## 4 Discussion
According to the authors of [60], the geomorphological traces of past earthquakes could be erased, even over historical timescales, so, a holistic research should combine among others: seismic hazard analysis, geotechnical site investigations, and regional earthquake-induced landslide susceptibility analysis. Based on this point of view, the major improvement of the current study relies on the fact that we evaluated the automatically derived results from the medium resolution data (Landsat imagery on GEE) with very high-resolution imagery (Ikonos and Pleiades) and in situ measurements from the sediment trap. Another important asset of this research is that we managed to define the minimum seismic threshold that affects the integrity of the cliff. In addition to the effects of 2015 earthquake, we also investigated the diachronic impact of the seismicity on the evolution of the western coast of Lefkada. In light of this, we have compared high-resolution satellite data (IKONOS bundle imagery) and airphotos before and after the 6.2 Mw earthquake, which occurred on 14 August 2003. As shown in Figure 23, significant topographic changes can also be detected on the Egemni cliff. Specifically, the left part of Figure 23 represents the coast of Egemni after the 2003 earthquake, while the right part corresponds to the same area in 2000. The slope has transformed in a steeper manner in the left image. The crosshair points out the same position on the cliff. The inclination of the cliff has been increased on the 2004 orthophoto. Additional photos of the coastal evolution can be found in the Supplementary Materials. Summarizing the outcomes of the current study, the evolution of the western cliff of Lefkada Island is directly related to the two strong earthquakes, which occurred in the north end and close to the south, respectively. Currently, this area is affected by minor earthquakes, i.e., magnitudes \(<4.1\). Although these earthquake magnitudes are considered weak to trigger mass movements, they are an exception for this specific area. The analysis of the sediment trap samples proved that such magnitudes affect the west coast, which remains active in terms of the sliding phenomena. Figure 24 depicts the trap sediment concentration along the four semesters of observation in accordance with the recorded precipitation and seismicity. As can be observed, earthquakes greater than 3.0 occurred throughout the investigated era. On the contrary, 4.1 M earthquakes were recorded during the first and the fourth semesters, when the sediment concentration was  
Figure 23: Relief changes on the Egemni cliff after the 2003 earthquake. On the **left** part, an orthophoto mosaic of the coast of Egemni after the 2003 earthquake is presented, while the **right** part corresponds to the same area in 2000. The slope has transformed in a steeper manner in the left image. quite high. Although the highest precipitation was observed during the third semester, sediment accumulation remained low. Hence, seismicity is the main triggering factor underlying the geomorphological evolution of the cliff in this specific area. It is a commonplace that strong earthquakes trigger landslides on a national [31; 63] or global level [64]. However, how low-magnitude earthquakes (i.e., M < 5.0) affect the stability of pre-existing or newly formed landslides has not been well investigated. In some recent publications, such as a study carried out in Norway, it was demonstrated that a critical magnitude of >4.5 in combination with precipitation and ground shaking can cause landslides [65]. Even smaller magnitudes, i.e., >2.5, in cases of soft sediments can provoke mass movements, as it was observed in a respective case study in Poland [66]. Regarding the national level, i.e., Greece, the threshold intensity was as low as 4.0 in the case study of the Pyrgos 1993 earthquakes [67]. These aforementioned observations are in line with the results of the current research, which attest that the 4.1 magnitude earthquakes are critical in increasing the ravel activity in the area.