Summary
Long-period (T > 10 s) shear-wave reflections between the
surface and reflecting boundaries below seismic stations are useful for
studying phase transitions in the mantle transition zone (MTZ) but
shear-velocity heterogeneity and finite-frequency effects complicate the
interpretation of waveform stacks. We follow up on a recent study by
Shearer and Buehler (2019) (SB19) of the top-side shear-wave reflection
Ssds as a probe for mapping the depths of the 410-km and 660-km
discontinuities beneath the USArray. Like SB19, we observe that the
recorded Ss410s-S and Ss660s-S traveltime differences are longer at
stations in the western US than in the central-eastern US. The 410-km
and 660-km discontinuities are about 40–50 km deeper beneath the
western US than the central-eastern US if Ss410s-S and Ss660s-S
traveltime differences are transformed to depth using a
common-reflection point (CRP) mapping approach based on a 1-D seismic
model (PREM in our case). However, the east-to-west deepening of the MTZ
disappears in the CRP image if we account for 3-D shear-wave velocity
variations in the mantle according to global tomography. In addition,
from spectral-element method synthetics, we find that ray theory
overpredicts the traveltime delays of the reverberations. Undulations of
the 410-km and 660-km discontinuities are underestimated when their
wavelengths are smaller than the Fresnel zones of the wave
reverberations in the MTZ. Therefore, modeling of layering in the upper
mantle must be based on 3-D reference structures and accurate
calculations of reverberation traveltimes.
Keywords : Composition and structure of the mantle; Phase
transitions; North America; Body waves; Computational seismology.