Abstract
Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave data set near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S‐to‐P and P‐to‐S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0–10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well‐resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (<6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0–60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.
Highlights
The relatively short and simple history of the ocean lithosphere makes it the ideal place to study the plate
Reflected and converted seismic phases image a strong, sharp discontinuity, typically !6% drops in seismic velocity with depth over
The multilayer model has a strong peak in azimuthal anisotropy of up to 6% relative to the VSV model at 26 km depth, decreasing to 2–4% from 50 to 200 km depth, rapidly decaying with depth
Summary
The relatively short and simple history of the ocean lithosphere makes it the ideal place to study the plate. First-order observations such as bathymetry and inverse heat flow increase as expected for a cooling plate, i.e., according to the square root of age [Turcotte and Oxburgh, 1967] at least beneath seafloor
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have