Progressive failure of a freestanding rock pillar
Abstract
A freestanding rock pillar with a volume of approx. 20 cubic meters on the Matterhorn Hörnligrat ridge failed on 13 June 2023. Based on comprehensive multi-method monitoring of the pillar and the surrounding ridgeline environment starting in 2008, we perform a detailed analysis of the progressive failure from kinematic precursors to seismic response. The rock pillar was instrumented with a differential GNSS station and inclinometers, showing a strong seasonality in displacement rates and an acceleration with regime change starting in 2022 almost two years prior to the failure. The pillar was in the field of view of a stationary camera, whereby the time-lapse images show a visually apparent acceleration two weeks before the collapse. Seismic precursors and response were characterized employing three seismometers in the vicinity. Weather data as well as permafrost ground temperatures enable us to characterize the temporal variation and identify anomalies at the site. The data analysis suggests that snowmelt water percolating into frozen fractures acts as the main driver for the strong seasonality in displacement patterns observed, eventually resulting in failure. This is supported by controlled laboratory experiments using rock samples from the Matterhorn site and thermo-mechanical modeling.