Figure 4. (a) Vertical section of the CRP image along 40°N.
Blue and red colors indicate reflectors with positive and negative
impedance contrasts, respectively. The color intensity is highest when
the impedance contrast is strongest. All record section profiles use
this color scale. (b) Depth of the 410-km discontinuity (top) and the
thickness of the mantle transition zone (bottom).
Figure 4a shows a vertical section of the CRP image along the 40°N
parallel. As expected from Figure 3, the 410-km and 660-km
discontinuities are the clearest reflectors. Variations of the Ss410s-S
and Ss660s-S difference times project as spatial variations in the depth
of 410-km and 660-km discontinuities. The 410-km and 660-km
discontinuities are deeper and more complex beneath the western US (west
of -100°E) than beneath the central and eastern US. This is also
apparent in other sections through the CRP images shown in Supplementary
Figure S4. The 410-km discontinuity is strongest between longitudes
-100°E and -75°E. The 520-km discontinuity may be responsible for a
relatively weak Ssds reflection between the 410-km and 660-km
discontinuities. The CRP images near the 410-km and 660-km
discontinuities west of -100°E are complex, which was also noted by
SB19. Strong reflectors corresponding to the Ssds signals in region B of
the record section of Figure 3 are mapped at about 100 km and 150 km
depth, but their depths and strengths vary. The incoherent structures at
depths larger than 800 km are most likely imaging artifacts because
these structures correspond to the amplified signals in region C of
Figure 3, where S are diffracting waves and the slowness resolution is
relatively poor.
Figure 4b shows maps of the depth of the 410-km discontinuity and the
thickness of the MTZ. These are estimated from the absolute minimum
values of the mean displacements in the CRP image in the depth ranges of
350–470 km (for the 410-km discontinuity) and 620–730 km (for the
660-km discontinuity) by cubic spline interpolation. We do not estimate
the depth of the 410-km and 660-km discontinuities where a secondary
absolute minimum is stronger than 40% of the absolute minimum in these
depth ranges. The depth of the 410-km discontinuity varies by 40–50 km.
The 410-km discontinuity is deepest beneath the southern Basin and Range
and the Colorado Plateau and shallowest beneath the central plains and
the Atlantic coast. The thickness of the MTZ varies less than 10 km
because the 410-km and 660-km discontinuities depth variations are
similar. The MTZ is thinnest beneath California and thickest beneath the
Southern Rocky Mountains and the Colorado plateau. The MTZ thickness is
anomalous in small regions near the margins of our model domain. This
includes the extremely thin (210 km) MTZ beneath the west coast of
central California which was also resolved by SB19. However, the CRP
images have low resolution here because the data coverage is poor.
SB19 resolved similar maps as Figure 4b, indicating that our and SB19’s
data sets contain consistent variations of the Ss410s-S and Ss660s-S
difference times and that estimates of the depths of the 410-km and
660-km discontinuity do not strongly depend on the applied mapping
method.
The S410S and S660S precursors interfere with the reverberations (Figure
3). To test their influence, we compare CRP images with and without
seismograms for which the reflecting time falls within 50 s of either of
the precursors (Supplementary Figure S6). The maps of the depth of the
410-km discontinuity and the MTZ thickness are similar so interference
with precursors has a minor effect on the imaging results. The large
difference near the Pacific coast is due to lack of data coverage.