Anisotropic models of the upper mantle

Abstract

.3-that Love wave and Rayleigh wave data were inconsistent unless SH> SV in at least the upper 125 km. He also believed that anisotropy might be present below 250 km. Schlue and Knopofi [1977] found that the LVZ was anisotropic but the crust and lid could be modeled as isotropic, Their LVZ extends front 180 km to the bottom of the lid (15 km -115 km depending on age). They comment that the observed P. velocity can be a;tiAained by a thin sub-moho layer that would not be resolved by their data. Schlue and Knopofl [ 1978] included anisotropy in the LVZ for a suite of calculated models. Yu and Mitchell [1979] and Mitchell and Yu [1980] find anisotropy predominantly in the lithosphere and possibly in the LVZ. These studies have found that lithosphere thickness and lithosphere velocity generally increase with age, except Schlue and Knopofi [1977] who constrained their velocities to remain constant with age. The differences in the depth where anisotropy is located in these studies can be attributed to differences in the assumptions, the constraints, the inversion methods, or to some systematic.The most serious source of systematic error is the separate isotropic inversion of Love and ?ayleigh waves to give an anisotropic structure. The studies discussed above are almost all based on pseudo-isotropic inversions that determine SV velocity from isotropic Rayleigh wave inversion and SH velocity from isotropic Love wave inversion. The differences between the two models are then used as a measure of the anisotropy. Separate isotropic inversions make no allowance for P-wave anisotropy, and include neither the effects of SV velocity on Love wave velocities nor the effect of changes in PV, PH, and SH velocities on Rayleigh waves. Thus, the procedure of using separate isotropic inversions is useful only to indicate the probable presence of anisotropy, or to calculate responses for propagation in planes perpendicular to the symmetry axis in a transversely isotropic medium [Crampin, 1976]. Some studies [Yu and Mitchell,

Global mapping of the uppermantle by surface wave tomography

“…The slow regions at this depth generally correlate with broad geoid highs. The reversal in the sense of anisotropy as the ridge is approached is consistent with the detailed modelling of the Pacific (Regan and Anderson, 1984). At 340 km depth (Fig.…”

“…The shields are particularly fast. Anderson and Regan (1983) and Regan and Anderson (1984) give velocity and anisotropy models for the Pacific lithosphere.…”

Lithosphere, asthenosphere, and perisphere

“…The depths of intraplate oceanic earthquakes (Figure 1) are generally less than the inferred 600øC isotherm but occasionally reach the 750 ø isotherm [Chen and Molnar, 1983;Wiens and Stein, 1983]. This corresponds approximately to the thickness of the seismic LID [Regan and Anderson, 1984], that is, the depth to the top of the LVZ. This in t rnun corresponds to the effective elastic thickness of the oceanic plate.…”

Recent Seismological Studies on Upper Mantle Structure.

“…6. The plate thickness was determined to be 40-45km including oceanic crust, which is comparable with Regan and Anderson (1984). The S-wave velocities decrease and the P-wave velocities increase with depth in the low-velocity zone, suggesting an increase of the partial melting or dislocation relaxation with depth in the low-velocity zone.…”

“…Regan and Anderson (1984) used transverse isotropy with a vertical axis of symmetry as the basic framework for inverting surface wave data for the oceanic upper mantle structure to reconcile the VSH-VSV discrepancy. They showed that introducing the anisotropy in the surface wave inversion leads to a dramatic change in thickness of the oceanic lithosphere: The average plate thickness is 35-40km, far thinner than previous estimates of about 100km from isotropic inversions.…”