward dipping reflectors (LCDR), which are observed with constant dip angles (20-25°) and intervals (2.5 km). From the observations, we conclude that the LCDR and the strong anisotropic mantle were formed by an active-mantle flow near a paleo-spreading center, which drags the base of the crust away from the spreading center. This could be direct evidence of an active mantle flow at a spreading center. Another important observation is a systematic structural change toward the trench from the outer rise region. Recent seismic studies found that P-wave velocities decrease and VP/VS ratios increase toward the trench from a distance of 200 km. The observations suggest that the water content within the incoming ocean crust increases toward the trench, accompanied by the development of bending-related fractures at the top of the oceanic crust. P-wave reduction (2-5%) in the uppermost mantle is observed at the near trench. This might suggest serpentinization of the mantle of ～20%, but it is necessary to have precise S-wave information to further interpret the mantle velocity reduction near the trench region.