Towards a better understanding of large-scale rock slope dynamics with seismic interferometry

The Kandersteg region, Switzerland, has a history of catastrophic rock slope failures that repeatedly occurred throughout the Holocene, with volumes reaching hundreds of millions of cubic meters. In recent years, the rock slope near "Spitze Stei" has exhibited elevated displacement rates exceeding 10 cm per day, suggesting a growing instability of up to 20 million m³.

Due to the destructive potential of the Spitze Stei rockslide, extensive monitoring has been put in place since 2018, including borehole temperature logging, water pressure measurements and surface displacement monitoring. Borehole temperature measurements and direct observations highlight the presence of degrading permafrost, possibly on planes of enhanced gliding and shear deformation. However, point-like borehole measurements and sensing technology focusing on the slope surface cannot fully describe processes influencing slope dynamics, such as freeze-thaw cycles, varying water pressure, and progressive damage within the slope. These processes have lateral and depth-dependent sensitivity, causing changes in the rock's elastic properties, thus impacting seismic velocities. Here, we aim to provide a better understanding of these primary processes driving the dynamics of Spitze Stei. To this end, we analyze variations in relative seismic velocities (dv/v) measured through continuous seismic data and seismic interferometry. With this technique we transform seismic noise into coherent signals through cross-correlations of data from five 3-component seismometers.

The initial results of the time series of relative seismic velocity variations (dv/v) constrain the lateral and depth-dependent extent of subsurface changes. The results indicate that a substantial decrease in relative seismic velocity occurs at the times of rather heavy rain (rainfall >10 mm d^-1). This suggests that dv/v reflects material changes caused by pore pressure increase and reduction in material strength. The shallowest dv/v measurements agree with surface displacements displaying cyclic slipping of material.

We discuss how our observations help identify the primary processes controlling the dynamics of the Spitze Stei rockslide, give quantitative insight into rock damage, and allow separating effects from irreversible damage growth and reversible thermoelastic and hydrologic variations. This knowledge is needed to better understand the development of large rock failures and potentially improve warning systems.