Anisotropic Rayleigh wave phase-velocity perturbations at four periods. Perturbations are shown as percent deviation from PREM at each period. The orientations of the black lines represent the fast azimuth direction for 2ζ anisotropy and the length of the bar is scaled to the anisotropy strength. Peak anisotropy strength is (a) 2.9% at 25 s, (b) 2.5% at 50 s, (c) 2.1% at 75 s, and (d) 1.5% at 125 s.

Anisotropic Rayleigh wave phase-velocity perturbations at four periods. Perturbations are shown as percent deviation from PREM at each period. The orientations of the black lines represent the fast azimuth direction for 2ζ anisotropy and the length of the bar is scaled to the anisotropy strength. Peak anisotropy strength is (a) 2.9% at 25 s, (b) 2.5% at 50 s, (c) 2.1% at 75 s, and (d) 1.5% at 125 s.

Anisotropy and age dependence of the Pacific upper mantle

I use surface-wave tomography to investigate the structure and dynamics of the oceanic upper mantle. I perform anisotropic surface-wave tomography to make phase-velocity maps and three-dimensional models of the anisotropic elastic structure in the Pacific Ocean basin. Simple models of halfspace cooling predict seismic velocities that increase with age as the lithosphere cools and gets thicker; our anisotropic phase-velocity maps and 3D models are consistent with this cooling history and also show evidence for occasional reheating of the lithosphere.

Flow in the Earth’s upper mantle leads to the alignment of olivine crystals and the resulting anisotropy can be imaged with seismic techniques. Our three-dimensional models of seismic anisotropy can be used to investigate frozen-in anisotropy within the cooling lithosphere and anisotropy related to present-day flow within the asthenosphere. We find evidence for small-scale convection or pressure-driven flow below the base of the lithosphere.

Eddy, C. L., G. Ekström, M. Nettles, and J. B. Gaherty, Age dependence and anisotropy of surface-wave phase velocities in the Pacific, Geophys. J. Int., ggy438.


Surface-wave amplification across the continental United States

Although most seismic imaging makes use of travel-time or phase measurements from seismic waves, the amplitude of these waves also contains useful information about the structure of the Earth’s interior. In this work, I use measurements of surface-wave amplitude recorded on stations in the USArray seismic network to make maps of surface-wave amplification, a quantity that describes how much wave amplitudes are affected by elastic structure local to the station recording the wave. We have found that local structure can have a large effect on seismic wave amplitudes and that lateral variations in amplification have good spatial correlation with geologic features such as the Colorado Plateau or the Gulf of Mexico. Amplification can potentially be used as a complementary measurement to phase velocity when investigating the structure of the Earth’s upper mantle. 

Eddy, C. L. and G. Ekström, Local amplification of Rayleigh waves in the continental United States observed on the USArray, Earth Planet. Sci. Lett., 402 (2014): 50-57.

 Observed local Rayleigh wave amplification factors at periods of 35 s, 50 s, 75 s, and 125 s. Each symbol corresponds to one USArray station and the color represents the derived amplification factor.

Observed local Rayleigh wave amplification factors at periods of 35 s, 50 s, 75 s, and 125 s. Each symbol corresponds to one USArray station and the color represents the derived amplification factor.