Tw~st Array

The array of the Teleseismic Western-Superior Transect experiment (Tw~st array, spoken twist-array) was operated during an experiment on the structure of the Superior Province in northern Ontario (Canada). The aim was to study the western Superior transect with teleseismic data. The location of the Tw~st stations are displayed in Figure 4.3. The stations form a linear array oriented roughly south-north, stretching from Lake Superior to Hudson Bay (Kay et al., 1999). Due to the line configuration, the slowness resolution for events from the western Pacific rim is sparse, but the large aperture of the array still enables some slowness resolution.
The array was in operation for only a few months, from May to November 1997. Therefore, not many events suitable for this study were recorded.

Figure 4.3: Configuration of the Tw~st array in Ontario (Canada). The stations form a line oriented north-south. This array configuration was chosen to study structure beneath the Caribbean. The slowness resolution for events from the western Pacific Rim is sparse. The experiment was realized by scientists of the GSC, Queens University (Canada) and University of Leeds (UK).

The aperture in N-S direction was $\sim$850 km. Due to the linear configuration of the array the E-W aperture was only a few kilometres. The mean spacing between the stations 5010 to B14 is $\sim$40 km, but larger for the stations BBT, BPW and BFS. Long period waves (T = 10 - 20 s) are coherent for this configuration. The coordinates of the Tw~st array stations are listed in Table A.2
The slowness resolution of linear arrays for waves arriving with a backazimuth perpendicular to the strike of the array is sparse. For the Tw~st array the large interstation spacing enables a reasonable slowness resolution for wave from a backazimuth some degrees ($\sim$15$^{\circ}$) different from 270$^{\circ}$. This resolution is sufficient to identify the S$^d$S phases from events in the western Pacific. The Tw~st array is used to study anisotropy within the upper mantle beneath the reflection point of SS phases. The knowledge of the upper mantle anisotropy structure beneath the station is essential to be able to correct for this local anisotropy. The splitting of horizontally and vertically polarized waves in the seismogram after the correction for the upper mantle anisotropy beneath the station results from anisotropy beneath the SS surface reflection point, if deep focus earthquakes are used. The splitting parameters for the Tw~st array stations are listed in Table A.2. The anisotropy beneath the stations was studied by shear wave splitting observations (Kay et al., 1999).