Quantifying irreversible movement in steep fractured bedrock permafrost at Matterhorn (CH)

Abstract. Identifying precursors of gravity-driven slope instabilities in inhomogeneous fractured rock masses is a challenging task. Recent laboratory studies have brought upon an enhanced understanding of rock fatigue and fracturing in cold environments but were not successfully confirmed by field studies. In this study we monitor environmental conditions, rock temperatures and fracture dynamics at 3500 m a.s.l. on the steep, strongly fractured Hörnligrat of Matterhorn (Swiss Alps). Here we analyze seven years of continuous data of the long term evolution of fracture dynamics in permafrost offering unprecedented level of detail and observation duration. The fracture dynamics consists of reversible and irreversible movement components resulting from a combination of temporal varying driving and resisting forces. As irreversible motion is suspected to occur prior to global gravity-driven slope failure, we developed a statistical model, assuming the reversible deformation is caused by thermo-mechanical induced strain, and tested it successfully with field measurements from steep permafrost bedrock. We apply this linear regression model to our data set of fracture dynamics and rock temperature in order to separate the residual irreversible movement component. From this, we produce a new metric that quantifies relative irreversibility of fracture dynamics and enables a better interpretation of the data. This index of irreversibility is based on in situ measurements and enables a local assessment of rock wall stability. Here we show how environmental forcing causes reversible and irreversible rock mass deformations that might be relevant in preconditioning rock slope instability. In general, all locations instrumented show a trend of fracture opening, but at variable rate between locations. At each individual location, the temporal pattern of deformation is very similar every year. All but one sensors show a reversible deformation component caused by thermo-mechanical induced strain. For many sensors, we observe an irreversible enhanced fracture deformation in summer, starting when rock temperatures reach above zero. This likely indicates thawing related process, such as melt water percolation into fractures, as a forcing mechanisms for irreversible deformation. Most likely, such water or thawing leads to a decrease of the cohesion and friction along fracture in the shear zone. For a few fractures instrumented, we find an irreversible deformation with the onset of freezing period, which suggest that cryogenic processes act as a driving factor through increasing ice pressure. It further highlights that irreversible fracture deformation can even at locations in close proximity not be explained by one single process.