Anisotropy beneath Hawaii from surface wave particle motion observations

Observed polarization ellipses for fundamental-mode surface waves observed at a digital station in Hawaii deviate from those expected for isotropic models of crust and mantle structure for that region. The anomalous motion occurs as rotations of the ellipse about all three axes in a cartesian corrdinate system. The largest and most consistent deviations occur as anomalous slopes of the ellipse about the horizontal axis transverse to the direction of propagation.The observed orientations and magnitudes of these angles can be explained by models of the upper mantle which contain olivine for which thea-axis dips significantly from the horizontal and which includes a sufficiently thick sedimentary layer (1 km) and a thicker than normal oceanic crust (15 km). The ellipses are also generally inclined from great circle paths about the vertical axis and are tilted about the axis aligned with the propagation direction. Both angles are small and difficult to measure, but the inclination angles are consistent with a model of the upper mantle in which thea-axis of olivine is preferentially oriented in an east-west direction.