Tw~st dataset

Only the material of the upper 300 km of the mantle mantle is anisotropic (Nishimura and Forsyth, 1989). Therefore, deep events (>300 km) are most suitable to study the mantle anisotropy, because the source is then located in isotropic material and the source side anisotropy can mostly be neglected.

Figure 4.7: Sources used for the anisotropy study with the Tw~st array. The sources are displayed as stars. Due to the short operation time of the Tw~st array, only few events were recorded. The source parameters of these events are listed in Table A.5. The lines indicate the great circle paths to the Tw~st array.

As a result of the short time the Tw~st array stations were in operation, only very few events with reflection points in the central Pacific were recorded. For this reason also shallow events were selected. The extent of the source side anisotropy for the shallow events can be estimated during the processing.
No magnitude selection of the events has been done, because of the sparse recordings during the operation time of the array. In Table A.5 the selected events are listed. The events have depths from 7 km - 620 km and magnitudes of m$_b$ = 5.5 - 6.7. The location of these events is shown in Figure 4.7. The sources of the events recorded at Tw~st cover a smaller area from Kermadec to New Guinea than the P$^d$P events recorded at YKA. Since the Tw~st array is located further to the east, the surface reflection points cover a different area. The reflection points are shown as circles in Figure 4.8. The size of the circles is considerably smaller than the first Fresnel zone of the long-period data. The size of the circles in Figure 4.8 was chosen to allow the visibility of the location of the SS reflection points of all events. The heavy black line indicates the Hawaii-Emperor seamount chain. The location of the surface reflection points are listed in Table A.6.

Figure 4.8: Surface reflection points for the events shown in Figure 4.7. The reflection points are indicated by grey circles. The black line indicates the Hawaii-Emperor seamount chain. The size of the circles is smaller than the size of the first Fresnel zone to enable the visibility of all reflection points. The location of the surface reflection points are listed in Table A.6.