Back-analyses of co-seismic landslides in carbonate fault rocks due to the Mw 6.5 earthquake (2015) in Lefkada island, Greece

Abstract

During the Mw 6.5 earthquake (2015) in Lefkada Ιsland, Greece, nearly 700 co-seismic landslides occurred along the western coastline of the island, where carbonate fault rocks, primarily fractured limestones and fault breccias, are encountered on steep slopes. The main objective of the present study is the investigation of a selected number of these landslides considering the local topography, ground conditions and seismicity of the area. The main challenge is the limitations due to the accessibility of the slopes and the difficulty of sampling and laboratory testing the carbonate fault rocks. For this reason, back-analysis of the failures is the only method for the determination of the upper limit of shear strength of the materials comprising the slopes. Remote sensing techniques were used for mapping the investigated landslides, since their locations were not accessible, especially after the occurrence of the landslide events. Three-dimensional models (Point Clouds) were created and used to define the geometric characteristics of eleven (11) major landslides that were back-analyzed using the limit equilibrium method (LEM). The findings of the present study indicate that the studied materials are very weak, having a Uniaxial Compressive Strength (UCS) which approximates the lower strength threshold (0.25 MPa) of weak rocks as defined by ISRM (1981). This can be attributed to the intense tectonic disturbance that these limestones have undergone in the past. Moreover, further LEM analyses were conducted for neighboring slope surfaces that did not fail during the same earthquake and proved that the mechanical properties of the ground improve with depth, a fact that can be associated with more intense weathering and relaxation near the ground surface.