Post-seismic monitoring of cliff mass wasting using an unmanned aerial vehicle and field data at Egremni, Lefkada Island, Greece

Abstract

On 17th November 2015, a strong earthquake affected Lefkada Island, Greece, leading to extensive translational debris slides along the cliff overhanging Egremni beach and the re-activation of previously deposited slope material. The cliff is crossed by the Cephalonia Fault Zone, with mass-wasting processes inducing dry flows occurring in association with long-term tectonic activity. Large-scale co-seismic and post-seismic disturbance of the forest on top of the cliff has also resulted from the seismicity. Three years after the 2015 event, the cliff had retreated due to active translational debris slides and the forest disturbance, which represents a safety concern for the tourist beach below. We aimed to qualitatively assess the transient evolution of debris slides at Egremni and clarify the level of risk posed to the beach below. Annual monitoring was performed between 2015 and 2018 including extensive field measurements and unmanned aerial vehicle (UAV) flight campaigns. Based on the high-resolution images acquired by the UAV, we created orthophoto maps and digital surface models (DSMs) of the cliff using Structure-from-Motion (SfM) photogrammetry. The cliff slope models indicate that the overhanging cliff is progressively entering a stage of decline and approaching equilibrium due to climatic forcing alongside minor seismicity. The DSMs were combined with field survey data to further investigate the role of erosion and deposition across the cliff. Our results indicate that the cliff dynamics at Egremni are primarily controlled by earthquake- and storm-induced debris slides, with rock material moving within the cliff formation via rock falls, and cliff inclination decreasing as a result of primitive slope ‘self-healing’ process. Mass-wasting processes remained widespread across the western part of the Lefkada coast three years after the 2015 earthquake sequence. For safety reasons, we suggest that pre-emptive inspection of the cliffs should be carried annually, after humid and winter periods, and following strong earthquakes using a combination of field surveys and UAV campaigns.