Anisotropy and the splitting of PS waves (original) (raw)

1994, Physics of the Earth and Planetary Interiors

The PS wave is radially polarized in an isotropic spherically symmetric Earth owing to the P to SV conversion at the free surface. If we choose stations with weak SKS splitting to minimize the effect of the receiver, we can measure the anisotropy beneath the bounce point of the PS wave. We find that with a deep source and epicentral range of 90°< ~< 125°,the PS phase can be distinguished from other phases. Owing to the limitation of recent large deep events with epicentral range between 90°and 125°,we analyze data from stations in North America and events beneath the Bonin Islands and the Fiji Islands. For events in these two regions, the bounce points of the PS wave are roughly located northwest of the Kuril Islands and near the Nova-Canton Trough in the central Pacific, respectively. Because the first Fresnel zone for the PS wave at this distance range is roughly 700 km across, each PS splitting observation must be interpreted as a local average of anisotropic properties. The average direction of the fast axis of anisotropy below the Nova-Canton Trough region is N104.2°Ewith a standard deviation 12.7°and a typical delay time of 1.8 s. Landward of the Kuril Islands, the average direction of the fast axis is N127.6°Eclockwise from north, and the standard deviation is 11.3°,with a typical delay time of 1.2 s. We have compared the fast axis with the direction of fossil seafloor spreading and with the present-day absolute plate motion. The fast axis of anisotropy at the Nova-Canton Trough agrees well with the direction of absolute plate motion, and poorly with the complex fossil spreading pattern. This suggests that significant upper-mantle shear anisotropy exists in this part of the central Pacific, and is consistent with mineral alignment owing to strain associated with present-day plate motion. Similarly, the fast axis beneath the Kuril Islands parallels the convergent motion of the slab. Coupled-mode synthetics constructed with the 'strong' Born approximation can be used to model the interaction of the PS waves with anisotropic structure that has a horizontal axis of symmetry. We use all free oscillations up to 35 mHz in the calculation. The coupled-mode synthetics show shear-wave splitting with delay time and fast axis consistent with the ray theory prediction.