SUPPLEMENTARY MATERIAL
S1.
Resolution test for phase velocity
A checkerboard test has been carried out to test the spatial resolution of the phase
velocities at each period. We constructed an initial velocity model with a velocity
perturbation of 10% from the phase velocity of the AK135 model for this period (Figure
S1a). The cells of the checkerboard velocity model are 2ºx2º and 4ºX4º for longer
periods. The position of the same sources used in the study is used to calculate the
synthetic incoming phase and amplitude of Rayleigh waves at each station.
The synthetic data has been inverted using the same methodology applied to the real data
to recover the phase velocities. Resulting 2D phase velocity maps are shown in figure S1.
The pattern of the velocity anomalies is well recovered for the area covered by the
stations. Outside, the pattern is smeared, especially in the southeast border of the study
area where there is a lack of stations.
S2.
Estimation of the error of the LAB depth
We create a set of modified 1D shear velocity models by incrementing the LAB depth on
a 1D model at a selected node (figure S2a and S2b). The dispersion curve of each new 1D
model has been calculated and compared with the observed one with measurement
uncertainties. For changes of the LAB depth greater than 15 km, the dispersion curves
exceed the uncertainties of the observed values. The range of variations in the LAB depth
that do not exceed the uncertainties of the observed values increases when the gradient
diminishes.
We also run the inversion using the 1D models with the modified LAB depth as an initial
model (figure S2c and S2d). The inversion pushes up the LAB depth to the position of
the unperturbed 1D model in order to fit the observed dispersion curve. The models
resulting from the inversion converge to the LAB depth of the unmodified model with
differences less than 20 km. This indicates that the position of the LAB in depth is well
constrained by the data.
Figure S1: Resolution test for the Rayleigh wave phase velocities. Input (left panel) and
recovered models at different periods are shown. Initial model is created by perturbing
the average phase velocity +/-10% in 2ºX2º anomalies (top panel) and 4ºX4º anomalies
(bottom panel). The dashed line indicates the limits of the area with good resolution.
Figure S2: a) and b) Calculated dispersion curve (top) for each of the 1D perturbed
velocity models (bottom). c) and d) Recovered 1D velocity model (bottom) after
inversion using as a initial model different perturbed models (bottom). a) and c)
correspond to the same, as well does b) and d). Triangles on the dispersion curves
represent the observed phase velocity with the error bars indicating their uncertainties.