Shear-wave splits

Figure 13.30: Splitting results across Oregon. The thick gray line indicates the result after stacking all useful events at each station. The line orientation gives the fast splitting direction, and its length is linearly proportional to splitting time. The thin black crosses indicate null results. Inset: Distribution of the 27 events used in the SKS splitting study.
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To constrain the possible flow fields beneath the Newberry track, preliminary SKS splitting measurements were made for 27 events at 12 OATS stations. A gradual rotation of fast direction is observed from ENE-WSW at the northwest end of the array to E-W to the southeast and the delay times average 1.65 sec (Figure 13.30). The SKS splits imply anisotropy primarily comes from asthenosphere and anisotropy orientation does not vary with depth beneath the track. Since the observed fast directions are not parallel to the Newberry track, and both the subduction counterflow model and the gravitational flow model require flow along the track, the lithospheric models are the most likely causal processes.

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