Anisotropy structure

In this thesis a new method to resolve the depth structure of the anisotropy beneath the reflection point using SS precursors has been proposed. The aperture of YKA is too small to identify the long-period precursors to SS in vespagrams. Therefore, the temporary Tw~st array was used to study these phases. From the 16 available events in the Tw~st dataset only one contained clear precursors to SS with appropriate slownesses to be interpreted as underside reflections from the discontinuities. This event was at a depth of only 168 km, and therefore within the region of the upper mantle, where anisotropic material is expected. Nevertheless, splitting time studies of the S$_{diff}$ showed that the upper mantle correction for the receiver side removed most of the splitting time and that the particle motion of S$_{diff}$ is mostly linear.
In contrast the SS phase showed strong elliptical polarizations, because it travels through the anisotropic upper mantle material at the reflection point. The two detected precursors from the 410 and the L show only small amplitudes. This makes the identification of the polarization quite difficult.
The application of this method to Tw~st array data is problematical. First of all, the slowness resolution is poor as a result of the backazimuths close to 270$^{\circ}$ resulting in nearly perpendicular incidence of the wavefront. This complicates the detection of the precursors. Secondly, the amplitudes of the precursors are too small to be identified on a single seismogram. The particle motions are computed for a single station only. This problem can be solved by using beam traces of the array for the investigation. The beam forming destroys the absolute time information of the single traces. Therefore, the beam traces must be aligned on a reference phase, e.g. SS. The alignment produces linear polarization for SS. The precursors will show different polarizations, because they travel a shorter distance within the anisotropic medium. Synthetic tests may help to study the connection of the deviation from the linear polarization and the path length the phase travels in the anisotropic medium. New methodologies are required to quantify the deviation from the linear polarization and to deal with the trade-off between the length of the travel path in the anisotropic medium and the intensity of the anisotropy.
The slowness resolution can be improved by using data from different arrays with a better alignment of the stations to the incident waves. An appropriate array for this purpose would be the array of Missouri to Massachusetts Broadband Seismometer Experiment (MOMA) (Li et a., 1998). This array with 20 stations along a 1740 km long line from Massachusetts to Missouri, was in operation from January 1995 to April 1996. The orientation of the array is ideally suited to study events from New-Zealand to Tonga-Fiji. This new source of information on the structure of the upper mantle structure will be used in the future.