Abstract
AbstractLarge 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. Modal analysis, horizontal‐to‐vertical spectral ratio data and end‐member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of a 260,000 m3 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 increased over multiple days and episodic stress release within a few days, expressed in an increased emission of short seismic pulses indicative of rock cracking. Our data provide essential first order information on the development of large‐scale slope instabilities towards catastrophic failure. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Highlights
Gravitational mass wasting is the dominant geomorphic process shaping mountain peaks
Once a rock mass starts to move, deformation accumulates along that critical path, and shear stress concentrates on a decreasing number of remaining rock bridges, initiating subcritical and critical fracture propagation in these preferential damage zones (Kemeny, 2003)
Meteorological dynamics define an important set of boundary conditions for the activity of the summit
Summary
Gravitational mass wasting is the dominant geomorphic process shaping mountain peaks. For modal analysis we calculated averaged Welch (1967) spectra for non-event periods (Bottelin et al, 2013) of the summit network sensors. A further rejection criterion imposed that an event had to be detected by at least two stations with a time difference of not more than 0.1 s This rule enforces that signals must travel across the entire network in a time that corresponds to an apparent seismic wave velocity of at least 730 m/s, a threshold well below typical values for limestone of different origin and degree of fracturing (Assefa et al, 2003; Dietze et al, 2017; Helmstetter and Garambois, 2010). We calculated a transfer function between gridded values at the peak and the Oberstdorf data for adjacent periods with data and use this relationship to infer precipitation data for the peak during relevant data gaps
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