Seismic Transients of Internal Deformation in an Active Rockslide (Spitze Stei, Switzerland): First Insights from a Dense Seismic Nodal Array

The unstable rock slope "Spitze Stei" (Kandersteg, Switzerland) has shown significantly increased activity for several years. Since 2018, observed displacement rates can exceed 40 cm per day seasonally. The instability covers a total area of ​​approximately half a square km. The volume of the moving rock and debris mass is ~16 million m³, distributed across several rock compartments. Driven by degrading permafrost and enhanced gliding planes, these primary gravitational instabilities result in secondary, often destructive, debris flows into the Oeschibach channel. While continuous monitoring is essential for risk management, traditional visual and radar methods are often constrained by adverse weather conditions, limited temporal resolution and limited sensitivity to subsurface processes. To overcome these limitations and monitor rockslide internal deformation, material damage, and ongoing mass-movement processes at high spatial resolution, a dense temporary seismic network consisting of 64 SmartSolo nodes (natural frequency 5 Hz) were deployed across the slope at the end of June 2025 and operated for nearly three months. This dataset is complemented by recordings from three semi-permanent seismometers that have been operating since October 2021, providing a longer-term reference for background seismicity and site-specific noise characteristics.

We analyze the continuous seismic records to detect and characterize signals from a variety of mass-movement phenomena, including rockfalls, granular flows, debris flows and avalanche-related activity. Signals are evaluated based on waveform properties, duration, amplitude evolution, and spectral content, with comparisons across sensor types and periods. A key objective is to isolate and cluster internal microseismic activity, distinguishing it from background noise, external sources (e.g., icequakes), and transient permafrost-related signals.

Our preliminary results highlight a diverse set of seismic signal types linked to both surface processes and internal rockslide dynamics. This observed variability suggests changes in deformation style across different rock compartments, demonstrating the potential of dense nodal seismic arrays to resolve internal rockslide processes relevant for hazard monitoring.