# Crust and Upper Mantle Structure of Mars Determined from Surface Wave Analysis
## 3 Interpretation and Discussion  
**Depth range: 0-5 km. ** There is a heterogeneous shallow crust that is constituted by several types of rocks, including sedimentary and igneous. This shallow crust is characterized by a wide range of densities and seismic velocities due to the presence of altered basalts, mudstones, shales, clays, sands, and loess, and it is expected to exhibit seismic anisotropy, as determined in the present study (7% of S-velocity variation for the layer 1 of Table 1), due to the presence of sediments or lava flow layering and wide damage zones surrounding the long-term fault networks [14]. **Depth range: 5-15 km. ** The observed anisotropy at depths greater than 5 km is unlikely due to sedimentary layers, as in the previous depth range. For the present depth range, the presence of volcanic materials in the study area (Figure 4), such as andesite or altered basaltic rocks, could probably be the cause of the observed anisotropy (17% of S-velocity variation and \(\xi\sim\) 1.4 (layer 2 of Table 1)), which was also determined by [PERSON] et al. [3] for another region of the Martian crust with 12% of S-velocity variation and \(\xi\sim\) 1.3. The layering of lava flows that could still exist for the present depth range could also be associated with this seismic anisotropy. [PERSON] et al. [21] have suggested that layered silk complexes can produce strong anisotropy (>8% S-velocity variation) at 5-18 km depth, but these complexes are usually very localized in depth and spatial extent, and are unlikely to be the cause of the observed anisotropy for this study area. Another cause for this anisotropy could be the presence of layered intrusions due to a single or multiple impacts [4]; such impacts could have caused internal layering within the crust, and the existence of seismic anisotropy. **Depth range: 15-100 km. ** In Figure 8, it is observed that all S-velocity models determined for Mars show similar values down to a 90 km depth, while these models show very different values up to this depth. This result is expected for Mars, because its Moho depth shows a great diversity of values from very shallow depths (< 10 km) to 90-100 km, depending on the region [14]: the northern lowlands show a uniformly thin crust (\(\sim\)30 km thick), the southern highlands consist of a thick crust (\(\sim\)60 km thick), and the Tharsis Rise (at the boundary between these two domains) is formed by a thick basaltic crust (\(\sim\)100 km thick). For the study area (Figure 4), the Moho depth is determined at 50 km depth (Figure 8, black line), and this discontinuity is not sharp in terms of S-velocity, but a gradual transition from crust to mantle S-velocities, in which the S-velocity jumps from the crustal values (3.4 km/s) to mantle values (4.35 km/s) through an intermediate value (3.9 km/s) determined from 50 to 60 km depth. This Moho depth agrees well with that calculated in previous studies [6; 22; 23; 24]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline Layer & \(\alpha_{\mathrm{H}}\) & \(\beta_{\mathrm{H}}\) & \(\alpha_{\mathrm{V}}\) & \(\beta_{\mathrm{V}}\) & \(\xi\) & \(\varphi\) & \(\beta_{\mathrm{H}}-\beta_{\mathrm{V}}\) \\ (n) & (km/s) & (km/s) & (km/s) & (km/s) & (\(\beta_{\mathrm{H}}\)/\(\beta_{\mathrm{V}}\))\({}^{2}\) & (\(\alpha_{\mathrm{V}}\)/\(\alpha_{\mathrm{H}}\))\({}^{2}\) & \(\beta_{\mathrm{H}}-\beta_{\mathrm{V}}\) \\ \hline \(1\) & 4.35 \(\pm\) 0.12 & 2.51 \(\pm\) 0.07 & 4.07 \(\pm\) 0.14 & 2.35 \(\pm\) 0.08 & 1.14 & 0.88 & 6.7 \\ \(2\) & 5.53 \(\pm\) 0.16 & 3.19 \(\pm\) 0.09 & 4.69 \(\pm\) 0.14 & 2.71 \(\pm\) 0.08 & 1.39 & 0.72 & 17.1 \\ \hline \end{tabular}
\end{table} 
Table 1: S-velocities (\(\beta_{\mathrm{H}}\), \(\beta_{\mathrm{V}}\)) calculated from the anisotropic inversion of the phase velocities shown in Figure 7 (black rectangles) for the first two layers of the isotropic model shown in Supplement S4.