Seismic Anisotropy in the Upper Mantle Beneath Adak Island Records Relative Plate Motion.
Abstract
Along the Aleutian Arc, convergence between the Pacific and the North American plates becomes progressively more oblique from east to west. At the Adak Island, in the center of this arc, the Pacific plate subducts at an angle of nearly 45° to the trench. Using data from a long-running seismic observatory on the Adak Island, we carried out a study of birefringence in 41 core-refracted shear waves (SKS, SKKS, PKS) observed betweed 1994 and 2003. Single-phase estimates of birefringence performed using a cross-correlation algorithm show considerable scatter, although a large fraction of them yields fast directions in the 120° - 150° SE range. Values of the delay never exceed 1 s. Direction-dependent scatter in shear-wave birefringence measurements may indicate that more than one anisotropic system is contributing to the signal, or that the axes of anisotropic symmetry are tilted. To investigate these possibilities, we selected a subset of records well distributed in backazimuth, and performed group inversions for four model classes: (1) one layer with horizontal fast axis; (2) two layers with horizontal fast axes; (3) one layer with arbitrarily oriented fast axis; and (4) one layer with arbitrarily oriented slow axis. Best single-layer solutions cluster around the fast direction of 130° SE, with delay < 1 s, consistent with an apparent trend in individual measurements. The addition of an extra layer does not yield a significant improvement in data fit. All best fitting two-layer models contain a layer with a fast direction in the 120° - 150° SE range, while the fast axis of the second layer is either very close, or else nearly orthogonal to the first. As splitting effects of two orthogonal fast axes cancel out, such two-layer models cannot be considered distinct from those of the single-layered class. Finally, models where the anisotropic symmetry axis (fast or slow) can plunge provide somewhat better fits. The preference for the fast axis at ~130° SE is retained, however, and is further confirmed by slow-axis inversion that yields NE (i.e., orthogonal to the SE fast) directions. Therefore, the finding of fast shear wave speed direction of ~130° SE appears to be very robust. It is very close to the relative direction of motion between the Pacific and the North American plates. Thus birefringence of shear waves observed at Adak likely reflects mantle corner flow induced in the supraslab mantle wedge.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.S51B..07B
- Keywords:
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- 7203 Body wave propagation;
- 7218 Lithosphere and upper mantle;
- 8120 Dynamics of lithosphere and mantle: general