Mapping an earthquake-induced landslide based on UAV imagery; case study of the 2015 Okeanos landslide, Lefkada, Greece
Introduction
Documentation of landslides is a very critical issue since based on the accuracy of the provided information, protection and mitigation measures can be designed. This effort comprises to a detailed description of the basic geomorphological features e.g. detection fissures, edge of landslide area, rate of displacement, scarp etc. In addition, quantitative data such as the landslide area and the volume that was removed are also evaluated.
However, it is well known that mapping of these features is not always feasible due to several adverse factors e.g. near-vertical slopes. In order to overcome this issue, remote sensing techniques were applied during the last decades. In particular, Interferometric Synthetic Aperture Radar (InSAR), Light Detection and Ranging (LiDAR), Terrestrial Laser Scanners (TLS) and photogrammetric surveys are used for geomorphic mapping in order to quantify landslide processes (Haugerud and Harding, 2001; Schulz, 2004, Schulz, 2007; Scaioni et al., 2014; Lindner et al., 2016). The latter one (i.e. photogrammetric survey) is frequently based on Unmanned Aerial Vehicles (UAV) that uses flexible multicopters equipped with webcams, digital cameras and other various sensors (Colomina and Molina, 2014; Lindner et al., 2016; Rossi et al., 2017; Yu et al., 2017). UAVs provide a convenient remote sensing platform for landslide studies given their ability to collect ultra-high resolution imagery over terrain that is often difficult to access (Lucieer et al., 2014, Rothmund et al., 2017).
Having captured images of the landslide area using an UAV, the so-called Structure from Motion (SfM) algorithms can be combined with UAV for the landslide mapping and monitoring (Turner et al., 2015; Casagli et al., 2017). In particular, based on the SfM image processing technique, which is considered as a cost-effective and efficient means to acquire dense and accurate data of Earth's surface (James and Robson, 2012; Lucieer et al., 2014; Carrivick et al., 2016), a 3D point cloud can be created by intersecting the matched features between the overlapping offset images. These 3D point clouds are generated in a relative image-space coordinate system, which must be aligned to a real world co-ordinated system based on a small number of known ground-control points of known coordinates (Westoby et al., 2012). Afterwards, a Digital Surface Model (DSM) and orthomosaics can be exported based on the dense point cloud (Lucieer et al., 2014). Three dimensional measurements of landslides, such as the creation of DSM, allow volume displacement to be calculated and are useful for predicting future movements (Turner et al., 2015).
This study examines in detail an earthquake-induced landslide that was triggered by the November 17, 2015 earthquake, which occurred in the island of Lefkada, Greece. The November 17, 2015 07:10 UTC earthquake (Mw 6.5, depth 10.7 km) occurred on a near-vertical strike-slip fault with dextral sense of motion. The fault plane strikes N20 ± 5°E, dips towards east with an angle of about 70–80° and is part of the Cephalonia Transform fault zone (Ganas et al., 2016; Sokos et al., 2016; Chousianitis et al., 2016; Melgar et al., 2017; Papathanassiou et al., 2017a; Avallone et al., 2017). The epicenter of the earthquake (38.6755°N, 20.5930°E) was located at the southwestern part of Lefkada island between the villages Athani and Agios Petros (Fig. 1). The generated strong ground motion was recorded at Vassiliki (Fig. 1) and the city of Lefkada at the instruments of EPPO-ITSAK (Earthquake Planning and Protection Organization- Institute of Engineering Seismology & Earthquake Engineering) with values of Peak Ground Acceleration (PGA) equal to 0.36 g and 0.1 g, respectively. The dominant earthquake-induced geological effects were related to slope failures and particularly, rock falls and slides, and shallow and deep-seated landslides that were mainly documented at the western part of the island as well as at its central area, on both natural and cut slopes. The denser concentration of these phenomena was found at the coastal zone from Porto Katsiki to Egremnoi-Gialos beach and along the 6 km long coastal road of Tsoukalades - Agios Nikitas (Papathanassiou et al., 2017a; Fig. 1 and Fig. 2). At the eastern part of the island and particularly along the road from Nydri to Vassiliki, few slope failures including small size rock-falls and planar slides were mainly reported at road cuts. For a detailed description of the earthquake-induced geological failures, the reader is referred to Papathanassiou et al. (2017a). Furthermore, it is important to highlight that at the zones of Egremnoi and Gialos (Fig. 1, Fig. 2), most of the sliding material moved downwards along pre-existing tectonic discontinuities e.g. fault planes that were acting as sliding surfaces (Grendas et al., 2018). In particular, in the case of Egremnoi, our fieldwork showed that the fault planes are located at a distance of 20–30 m below the crest of the slope towards the mainland, and accordingly deep-seated landslides were developed. At the zone of Gialos beach, the sliding surfaces were not as deep as the ones mapped at Egremnoi and accordingly the width of the failure zone was shorter than the one observed at the area of Egremnoi. In addition, shallow landslides and rock slides were reported in areas where clastic material covered the bedrock and particularly in places where the rock mass was heavily jointed (Papathanassiou et al., 2017a).
The Okeanos landslide occurred very close to a touristic residence (the Okeanos villas complex) about 2 km southwest of Athani village in South Lefkada (Fig. 1; Valkaniotis et al., 2017) In this study, we briefly present the methodology that was applied in order to estimate the volume of the accumulated and removed material within the landslide area. In particular, we present the pre-field survey preparation, the steps that were followed in situ and finally the procedure that was applied after data acquisition. Finally, we compared pre-earthquake satellite images and aerial photos to post-earthquake UAV-acquired photos aiming to estimate the volume of the mass that moved downward.
Section snippets
Geology of the study area
The island of Lefkada is located in the northwestern part of the Hellenic arc, in the Ionian Sea, where complex crustal deformation results from the subduction of oceanic lithosphere of the African plate towards NE and the Apulian platform continental collision with Eurasia further to the northwest (Fig. 1; Hatzfeld et al., 1995; Clement et al., 2000; Ganas et al., 2013). The main active tectonic structure, accommodating the relative motion of the two lithospheric plates, is the dextral
Data acquisition and photogrammetric reconstruction
A UAV survey focusing on the site of Okeanos took place during June–July 2016. UAV imagery was collected using a DJI Phantom 3 with a 3.61 mm FC300X camera. A total of 75 images were acquired, covering the landslide area with both vertical and oblique views. Images were processed using the Agisoft Photoscan software, resulting in a point cloud dataset of 24 million points. Further processing of the point cloud data enabled the production of a) a detailed DSM with pixel dimensions 0.08 m and b)
Use of UAV for surveying earthquake induced landslides
The explosion of UAV usage in geosciences, and especially to landslide studies (Stumpf et al., 2013; Scaioni et al., 2014; Carrivick et al., 2016; Casagli et al., 2017; Giordan et al., 2017; Huang et al., 2017), led to the inevitable frequent use in post-earthquake surveys. For example, UAV were operated in order to document and study co-seismic landslides after recent large earthquakes such as the 2015 Mw 7.8 Ghorka, Nepal earthquake (Greenwood et al., 2016), the 2016 Kumamoto, Japan
Conclusions
Following the November 17th 2015 Mw 6.5 earthquake at Lefkada island, a series of post-earthquake field surveys took place in order to systematically identify, map and study the co-seismic effects. Of special interest were the large landslides along the western coast and especially the Okeanos site landslide because of its size and its effect on the villa complex. The Okeanos landslide was mapped during June–July 2016 using a UAV and a detailed point cloud was extracted. The use of UAV images
Acknowledgements
We thank Nikolaos Grendas, Elisavet Kolia, Gerassimos Katopodis and Vaggelis Katopodis for assistance in the field surveys. This research was partly funded by FP7 project RASOR (FP7-contract 606888 http://www.rasor-project.eu/). The CloudCompare (www.cloudcompare.org), QGIS (www.qgis.org) and SAGA-GIS (www.saga-gis.org) open software packages were used. We thank the Editor and two anonymous reviewers for their constructive comments and suggestions.
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