Using the natural oscillation of the 2592-meter-high Hochvogel peak in Austria to “hear“ how the rock becomes unstable

The summit of the Hochvogel in the Alps is breaking in two. Photo: TU München

Similar to a more or less taut violin string, the pitch of the mountain also changes depending on the tension in the rock

The entire summit of the 2592 meters high Hochvogel is sliced by a five meters wide and thirty meters long fracture. It continues to open up by up to half a centimeter per month. Throughout the years, the southern side of the mountain has already subsided by several meters; and at some point, it will fail, releasing up to 260,000 cubic meters of limestone debris down into the Hornbach Valley in Austria. Such a volume would roughly correspond to 260 family houses.

When this will happen is hard to predict by conventional methods. Researchers of the Helmholtz Centre Potsdam – German Research Centre for Geosciences and the Technical University of Munich have approached this question by seismic sensors. The devices record the subtle vibration of the peak: similar to a violin string which is pulled more or less does the pitch of the summit change as it becomes stressed, an effect that allows unique insight to the preparation phase of an upcoming rock slide. Thus, also a timely warning should become possible – even if human dwellings are not threatened directly at this site.

To understand when and why the instable rock mass at the Hochvogel becomes mobile, in 2018 researchers around Michael Dietze of the GFZ had deployed a network of six seismometers at the summit, each at a distance of thirty to forty meters from each other. For several months, the sensors have recorded the frequency with which the mountain swings back and forth. The vibrations are caused by wind and numerous small excitations of the Earth‘s surface, and the summit’s frequency is determined by factors such as temperature, rock stress and material weakening.

Typical sawtooth pattern in the mountain's frequency. Source: Dietze/GFZ

During the summer of 2018, the researchers were able to measure a recurring sawtooth-like frequency pattern: Over a period of five to seven days, it rose repeatedly from 26 to 29 Hertz, only to drop back to its original value within less than two days. The increase in frequency is caused by stress increase within the rock mass. As the frequency drops, the sensors also recorded an increased rate of crack signals, as they are known to happen when rock is being torn apart.

This cyclic increase and decrease of stress by jerky movement is also called stick slip motion. It is a typical precursor of large mass movements. The decisive factor here is that the closer this event comes, the shorter the observed cycles become, making them an important hazard indicator.

In the course of their measurements, which – with interruptions due to lightning strikes – extended from July to October, the researchers made another interesting discovery: While the sawtooth-like build-up and release of stress was clearly visible in the first few months after snow melt, it disappeared in late summer of the drought year 2018.

Apparently, the summit ran short in an essential lubricant during the summer: water.

By then, only a diurnal up and down of the summit’s vibration frequency played a role: during the cold night hours the rock contracts, fissures become larger and the connection to the solid rock becomes less rigor, resulting in a decreasing vibration frequency. In turn, the heat of the sun lets the rock mass expand, closing small fissures and thus causing a rise of the vibration frequency.

Over a period of two more years, the researchers will now investigate how these diurnal and longer period cycles interact and how the chilly winters will affect the deep, water filled crevices that cut the Hochvogel.

This includes investigating the consequences of rock mass activity at the summit for the south facing hillslope by a larger seismic network that stretches down towards the Hornbachtal. Settlements in that valley will not be threatened by mass wasting along the slopes, but the access of the peak from this area has already been closed years ago due to an imminent rockfall risk.

The study has been published in the journal Earth Surface Processes and Landforms.

Source: Helmholtz Centre Potsdam – German Research Centre for Geosciences

(29.12.2020, USA: 12.29.2020)