SHEAR-WAVE SPLITTING ABOVE THE SUBDUCTING JUAN DEFUCA PLATE

Shear-wave splitting observed in three-component broadband recordings of local earthquakes near southern Vancouver Island, British Columbia, provide the first constraints on shear-wave anisotropy in the contine ntal crust above the subducting Juan de Fuca plate. Waveforms of 22 '' shallow'' earthquakes (15-30 km depth) and 16 ''deep'' earthquakes (40 -60 km depth) were examined for shear-wave splitting. The fast-polariz ation direction is well-defined at 312 +/- 19 degrees for the deep ear thquakes, and 293 +/- 27 degrees for the shallow earthquakes. These fa st polarization directions are approximately parallel to the continent al margin and coincide with the orientation of the pressure axes of sm all crustal earthquakes in this region. Interpreting this polarization pattern in terms of the extensive-dilatancy anisotropy model indicate s a margin-parallel compressive stress regime. Comparing the average t ime delays between the fast and slow shear-waves for the deep earthqua kes (0.32 s) and the shallow earthquakes (0.20 s) suggests that the so urce of the seismic anisotropy is stronger in the upper 20 km of the c ontinental crust. Here, the average differential shear-wave anisotropy is estimated at 2.3%. These observations support the theory of weak c oupling between the downgoing Juan de Fuca plate and the overlying Nor th America plate, as the principal stress is perpendicular to the dire ction of subduction, and the S-wave splitting above the subducting pla te is nearly perpendicular to the SKS splitting direction in the upper mantle beneath the subducting plate.