JGR - Solid Earth
Supporting Information for
Validation of recent shear-wave velocity models in the United States with full-wave
simulation
Haiying Gao1 and Yang Shen2
1
Department of Geosciences, University of Massachusetts Amherst, Amherst, MA 01003,
USA, email: haiyinggao@geo.umass.edu.
2
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882,
USA.
Contents of this file
Text S1
Figures S1 to S5
Introduction
The supplementary material briefly discusses the effect of the crustal structure on the
accuracy of the finite-difference waveform simulation used in this study.
Text S1.
Seismic tomographic results are sensitive to the shallow crustal structure, especially for
shorter wavelength waves (e.g., Capdeville and Marigo, 2007 and 2008; Qin et al., 2008;
Lekic et al., 2010). None of the five models selected in our model validation provides
well-constrained crustal structure and distribution of sedimentary layers, even though all
provide velocities starting from the surface. Because the point of this paper is to evaluate
how the structures in the models predict the observation, the question of a thin sediment,
though interesting, is not directly relevant for velocity models that do not include such a
thin sediment layer. In this study, we simply interpolate the original models in a linear
way to each grid points for the model configuration and wave propagation simulation,
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which may not accurately account for the very shallow crustal layer (e.g., Qin et al.,
2008). Considering the uncertainty in the shallow crustal structure and with the horizontal
grid spacing of ~3 km, the shortest period of surface waves in our wave simulation is set
to be 15 s. Nevertheless to put a bound on the effects of finite grids in models with a thin
sediment layer on the phase arrivals, we carried out simulations with a fine grid (500 m
horizontal grid spacing and 150 m vertical grid spacing near the surface) and a coarse
grid (3 km horizontal grid spacing and 1 km vertical grid spacing near the surface) for a
model with a 500-m thick sediment layer and compared the waveforms at 15-75 s period
range. The results (see examples in Figures S1-S4) show that the sedimentary layer has
only small effects on the phase arrival times for Rayleigh waves longer than 25 s periods.
This is due to the long wavelength of surface wave, relatively low sensitivity of
intermediate- to long-period Rayleigh waves in the shallow depth, and the harmonic
averaging of elastic moduli at a subgrid scale (Moczo et al., 2007) that accounts for, to a
large extent, the elastic effect of the sediment by the coarse grid. Thus, the use of the
coarse grid for a model with a shallow sedimentary layer will likely shift the trend of the
phase delays slightly (see Figure S5), but will not change the general patterns among the
models. Shorter than 25 s periods, the waveforms appear more complicated with the
existence of a sedimentary layer, especially for inter-station distance within the range of
~200-300 km (see the example in Figure S3), which may indicate the overlapping of S
wave with Rayleigh waves. Thus both the unaccounted or inaccurately accounted
sedimentary layer in the models and the coarse grids may contribute to the larger scatter
of phase delays at short periods. At larger inter-station distances, the effect of body wave
becomes smaller as it decays faster with distance compared to Rayleigh waves. Also a
larger travel time separates S wave and Rayleigh waves (see Figure S4), providing a clear
signal window for Rayleigh waves.
2
Figures S1-S5.
Figure S1. Comparison of synthetic waveforms for station pair of IU.COR.TA.H04A for
a model with a 500-m thick sedimentary layer and calculated with a coarse (3 km
horizontally, red lines) and a fine (500 m horizontally, black lines) grid, respectively.
Figure S2. Same as Figure S1 except for station pair of IU.COR.TA.H05A with a 165-km
inter-station distance.
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Figure S3. Same as Figure S1 except for station pair of IU.COR.TA.I06A with a 256-km
inter-station distance. The synthetic waveform at 15-35 s periods appears to be more
complex for the finer grid, which may indicate overlapping between S wave with
Rayleigh waves.
Figure S4. Same as Figure S1 except for station pair of IU.COR.BK.WDC with a 450-km
inter-station distance. As shown, the larger distance allows the separation of S wave and
Rayleigh waves at shorter periods.
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Figure S5. Phase delay versus source-receiver distance for the synthetic waveforms of a
model with a 500-m thick sedimentary layer calculated with a coarse (3 km horizontally)
and a fine (500 m horizontally) grid, individually, as demonstrated in Figures S1-S4.
Waveforms are filtered at three period ranges. The source-receiver distance is required to
be at least 1.5 times of the wavelength. The larger phase delays at periods of 25-50 s and
15-35 s within ~200-300 km inter-station distances result from the difficulty of separating
S wave with Rayleigh waves. Longer than 300 km, the phase delays at all the three period
ranges are well constrained.
References
Capdeville, Y., and J.-J. Marigo (2007), Second order homogenization of the elastic wave
equation for non-periodic layered media, Geophys. J. Int., 170, 823-838.
Capdeville, Y., and J.-J. Marigo (2008), Shallow layer correction for Spectral Element
like methods, Geophys. J. Int., 172, 1135-1150.
Lekic, V., M. Panning, and B. Romanowicz (2010), A simple method for improving
crustal corrections in waveform tomography, Geophys. J. Int., 182(1), 265-278.
Moczo, P., J. O. A. Robertsson, and L. Eisner (2007), The finite-difference time-domain
method for modeling of seismic wave propagation, in Advances in Wave
Propagation in Heterogeneous Earth, 421–516, 48, eds Wu, R.-S., Maupin, V. &
Dmowska, R., in the series Advances in Geophysics, Elsevier/Academic Press, San
Diego.
Qin, Y. L., Y. Capdeville, V. Maupin, J.-P. Montagner, S. Lebedev, and E. Beucler
(2008), SPICE benchmark for global tomographic methods, Geophys. J. Int., 2, 598616.
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