A Long-term Monitoring Array for Landslide Precursors: A Case Study at the Ligurian Slope (Western Mediterranean Sea)

摘要

The Nice Slope, situated in the French part of the Ligurian Margin, represents a landslide-prone area, where different factors favour slope instability: (i) Seismicity up to ~M6, (ii) rapid sediment deposition in the Var fluvial system, (iii) groundwater discharge along the coastal aquifer, (iv) creeping of sensitive clay layers, and (v) human activity (land reclamation, construction). In 1979, a catastrophic failure occurred in the Var pro-delta, that destroyed the Nice new harbour extension, mobilised ~8.7x106 m3 material, and initiated a tsunami wave of 2-3m. This event led extensive research efforts first directed at understanding the triggers of the failure. In later years, some further studies allowed to detect the occurrence of mechanically weak sediments outside the area that failed in 1979. This, along with the detection of freshened pore water and excess pore pressure development raised concerns as to the present day and future slope stability. Hence, at present, progressive weakening of the shallow sediment is monitored using a long-term array in collaboration between Ifremer Brest and MARUM Bremen. It comprises two multi-level pore pressure probes (Ifremer piezometers, one installed in the 1979 scar, one in the unaffected slope further E) as well as a tilt-meter instrument (Ifremer) that will observe the deformation rate of the upper 8 m of sediment in real-time. In addition, an osmotically driven fluid sampling-lance (MARUM) collects pore fluids for geochemical studies at the levels of the pore pressure ports of the piezometers to identify ground-water discharge. With temporary OBS installations and onshore seismometers in place, the array will allow us to distinguish between the various factors that are presently affecting the stability of the Nice Slope.