The utilizing of time-lapse electrical resistivity method (tl-ERT) in modeling of landslide activity: Case studies from the west of Antalya

Natural events like earthquakes and landslides are caused by geological activities and they turn into natural disasters due to the high sensitivity of people to these events. Landslides, which are among the most important natural events in terms of number of occurrences for Turkey, are basically defined as the slope downward movement of the rock or soil. These movements can be triggered by natural external stimulus such as heavy rainfall or natural internal stimulus such as structural mobility or by artificial stimulus such as human-induced actions. In order to determine the landslide activity in the field, electrical geophysical methods can successfully be used to identify the unconsolidated soil - bedrock contact, which is an important indicator for such movements. Instead of traditional methods, the multi-electrode electrical resistivity imaging method (ERT), which is based on continuous measurement along a line, was applied by taking measurements at different times (tl-ERT). With the time-lapse electrical resistivity measurements, the differences of electrical resistivity were observed and interpreted depending on the date. It is known that west of Antalya is sensitive to landslides along the Kemer-Kumluca line. For this reason, the study area was selected in this context and time-lapse electrical resistivity analyses were performed on two possible landslides selected from the Üçoluk region on the 52nd km of the Kumluca-Altınyaka-Antalya highway. Five-month analyses were carried out on two vertical lines in the Üçoluk-1 landslide. In the Üçoluk-2 landslide, two-month measurements were made on a single line. With the field observations, a slide was observed in the Üçoluk-1 landslide towards the SW direction and the analyses were carried out within this framework. In SW-NE line measurements, it was observed that the areas showing high electrical resistivity relative to its surroundings decreased in the direction of sliding over time, and the low electrical resistive material (<4 ohm.meter) followed the elevation. In the first analysis, the values observed in the range of 40-55 meters and reached 30th meter in the next analysis and then reached to the 20th meter of the electrode array. It is seen that the decreasing of resistivity is moving in the direction of sliding. In this way, it has been determined that the region with the resistivity value of 10 ohm.meter regressed to 3-4 ohm.meter in later analyses. Similarly, a 20-meter wide resistivity difference was observed in the SE-NW line, and as the time progressed, the electrical resistivity values decreased compatible with the other line. It was determined that a 20-meter wide, approximately 5-meter deep slide could be observed and the length of the slide was observed as 20 meters by the analysis carried out. In Üçoluk-2 landslide, it was determined that a soft soil could not be stable on the steep topography and slide towards the valley by field observations. The electrical resistivity change was investigated by two different analyses performed in one direction in this landslide, and as a result, it was determined that there was a 15-meter-long and 4-meter-deep slide. The landslide width could not be discovered due to the one-way analysis, but it was determined that the material sliding into the valley as a talus was 6 meters wide. KEYWORDS: Antalya, Electrical resistivity imaging, Geophysics, Landslide, Time-lapse method