## 3 Results and Discussion
### Slope Erosion Versus Seismic Signals and Acoustic Signals (Baseline Case)  
In the Baseline Case, significant changes in seismic and acoustic signals were observed during Stage 3, compared to Stages 1 and 2. Observing Figure 4 reveals that the majority of slope damage due to wave erosion occurs in Stage 3. Figure 14a-f present the analysis results of seismic and acoustic signals. To understand the reasons behind the changes in the signals, we examined the variations in spectral magnitude and Power Spectral Density (PSD) (Figure 14c,f) and compared them with the erosion ratio, run-up height of waves (H\({}_{\rm w}\)), and retreat (Figure 14g). We also correlated these with video observations of the wave erosion behavior and slope damage over time. Figure 14b is the time-frequency spectrum of the seismic signal, and the spectral magnitude profile of 311 Hz is shown in Figure 14c. Figure 14e is the time-frequency spectrum for the acoustic signal; because the red and yellow energy traces are concentrated around 361 Hz during 4600 s-4800 s, we took the 361 Hz cross-section of the time-frequency spectrum, obtaining the spectral magnitude profile for the acoustic signal (Figure 14f). To observe the changes in energy from 0-600 Hz, we calculated the PSD for seismic and acoustic signals, respectively (Figure 14c,f). Figure 12: Duration: 4500 s–4600 s in Stage 3: (**a**) acceleration, Acc 2; (**b**) time–frequency spectrum; (**c**) spectral magnitude profile of 311 Hz (the red arrows represent the timing of collapse). Figure 13: Series front images of the slope model during 4500 s–4600 s (the red dotted lines represent the collapsing area). the spectral values and PSD of the acoustic signal (Figure 14f). This may be because the sound of soil particles rubbing against each other and the sound of wave erosion on the slope was relatively quiet during slope sliding. During 4188 s-4371 s, both H\({}_{\mathrm{w}}\) and the erosion ratio rose (Figure 14g), indicating that as H\({}_{\mathrm{w}}\) increased, the range of wave erosion also increased. This led to more soil being eroded away by the waves, raising the erosion ratio. Generally, it is expected that as the erosion ratio rises, both seismic and acoustic signals would increase. However, our test showed that the spectral magnitude and PSD of the signals (Figure 14c,f) did not increase correspondingly with the erosion ratio (Figure 14g). This is because the soil carried away by wave backwash during 4188 s-4371 s was just the soil that previously slid and remained on the slope surface. These soil masses were loosely structured, and the wave swash could erode and carry them away without generating impact. This is also why, during 4188 s-4371 s, the spectral magnitude and PSD of seismic and acoustic signals did not rise, while the erosion ratio continued to increase. Figure 15 shows the changes in slope profile during 4176 s-4438 s. According to Figure 15a-c, as the waves erode, the upper part of the slope gradually forms a steep cliff due to the decreasing amount of \"talus deposits\". The term \"talus deposit\" refers to materials from the upper slope that accumulate on the surface after sliding and/or collapsing, without being backwashed into the water. As shown in Figure 15a, the talus deposits covering the slope surface had previously experienced sliding but had not yet been eroded away due to the limited range of wave erosion. Because this talus deposit had experienced sliding, its structure was loose and easily eroded away as the range of wave erosion increased due to H\({}_{\mathrm{w}}\) rising (Figure 15b), even if the force of wave erosion was weak. This helps further explain why the energy of seismic and acoustic signals did not increase at high erosion ratios during 4188 s-4371 s. As most of the talus deposits were eroded away (Figure 15a,b) during 4176 s-4371 s, waves subsequently began to erode previously unaffected areas of the slope.