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Title: Seismic surface-waves and anisotropic alignments in the oceanic upper-mantle
Author: Kirkwood, Sheila C.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1978
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Observations and theories of elastic anisotropy in the Earth's crust and upper mantle are reviewod and discussed. Crampin and Taylor's (1971) procedure for computation of seismic surface-wave characteristics in a plane-layered anisotropic half-space is described and a modification to incorporate a surface water layer is developed. Simplified models of anisotropic ocean-basin structure are examined, with particular reference to surface-wave particle-motion. Two types of anisotropic alignment are considered, one resulting from syntectonic recrystallisation of olivine in a horizontal-shear-zone, the other from olivine glide-plane slip with horizontal or vertical slip-planes. Alignment of the first type can cause slightly anomalous particle motion, of tilted-Rayleigh-type, in all surface-wave modes. The variation of anomaly amplitude with period in the fundamental mode can indicate the approximate depth to the anisotropic layer. Alignment of the second type can cause highly anomalous particle-motion, of inclined-Rayleigh-type, in the third generalised mode, corresponding to the isotropic second-Rayleigh mode. The anomaly amplitude is rather insensitive to details of structure. For either type of alignment, the sense of tilt, or inclination, varies with direction of propagation, in a manner characteristic of the structural symmetry. Some practical problems in observing surface-wave particle-reaction in real ocean-basins are discussed. Several seismograms are presented showing tilted-Rayleigh-type particle-motion for modes corresponding to the isotropic fundamental-Rayleigh and Love modes. Observations are consistent with theoretical models if aligned olivine b-axes are tilted down in the direction of plate-motion. Carter's (1970) syntectonic recrystallisation model then suggests that the lithosphere is dragging the asthenosphere. Preliminary results for the variation of anomaly amplitude with period suggest a high degree of allignment, stronger in the lithosphere then in the athenosphere.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available