Monitoring seafloor Deformation and Assessing Landslide hazards associated with fluid pressures (Nice slope)

• Applicant: Professor Dr. Achim Kopf
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry ; Geophysics
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 390994250

• Final Report Year: 2022

Final Report Abstract

The Mediterranean Sea is impacted by major natural hazards (earthquakes, landslides, tsunamis, etc.). For the area surrounding the Nice study site, the 1887 earthquake and its 2 m high tsunami wave, then the 1979 submarine landslide and its local tsunami of less than 2 m high are two relatively recent examples that caused significant damage to infrastructures and populations. On a local scale, the study area has a potentially unstable slope where factors favouring instability include high seismicity, a high hydraulic load produced by coastal aquifers, the presence of so-called fragile sedimentary layers, high sedimentation rates and significant anthropogenic modification of the banks of the Var. An event equivalent to the 1979 landslide could be triggered again today. In this context, we have analysed the hazards associated with underwater sedimentary deformation under the action of fluid pressures and seismic shocks. Estimating where and when a submarine landslide could occur is all the more important as the Nice Urban Community is one of the largest economic centres in France, with more than half a million permanent inhabitants and millions of tourists visiting throughout the year. The main objective of the project was to analyse the coupling between sedimentary deformation and external factors such as fluid pressure and seismicity, the effects of which were monitored in-situ on the sea floor. For this purpose, various tools were developed or adapted and deployed during the project. Ifremer developed a new in-situ instrument called "TIPS" (Temperature, Inclination and Pressure Sensors) to measure the evolution of pore pressures, temperatures and deformations of the superficial sedimentary column (< 35 m below the seabed). MARUM has built and deployed a lance called a seismo-piezometer probe combining a 3-component seismometer with a multi-level pore pressure measurement. MARUM also built a lander containing the KATARINA Radon (Rn) probe by HCMR-HORST from Greece and a CTD, which was deployed to identify and quantify freshwater seepage through the submarine sediments. Finally, Geoazur deployed the PRIMA seismological station, consisting of a Guralp 360 s broadband sensor. Repeated multibeam bathymetric surveys carried out during the project allowed the evolution of the seabed morphology to be analysed. Finally, all of these observations and data were used as input for slope stability analyses carried out using 2D and 3D numerical calculations. The data acquired confirms the vulnerability of the underwater slope off Nice airport. The PRIMA station reveals a strong amplification effect of seismic waves in the surface sediments; geotechnical analyses indicate that a liquefaction rupture could occur under the effect of a magnitude 6.3 earthquake located 25 km from the area. The acquired pore pressure data show a seasonal variation that may affect the stability of the continental slope. Two TIPS installed in July 2021 and connected to the EMSO-Ligure Nice network already provide the data necessary to assess the stability of the Nice slope. Weak deformations identified near the location of one of the TIPS from the bathymetric surveys suggest the presence of precursors that would highlight a slip process in the making.

Publications

• Impact of seismicity on Nice slope stability—Ligurian Basin, SE France: a geotechnical revisit. Landslides, 16(1), 23-35.
  Roesner, Alexander; Wiemer, Gau­vain; Kreiter, Stefan; Wenau, Stefan; Wu, Ting-Wei; Courboulex, Françoise; Spiess, Volkhard & Kopf, Achim
  
• Strong Site Effect Revealed by a New Broadband Seismometer on the Continental Shelf Offshore Nice Airport (Southeastern France). Pure and Applied Geophysics, 177(7), 3205-3224.
  Courboulex, Francoise; Mercerat, E. Diego; Deschamps, Anne; Migeon, Sébastien; Baques, Marion; Larroque, Christophe; Rivet, Diane & Hello, Yann
  
• Transient Groundwater Flow Through a Coastal Confined Aquifer and Its Impact on Nearshore Submarine Slope Instability. Journal of Geophysical Research: Earth Surface, 125(9).
  Sultan, N.; Garziglia, S.; Bompais, X.; Woerther, P.; Witt, C.; Kopf, A. & Migeon, S.
  
• Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations. Journal of Geophysical Research: Earth Surface, 126(10).
  Garziglia, Sebastien; Sultan, Nabil; Thomas, Yannick; Ker, Stephan; Marsset, Bruno; Bompais, Xavier; Woerther, Patrice; Witt, Christoph; Kopf, Achim & Apprioual, Ronan