## 2 Materials and Methods
### Materials  
#### 2.2.1 Remote Sensing Data  
In the present work, the western cliffs of Lefkada Island were investigated using remote sensing techniques. In fact, the utilization of such data was considered imperative as the study area covers a 20 km-elongated part of the island. Multi-sensor data were selected due to the fact that very high-resolution data are not freely available nor can they be acquired on a regular basis, such as medium resolution imagery (Landsat and Sentinel-2 MSI).  
Figure 3: (**a**) Panoramic view of Egremni coastal area from the north to the south. Mass-wasting materials and rocks detached from the cliff are laying on the beach. (**b**) A close-up photo of the coast facing to the east. Several rocks detached from the upper cliff are accumulated on the coast.  
In this framework, our dataset was consisted of: (a) medium resolution imagery acquired from Landsat satellites and (b) high-resolution data, such as analogue aerial photographs on a 1:10,000 scale acquired by the Hellenic Military Geographical Service (HMGS), digital orthomosaics of the years 2008 and 2016 obtained from the National Greek Cadastre and Mapping Agency, as well as IKONOS and Pleiades satellite imagery (Table 1). In the following paragraph, more details have been provided regarding the processing of these different datasets. Specifically, Landsat images obtained from June to August of the years 2003, 2004, 2015, and 2016 were imported and processed onto Google Earth Engine (GEE). This specific acquisition period was selected due to the lower cloud coverage. Regarding the high-resolution data, the older set of data used was an IKONOS bundle image acquired in 2000; the spatial resolution was 1 m and the image was processed in Leica Photogrammetry Suite. The next dataset, dating back to 2004, was composed of twenty analogue airphotos obtained from the HMGS. The airphotos were scanned in Tagged Image File Format (TIFF) in 1200 dots per inch (DPI) without compression and afterwards they were orthorectified using a suitable bloc file created in LPS. To orthorectify the IKONOS imagery and the airphotos, an orthomosaic and a digital surface model (DSM) of the Greek cadastral were utilized. These specific products, dated back to 2008, are available upon request and they constitute the more accurate official dataset for the Greek territory. In addition, an orthorectified Pleiades bundle imagery was purchased from Airbus. The spatial resolution of the data was 0.5 m and the acquisition date was 13 June 2015, i.e., five months before the earthquake. Finally, a Pleiades stereo pair was obtained from the Enceladus Hellenic supersite. The stereo pair was acquired on 13 October 2016, i.e., a few months after the earthquake. There are both panchromatic and multispectral data with 0.5 and 2 m spatial resolution. The specific imagery covers the whole island and it was also processed in LPS using ground control points from the precise cadastral products and the fieldworks. Ground control points were measured using a Leica GS08 Real Time Kinematic sensor. The processing of the Pleiades stereo pair produced an orthophoto with a spatial resolution of 0.5 m and a DSM with a 1.5 m spatial resolution. This specific DSM was down-sampled in order to be comparable with the respective one from the Greek cadastral. The remote sensing data, as well as their sources and spatial resolution, are described in Table 1. More details about data processing via LPS are given in previous studies [20,49].