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Landslides are common landscape features in the Northern Apennine mountain chain and cause frequent damages to human structures and infrastructure. Most landslides in the area can be classified as earthflows, where the clay-shales form the substrate, whereas complex landslides with flow and sliding components are common on the slopes where fine-grained turbidites form the substrate. Most of these landslides move periodically with contained velocities and, only after particular rainfall events, some of them accelerate abruptly. Space-borne synthetic aperture radar interferometry (InSAR) provides a particularly convenient way for studying the periods before and after failures. In this paper, we present InSAR-results derived from the Sentinel 1 satellite constellation for two landslide cases in the Northern Apennines. The first case is a complex landslide that is hosted on a pelitic flysch formation, whereas the second case is an earthflow located in chaotic clay shales. Both cases failed catastrophically and threatened or damaged important infrastructures. In the case of the complex landslide, we report spatially variations of the deformation field between repeated periods of acceleration. The data illustrate that the deformation initiated in the upper part of the slope and expanded over the whole landslide body afterward. In the case of the earthflow, we describe spatial and temporal kinematics during the period before a catastrophic failure in March 2018. We discuss the temporal deformation signal together with rainfall and snowmelt data from a nearby meteorological station. Deformation and precipitation data highlight that high total precipitation can be considered the trigger of the failure.

Earthflows are clay-rich, slow-moving landslides subjected to periodic accelerations. During the stage of rapid movement, most earthflows exhibit a change in behavior from a solid to a fluid-like state. Although this behavior has been extensively documented in the field, the mechanism leading to the rapid acceleration of earthflows is still poorly understood. Some studies suggest that earthflows essentially behave as Coulomb plastic solids, attributing the flow-like appearance to distributed internal shearing; others believe that these landslides can be treated as viscous fluids, pointing out that the material undergoes a phase transition by increasing its moisture content. Minimal data are currently available to support these different findings. In this study, we present the results of periodic and continuous measurements of Rayleigh wave velocity carried out in an active earthflow located in the northern Apennines of Italy. Our data indicate that the material undergoes significant changes in shear stiffness and undrained strength during rapid movements. In particular, the material exhibits a substantial drop of Rayleigh wave velocity as the earthflow accelerates, followed by a slow return to predisturbance Rayleigh velocities as the landslide decelerates. Soon after a surge, the earthflow material is extremely soft and the estimated gravimetric water content is above the liquid limit. In the following months, the shear stiffness gradually increases and the water content decreases to the plastic limit following a nonlinear trend typical of a consolidation process. These data demonstrate that the earthflow transforms into a viscous fluid by softening of the material and by water entrainment.

Earthflows are a flow-like movement of plastic clayey soils characterized by long periods of slow motion (at rates averaging a few meters per year or less) alternated with short periods of rapid surges at high velocity (up to meters per hour). During rapid surges, most earthflows move over a long distance with a fluid-like behavior. Although the generation of flow-type failures is an important issue for hazard assessment, our knowledge is limited by the difficulty of monitoring the process in the field. This has led to different explanations for rapid earthflows including high pore–pressure generation along the basal slip surface, pervasive shearing, or material fluidization. One key question is whether or not earthflows can fluidize through remolding and water entrainment. If this occurs, the material can change from plastic to fluid as the soil moisture increases, causing the landslide to move as a viscous flow; if not, the material remains in a plastic state and, as suggested by many authors, the flow-like morphology shown by earthflows would result by distributed internal shears rather than real mass flow. In this study, we provide the first answer to this question by measuring the shear stiffness of four large active earthflows in the Northern Apennines of Italy. Shear stiffness was measured using two geophysical techniques, the multichannel analysis of surface waves (MASW) and the passive refraction microtremors (ReMi). Measurements were carried out just a few days after the mobilization of the landslides and repeated in the following 2–3years to evaluate the change of elastic properties with time. Field data show that soon after the mobilization, earthflows are characterized by very low values of shear stiffness (about 5–15MPa), typical of soft clay soils with the high-void ratio. Shear stiffness then increases 4–5 times in the following months (up to 40–60MPa) as the earthflows slow down and the material consolidates. These data indicate that during a rapid movement, earthflows undergo a dramatic increase of porosity and water content that probably drive the transition from a solid to a fluid-like state.