Mapping an earthquake-induced landslide based on UAV imagery; case study of the 2015 Okeanos landslide, Lefkada, Greece

On November 17, 2015 07:10 UTC an earthquake of Mw 6.5 induced slope failures mainly at the western part of the island of Lefkada, Ionian Sea, Greece. The most characteristic one is the Okeanos site deep-seated landslide (near village Athani) that affected the man-made environment; part of the site, including a dirt road, moved downwards along the coastal cliff and a luxury hotel suffered heavy damage. The goal of this study is to map in detail the Okeanos landslide and to provide relevant quantitative data i.e. structural data and volume of both the accumulated and removed material. In order to achieve this, a comparison between pre-earthquake and post-earthquake point clouds generated from aerial imagery and UAV-acquired photos took place. Using Structure from Motion, the steep coastal cliff site was surveyed in detail, even in areas that are difficult to approach. Results from this study show that the total landslide area covers a surface of ~36,000 m2, while a removed volume of ~93.000 m3 of landslide material was evaluated using change detection techniques. In addition, fieldwork and survey data revealed that the sliding surface was a pre-existing fault surface upon which the geological material moved downwards the coastal cliff. The sliding surface dips west and is exposed along a 115 m long and 25-30 m high section scarp. As a conclusion, this study underlines the importance of a thorough geological study in order to identify landslide prone areas prior to any building activity, and shows that a UAV survey is capable to provide adequate information during a post-earthquake field survey for co-seismic landslide mapping and modeling.