Geophysical investigation of a landslide: The Altındağ landslide site, İzmir (western Turkey)

A geophysical survey including electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) was carried out to study a landslide site in the Altındağ district of İzmir city, western Turkey. In the city, landslide is one of the causes of natural hazard, and one of the most important landslide sites is located in the Altındağ district. The ERT studies were performed along four profiles over the landslide body in the directions of N–S and E–W. A Wenner–Schlumberger configuration was used during the resistivity measurements. The SRT survey was carried out using vertical geophones along a profile coincident with one of N–S trending resistivity profiles. Both the resistivity and the seismic data sets were processed by least-squares inversion techniques. In the first-arrival traveltime inversion of the surface refraction data, a method which is not based on ray tracing was used. Instead, the method used the functional description of traveltimes to generate the Jacobian and this matrix was calculated by using a finite-difference approximation based on the perturbation of the cell slownesses. Traveltime calculations were performed by an eikonal solver. Each technique produced useful results on the internal structure of the landslide, physical properties of the landslide material and sliding surface geometry. The landslide material was characterized by low resistivity and low seismic velocity. The resistivity results also indicated the zones associated with the high water and clay contents within the landslide mass. A layer of consolidated clastic rocks was considered to be the landslide bedrock, and it was represented by relatively high (moderate) resistivity and high velocity. Integrated interpretation of the resistivity and the seismic refraction data along one of N–S trending profiles helped us identify the geometry of the failure surface and the changes in the thickness of the landslide material. An undulating sliding surface is present along that profile and the landslide material gets thicker in the middle and the northern parts (the toe area) of the profile. Moreover, the water content is thought to have a key role in the mass movement in the landslide site.