Long-term movement activity and internal structure of deep-seated landslide by using dendrochronology analysis and electric resistivity tomography in flysch rocks, Carpathians, Czech Republic

Complex or compound landslides, which combine different movement types with sliding planes at various depths and with varying movement acceleration frequencies, are highly demanding for landform mapping, movement monitoring and reliable hazard assessment. In this work, several techniques including dendrogeomorphological investigation were combined to describe surface morphology, underground structures and movement dynamics of the compound and deep-seated landslide aiming to provide reliable information for its hazard assessment. Interpretation of high-quality digital elevation model and detailed field morphological mapping along with geological information provided context for the interpretation of electric resistivity tomography profiles and enabled the description of properties of two distinct landforms, which are typically identified on a compound or complex deep-seated landslides in the studied region—shallow slides and landslide blocks. Dendrogeomorphological investigation proved for the first time the movement accelerations of the landslide blocks, which reactivate approximately half as often as shallow slides. It also showed different trees’ responses to the movements of these two landforms. Trees on the shallow landslide responded mainly with abrupt growth suppression (54.4%) to movements of its highly disturbed material. In contrast, trees on landslide blocks exhibited a dominant response (84.7%) with reaction wood to tilting of the landslide blocks composed of more coherent rock material. The research demonstrated that the dendrogeomorphological investigations provide reliable identification of years with accelerated movements, which corresponds well to instrumental, near-surface monitoring of the landslide. And at the same time, the method provided densely spatially distributed information about partial landslide reactivations during several decades in conditions (e.g. dense forests), where remote sensing methods are difficult to apply. Therefore, we argue that the dendrogeomorphological research is well applicable for hazard assessment of partial failures (cf., shallow slides and landslide blocks) of compound or complex landslides providing information also about the type of landslide movements (sliding vs. surface tilting) and character of the deformed material (highly disturbed vs. more coherent).