Figure 7 . (a) Maps of the shear-velocity variation at (top) 150
km and (bottom) 500 km depth according to S40RTS. The east-west contrast
across the US is similar for SEMUCB-WM1 and TX2015. (b) Harmonic
undulations of the 410-km and 660-km discontinuities for (from top to
bottom) models T8, T5, and T2 with spatial wavelengths of 8°, 5°, and
2°. (c) Distribution of hypothetical earthquakes (stars) and stations
(circles). For models PREM, S40RTS, T8 and T5 we compute waveforms for
the twelve earthquakes indicated by red stars. For T2, we compute
waveforms for these earthquakes and the additional 36 earthquakes
indicated by black stars.
For each of the eight structures, we compute waveforms at periods longer
than 10 s for 462 stations in a rectangular 2° 2° grid between
longitudes 130°–65°W and latitudes 25°–50°N (Figure 7c). We calculate
waveforms for 12 earthquakes uniformly distributed at a distance of 75°
from [-100°E, 40°N]. We use 48 earthquakes distributed in a spiral
for structure T2. All earthquakes have the same dip-slip source
mechanism (source parameters for event 080596G in the Global CMT
catalog) to ensure strong radiation of SH waves to teleseismic stations.
The uniform data coverage is sufficient to investigate the effects of
velocity heterogeneity on Ssds-S traveltimes and the resolution of
undulations of the 410-km and 660-km discontinuities using long-period
Ssds reflections. Because of the high computational cost, we cannot
afford to reproduce the source-station combinations in the data and,
therefore, we cannot estimate CRP mapping artifacts due to inhomogeneous
slowness and azimuthal sampling.