Method

We use linear and semi-enclosed arrays of 48-4.5 Hz vertical geophones separated every 5 and 2.5 m to conduct active source and ambient noise seismic acquisitions (Figure 1). From active seismic source records, 24 sources using a sledgehammer, we produce in-depth images of P wave velocity (Vp) and S wave apparent velocity (Vs) using seismic refraction and spectral analysis of surface waves (SW) methods, respectively. A first approximation of Vp distribution (using the linear array) was obtained using the seismic refraction method. Subsequently,  the area inside the semi-enclosed array was discretized with cells proportional to receiver separation. Then, the refracted time arrivals at each cell were linearly adjusted to obtain the slope inverse (Vp) and the intercept time to estimate the bedrock depth \cite{cardenas2022pseudo}
In the case of Vs, we calculate dispersion curves \cite{herrmann2013computer} of all refraction records with a source-receiver distance greater than 15 m, and we use the group velocity times to elaborate tomographies in the frequency range of 6 to 24 Hz using the open-source libraries of \cite{rucker2017pygimli}. Similarly,  ANT images were calculated in the same frequency range. To do this, we normalize the records by 1-bit and spectral whitening \cite{bensen2007processing} and conduct cross-correlations between all pairs of receivers in 8s time windows over 30 min to obtain the so-called EGFs \cite{shapiro2004emergence}. Subsequently, we stack acausal and causal parts of these functions and get group velocity dispersion curves.

Results

Figure 2 shows a SRT of the linear array on the apparently most stable side of the park. Deposits of soft materials are observed at the lower part of the slope with Vp values less than 600 m/s (typical velocities of weathered materials). The line extension allows defining a second layer with an irregular structure with poorly consolidated materials (Vp=800 m/s). Vp values greater than 1200 m/s can be associated with the bedrock at depths greater than 10 m. The materials susceptible to sliding correspond to a filling deposit, which is highly saturated.