AZIMUTHAL ANISOTROPY OF SURFACE WAVES AND THE POSSIBLE TYPE OF THE SEISMIC ANISOTROPY DUE TO PREFERRED ORIENTATION OF OLIVINE IN THE UPPERMOST MANTLE BENEATH THE PACIFIC OCEAN

抄録

The group velocities of surface waves in the period range 22-89 s were measured to retrieve the anisotropy pattern of the surface wave velocities in the Eastern Pacific Ocean. WWSSN long period records at KIP, Hawaii, from ridge events in the Gulf of California, the East Pacific rise, and the Easter Island Cordillera were used to cover propagation directions N60°E-N140°E. These station and epicenters are in very favorable positions to minimize the effects of the lateral heterogeneity owing to ocean floor ages. The fast direction of the Rayleigh wave group velocity is normal to ridge, showing 3-5% azimuthal anisotropy. The Love wave group velocity shows little azimuthal anisotropy. A model of a possible type of preferred orientation of constituent olivine beneath the Pacific Ocean is proposed, based on the elastic constants and the petrofabrics of dunite from Hidaka, Hokkaido, Japan, and that of olivine nodules in the ophiolite complex from the Bay of Islands and Oman. In this model, a-axis is concentrated along a particular direction and b- and c-axes form a girdle. Elastic constants of the model are C₁₁=2.44, C₂₂=C₃₃=2.05, C₁₂=C₁₃=0.72, C₂₃=0.67, C₄₄=0.69, and C₅₅= C₈₆=0.75 (10¹² dyn·cm), where X₁-axis, or the direction of the a-axis concentration, is normal to ridge and X₃-axis is vertical. If we assume that the type of the preferred orientation is an age-independent property of suboceanic lithosphere, this model can explain the overall features of the large-scale anisotropy in the Pacific Ocean: the azimuthal anisotropy of surface wave velocities as described above; the SH-SV anisotropy of surface waves of about 0.15 km/s; the P_n, anisotropy of 7.9 km/s to 8.6 km/s; the high S_m, velocity of about 4.8 km/s; and the ScS polarization anisotropy