Large rock slope failures play a pivotal role in long-term landscape evolution and are a major concern in land use planning and hazard aspects. While the failure phase and the time immediately prior to failure are increasingly well studied, the nature of the preparation phase remains enigmatic. This knowledge gap is due, to a large degree, to difficulties associated with instrumenting high mountain terrain and the local nature of classic monitoring methods, which does not allow integral observation of large rock volumes. Here, we analyse data from a small network of up to seven seismic sensors installed during July--October 2018 (with 43 days of data loss) at the summit of the Hochvogel, a 2592 m high Alpine peak.

We develop proxy time series indicative of cyclic and progressive changes of the summit. Fundamental frequency analysis, horizontal-to-vertical spectral ratio data and end-member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of a 126,000 m^3 large, instable rock volume, due to thermal forcing. Relative seismic wave velocity changes also indicate diurnal accumulation and release of stress within the rock mass. At longer time scales, there is a systematic superimposed pattern of stress increases over multiple days and episodic stress release within a few days, expressed in an increased emission of short seismic pulses indicative of rock cracking. We interpret our data to reflect an early stage of stick slip motion of a large rock mass, providing new information on the development of large-scale slope instabilities towards catastrophic failure.