## 3 Results and Discussion
### Slope Erosion Versus Seismic Signals and Acoustic Signals (Baseline Case)
Since the type of wave erosion began to change from swash to impact after 4371 s, this caused the spectral magnitude and PSD of both seismic and acoustic signals to begin to rise (Figure 14c,f). During 4600 s-4800 s, the wave erosion was in the form of impact, and the sound of wave erosion was noticeably louder during this period. This illustrates that the yellow and red energy traces in the time-frequency spectrum (Figure 14b,e) are caused by wave impact. Compared to swash-type wave erosion, impact-type wave erosion causes more significant vibrations on the slope and also produces louder sounds. We found that as H\({}_{\mathrm{w}}\) decreased, the PSD of both signals also decreased. Observing Figure 14c,f,g, during 4600 s-4800 s, the PSD of the seismic signal began to decrease at 4670 s, followed by a decrease in H\({}_{\mathrm{w}}\) at 4701 s, and finally, the PSD of the acoustic signal decreased at 4739 s. The timing differences of approximately 1-2 min are relatively minor compared to the entire duration of Stage 3, and the times of decline for the three factors (PSD of the seismic signal, PSD of the acoustic signal, and H\({}_{\mathrm{w}}\)) are closely aligned. This reflects that a decrease in H\({}_{\mathrm{w}}\) reduces both the force of wave erosion on the slope and the energy of the sound. Figure 14g shows that H\({}_{\mathrm{w}}\) first decreases and then increases, and the erosion ratio changes little during 3924 s-4188 s. Generally, each time H\({}_{\mathrm{w}}\) surpasses its previous maximum value, it indicates that a new range of the slope is being eroded. During 4188-4744 s, although H\({}_{\mathrm{w}}\) gradually rose, the erosion ratio only steadily increased. This indicates that the erosion rate during this duration is about the same, because the slope of the curve of the erosion ratio is approximately the same (as the light blue dashed line). So, although H\({}_{\mathrm{w}}\) increased, the erosion rate did not correspondingly increase. After 4744 s, the erosion ratio slowly changed from 16% to the final 16.5%. This indicates that after 4744 s, the force of wave impact on the slope had significantly reduced, causing only a small amount of erosion, and the erosion rate approached zero. This caused the curve of the erosion ratio to gradually level off, and the slope's profile tended toward a steady state. It is defined such that the slope reaches a state of equilibrium profile. As shown in Figure 15a-c, as the talus deposits gradually decreased due to swash, the top of the slope began to gradually form a cliff. In this process, the disappearance of the protection from the talus deposits relative to the top of the cliff meant that the cliff had a tendency to slide; for example, the sliding area is the yellow dashed zone shown in Figure 15a-e. When the cliff's profile is formed and the range of wave erosion can affect the stability of the cliff, the top of the cliff is prone to sliding or collapsing (Figure 15d-f). This section can be summarized as follows. (1) Intense wave swash and impact on the slope surface will increase seismic and acoustic energy. (2) Despite increases in H\({}_{\rm w}\) and erosion ratio, the spectral values and PSD of seismic and acoustic signals did not rise correspondingly. This was attributed to the erosion of loosely structured soil that had previously slid and remained on the slope without impact. (3) As erosion changed from swash to impact, there was a noticeable increase in the spectral values and PSD of both seismic and acoustic signals. (4) Wave erosion occurred through impact, producing louder sounds and greater seismic activity than swash erosion. (5) Despite the gradual increase in H\({}_{\rm w}\), the erosion rate remained roughly the same, and the erosion rate did not increase correspondingly with H\({}_{\rm w}\). (6) The turning points of the decrease in vibration and sound energy indicate that the force of wave erosion on the slope began to gradually diminish, due to the decrease in the run-up height of the wave (H\({}_{\rm w}\)).