Significance of very small-strain stiffness for interpreting the internal structure of flysch landslides

Very small-strain stiffness parameters derived from seismic methods are commonly used in landslide investigations to describe subsurface mechanical conditions. In practice, these parameters are often interpreted in terms of slope stability. However, their role in identifying the internal structure of landslide bodies is still not fully recognized, especially in geologically complex flysch terrains.

This study examines the significance of very small-strain shear modulus (G₀) for interpreting the internal structure of deep-seated landslides developed in the Carpathian Flysch. The analysis is based on two slow-moving landslides instrumented with deep inclinometer boreholes and monitored over periods of 9–10 years. Long-term inclinometer records provide information on cumulative deep-seated displacements and their vertical distribution within the landslide bodies.

Seismic surveys were carried out along profiles located within the landslides, and very small-strain stiffness distributions were derived from shear-wave velocity measurements supported by laboratory-based bulk density data. Instead of focusing on the integration methodology, the study compares stiffness profiles directly with long-term displacement patterns and geological information at borehole locations.

The results indicate that variations in very small-strain stiffness reflect differences in lithology, degree of weathering, and structural discontinuities within the landslide bodies. Zones characterized by relatively high stiffness values may correspond to less weathered but strongly fractured flysch units, while lower stiffness values are typically associated with colluvial material or highly disturbed rock masses. Importantly, similar stiffness values can be linked to different kinematic behaviors, highlighting that stiffness parameters alone do not describe landslide activity.

The comparison of geophysical stiffness data with long-term monitoring records demonstrates that very small-strain stiffness is particularly useful for identifying internal structural domains rather than for direct assessment of landslide stability. The study emphasizes the role of long-term inclinometer monitoring as a reference framework that constrains the interpretation of geophysical results. The findings support a more informed use of seismic stiffness parameters in landslide studies and contribute to improved characterization of landslide structure in flysch terrains.