Multi-technique approach to characterize ancient deep-seated landslides in seismic regions

Mountain ranges in seismically active regions often present vast numbers of deep-seated and voluminous landslides. In general, the comprehension of factors contributing to such slope failures can help better understand the dynamic history of a region. Here, we present the outcomes of our studies in the Buzau-Vrancea seismic region in the Romanian Carpathian Mountains, where massive slides are marked by "softened" hummocky morphologies that result from weathering and aging. The latter factors make it more difficult to understand the circumstances of slope failure development and to trace the limits between in-situ and displaced material. In particular, the uncertain subsurface structure of those large mass movements suggests the application of geophysical methods. We used several geophysical techniques, which allowed us to overcome limitations in terms of investigation depth and lateral resolution of single methods.

Electrical resistivity measurements can be used to obtain lithological information, and to characterise the water content of the landslide deposits and of in situ rocks. On most sites, we combined electrical surveys with seismic methods: actively triggered seismic energy used for seismic refraction surveys - analyzed as 2D P-wave tomography (SRT) and as 1D MASW - together with passive methods using ambient noise, i.e. small-aperture seismic array and single station HVNR measurements. In contrast to P-wave velocity prospections, surface and shear wave velocity measurements are able to reveal deeper and lateral contrasts that are independent of the soils water content and thus useful to define the mechanical properties of complex slope deformations. Ambient noise array methods, in addition, generally allow very deep sounding. With this approach, we are able to distinguish the basal shearing horizon of the studied landslide, their volume, geometry, and general geomechanical parameters.

The main focus of this work is on the combination of these methods, and on the advantage of interpreting these results in a full 3D geomodel of study sites based on the collected surface (UAV flights, LiDAR-DEMs) and geophysical subsurface data. The 3D geomodel will further be used as a basis for numerical calculations aiming at a back-analysis of the landslide development (in a static and possibly dynamic domain).