Seismic precursors reveal the role of internal processes in driving mobilisation of the 15th June 2023 Brienz/Brinzauls Rockslide

Early detection and monitoring of rock slope instabilities are critical due to their sudden onset and significant risks to life and infrastructure. Understanding the factors controlling the dynamic evolution of rock slopes towards catastrophic failure remains a major challenge as mechanisms driving the failure occur at depths inaccessible to surface-based measurement techniques. 

Once rock bridge failures grow and coalesce to a continuous failure plane under (sub)critical stress, a rockslide enters the mobilisation phase. From then, it creeps or slides until it evacuates the source area. For many hillslope instabilities, it is unclear how the interplay between internal mechanisms and external, often meteorological drivers governs the time to collapse and the extent of structural damage during displacement.

In Brienz/Brinzauls, Switzerland, near-field seismic data from a network of geophones and broadband sensors captured precursory signals originating on or within the active “Insel” compartment of a large landslide complex, as it accelerated from 50 mm/day in late April to over 5000 mm/day, before its collapse on 15 June 2023. During prolonged mobilisation, we analyse the link between precipitation and internal mechanisms and assess how these internal processes independently drive the unstable rock mass to catastrophic collapse in the absence of external meteorological forces.

We apply a supervised XGBoost machine learning model based on seismic features to detect and classify surface rockfall events and sub-surface micro-earthquake events (internal rock bridge failures and basal stick-slip) from continuous seismic time series. 

Initial increases in surface and sub-surface event rates were rainfall-driven, with sub-surface event spikes lagging behind surface events due to progressive water infiltration into the landslide mass. After rainfall ends, surface event rates decrease earlier than sub-surface event rates as water drains from the landslide mass. Rocksliding transitioned to a phase of internal control, leading to the nonlinear evolution of surface and sub-surface events until the main collapse, in the absence of rainfall. After the transition, subsurface activity accelerated without a corresponding change in rockfall activity. Rockfall activity from the "Insel" increased after a 9-day lag, likely driven by the upward propagation of stress imbalances caused by an enhanced rate of basal sliding. A continuous decrease in sub-surface events per unit slip indicates rate-weakening behaviour at the sliding surface with slip progressively eroding asperities, reducing frictional resistance. In this context, the disintegration of rock fragments along the sliding surface generates transient families of repeating seismic events characterized by near-identical waveforms.

Our observations underline the critical role of dynamic roughness evolution at the sliding interface in governing rock mass mobilisation, with the transition from meteorologically driven sliding to internally controlled acceleration predominantly reflected in basal stick-slip and internal cracking, rather than surface rockfall activity. This highlights the need for spatially extensive monitoring of rock-internal processes to understand the non-linear dynamics of large slope instabilities during failure preparation, beyond precipitation-based models.