A Surface-Wave Analysis of Seismic Anisotropy Beneath Eastern North America
A Surface-Wave Analysis of Seismic Anisotropy Beneath Eastern North America
Credit:
James B. Gaherty • Lamont-Doherty Earth Observatory of Columbia University/IRIS Consortium
Description
(a) Surface-wave raypaths used in this analysis, which propagate along the MOMA array from three earthquakes in western North America. Background shows shear-velocity perturbations at 150 km depth from van der Lee (2002). (b) Radially anisotropic shear-velocity models of the upper-mantle beneath MOMA, derived from inversion of surface-wave delays. Left panel displays mean shear speed (vS = (vSH+vSV)/2), while right panel displays shear anisotropy (vS = (vSH-vSV)/vS) in percent). Blue solid curves (CRTN) correspond to the average structure beneath the craton (the region between GSN station CCM and MM06), while green dash-dot curves (MRGN) represent the structure beneath the margin, from MM06 to GSN station HRV. Shown for comparison are isotropic reference model IASP91 (dotted), and radially anisotropic model AU3 (dashed) (Gaherty and Jordan, 1995).
The nature of upper mantle seismic anisotropy beneath central and eastern North America has been evaluated using the phase velocities of surface waves traversing the length of the Missouri-to-Massachusetts (MOMA) PASSCAL broad-band array. Frequency-dependent phase delays of fundamental-mode Love and Rayleigh waves, measured across the array using a cross-correlation procedure, require the presence of anisotropy in the upper mantle. Two-dimensional radially anisotropic structures obtained via linearized inversion of these data contain shear anisotropy with a magnitude of ~3% between the Moho and at least 180 km depth. This anisotropy is approximately constant across the tectonic transition from the craton to the Atlantic margin (Figure 1). Forward models consisting of a shallow, lithospheric layer of azimuthal anisotropy derived from previous shear-wave splitting observations (Fouch et al., 2000) fail to satisfy the surface-wave delay times. The combined surface-wave and splitting results suggest the presence of two layers of anisotropy: a lithospheric layer that produces the phase-delay differences observed in Love and Rayleigh waves but is transparent to vertically propagating shear body waves, and a deeper (presumably asthenospheric) layer that generates the shear-wave splitting. One plausible model is that the lithosphere is characterized by vertically heterogeneous anisotropic fabric, which would produce minimal splitting in vertical shear waves. Such fabric has been hypothesized for regions of weak and complex splitting such as South Africa and Australia; the results here imply that it may be appropriate for North America as well.
Fouch, M.J., K.M. Fischer, E.M. Parmentier, M.E. Wysession, and T.J. Clarke, Shear-wave splitting, continental keels, and patterns of mantle flow, J. Geophys. Res., 105, 6255-6276, 2000.
Gaherty, J.B. and T.H. Jordan, Lehmann discontinuity as the base of an anisotropic layer beneath continents, Science, 268, 1468-1471, 1995.
Gaherty, J.B., A surface-wave analysis of seismic anisotropy beneath eastern North America, Geophys. J. Int., 158, 1053-1066, 2004.
van der Lee, S., High-resolution estimates of lithospheric thickness from Missouri to Massachusetts, USA, Earth Planet Sci. Lett., 203, 15-23, 2002.
Photographer / Contributor: James B. Gaherty • Lamont-Doherty Earth Observatory of Columbia University
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