Seismic constraints on rock damaging related to a failing mountain peak: the Hochvogel, Allgäu

Abstract

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. 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.

Structural Characterization of a Toppling Rock Slab From Array‐Based Ambient Vibration Measurements and Numerical Modal Analysis

“…While conventional geotechnical techniques (Andrew et al., 2012; Eberhardt, 2012; Loew et al., 2017; Scaioni, 2015) can be effective for characterizing unstable rock slopes, they are often costly or invasive, and obtaining crucial information on structural conditions at depth typically requires drilling of deep boreholes with limited spatial coverage. Ambient vibration measurements can be used to detect subsurface changes associated with slope instabilities through identification of modal properties derived from spectral analysis (e.g., Bottelin et al., 2013; Burjánek et al., 2010; Colombero et al., 2021; Lévy et al., 2010) or by using ambient noise cross‐correlation to examine shear wave velocity changes (Colombero et al., 2018; Dietze et al., 2021; Fiolleau et al., 2020; Larose et al., 2015; Mainsant et al., 2012). In particular, ambient vibration modal analysis can provide information on the dynamic and material properties of slope instabilities, and can reveal fracture information from spectral and polarization analyses of in situ single‐station or seismic array data.…”

Rock Glacier Characteristics Under Semiarid Climate Conditions in the Western Nyainqêntanglha Range, Tibetan Plateau

“…As an example, the data for hammer blows to deploy ERT probes are shown in Figures 3a and 3b. Seismic location estimates of these regular signals (Figure 3a) were closest to the known origins (Figure 3b) when using an apparent seismic wave velocity of 1,600 m/s for the signal migration approach (used here only to constrain the velocity estimate, for details on the method see Dietze et al [2020]), and a ground quality factor of 80. Both approaches yield the correct location with a deviation of less than 20 m, that is, about 7% of the average station spacing, a value in the range of other studies (Dietze, 2018a(Dietze, , 2018bPolvi et al, 2020).…”

“…If all cracks were just caused by thermal stress (e.g., Polvi et al, 2020;Weber et al, 2018) the distribution of their signals in space would need to be much more homogeneous than visible in our clustered results (Figure 9). The link between crack activity of seismic events and brittle deformation has been demonstrated and justified by physical considerations before (Dietze et al, 2020;Guillemot et al, 2020;Zeckra et al, 2015).…”

Special issue: German Society for Geomorphology

“…Dietze et al ( 2021 ) investigated the preparation phase of rock slopes prior to failure at the Hochvogel summit in the Dolomite Alps at sub‐diurnal to seasonal timescales. Using environmental seismological methods, they continuously measured material properties, stress evolution and fracture propagation.…”