## 2 Data, Methodology and Results  
The traces of 109 earthquakes (Supplement 1), registered by the station: RAYN (Supplement 2), have been analyzed to calculate the fundamental-mode surface-wave group velocities. The earthquakes' epicenters have been grouped into 56 source zones (Supplement 3), as described by [PERSON] et al. [7]. These source zones are defined as a location in which seismic events with similar epicenter coordinates have occurred, i.e., seismic events with a coordinate difference less than or equal to 0.5 degrees in latitude and longitude. Thus, a path coverage of the study area of 56 source-station paths of surface-wave group velocities is determined (Figure 1a). Figure 1b,c show that the number of paths of Love waves decreases more rapidly than that corresponding to the Rayleigh waves. The group velocities corresponding to these paths have been determined by means of a combination of the Multiple Filter Technique (MFT; [8]) and the Time Variable Filtering (TVF; [9]), as described by [PERSON] et al. [7]. Figures 2 and 3 show examples of the group-velocity calculation for the Love and Rayleigh waves, respectively. However, the methodology used to determine the seismic anisotropy [5], in the present study, considers phase velocities as observed data. Then, the phase velocities must be determined from their corresponding group velocities. This computation is performed as described by [PERSON] [6], and Figure 4 shows an example of the phase velocity computation for the group velocities shown in Figures 2c and 3c (magenta line). Figure 5 shows the Love and Rayleigh phase velocities (dispersion curves) determined for the events involved in the path S23-RAYN (Supplements 2, 3 and Figure 1a), and this method is followed to calculate the dispersion curves for all paths shown in Figure 1a, and these curves are fitted to the formula [5]. \[\mathrm{c}(\omega,\,\theta)=\mathrm{A1}(\omega)+\mathrm{A2}(\omega)\,\cos 2 \theta+\mathrm{A3}(\omega)\,\sin 2\theta+\mathrm{A4}(\omega)\,\cos 4\theta+ \mathrm{A5}(\omega)\,\sin 4\theta \tag{1}\]  
where \(c\) is the phase velocity, \(\omega=2\pi/\mathrm{T}\), \(\mathrm{T}\) is the period and \(\theta\) is the azimuth of the wave-number vector. Formula (1) expresses the azimuthal variation in Love and Rayleigh phase velocities through a set coefficients \(\mathrm{Ai}\) (with i varying from 1 to 5) for each type of wave: Love or Rayleigh. Figure 6 shows this azimuthal variation for the periods: 20, 40, 60 and 80 s; and Figure 7 shows the values of the coefficients \(\mathrm{Ai}\) with the period \(\mathrm{T}\). Figure 8a shows the error \(\varepsilon\) determined between the phase velocity \(\mathrm{cth}\) given by Equation (1) and the phase velocities \(\mathrm{cob}\) (the dispersion curves) determined for the paths shown in Figure 1a. This error \(\varepsilon\) is calculated by  
\[\varepsilon(\mathrm{T})=\sqrt{\frac{\sum\limits_{i=1}^{N}\left[\mathrm{c_{ob} (T)-c_{th}(T)}\right]^{2}}{N}} \tag{2}\]  
where N is the number of paths for each period \(\mathrm{T}\) (Figure 1b,c) and for each type of wave (Love or Rayleigh). The error \(\varepsilon\) reaches a minimum value at \(\sim\)40 s for Love and Rayleigh waves (Figure 8a), and from this value, it remains approximately constant with the period for Rayleigh waves (Figure 8a, blue line), while it increases with the period for Love waves (Figure 8a, red line), because the number of paths of Love waves (Figure 1b) decreases drastically with the period from \(\sim\)40 s. A new fit of the dispersion curves using Equation (1) is calculated considering only three coefficients \(\mathrm{Ai}\) (i varying from 1 to 3). Figure 8a (dashed lines) shows that the error \(\varepsilon\) is greater than that corresponding to the former fit, i.e., the fitting given by Equation (1) with five coefficients is better than that with three coefficients (Figure 8b).