Literally exciting music: the “melody” of a summit can provide clues about the development of landslides, researchers report. Using seismic methods, they overheard the mountain peak of the Hochvogel in the Allgäu Alps, which is slowly breaking in two. Similar to a more or less tightly drawn instrument side, the pitch of the mountain also changes depending on the tension in the rock, the results show. This allows conclusions to be drawn about the development of an impending rupture, say the scientists.
It belongs to the prominence of the Allgäu Alps: At an altitude of 2592 meters, the summit of the Hochvogel dominates the mountain panorama on the border of Germany and Austria. The prominent rock spike is also famous for a noticeable crack: In it there is a gap five meters wide and 30 meters long, which opens by almost another centimeter every month. That means: at some point there will be a big crash up there. The southern side of the massif threatens to plunge into the Austrian Hornbachtal. Fortunately, the roughly 260,000 cubic meters of rock will probably not reach any settlements.
New monitoring procedure
But this is definitely the case with other unstable rock formations in the world. So far, however, it is difficult to predict when the sudden landslides will occur. While the demolition processes themselves have already been well investigated, the longer-term harbingers could not yet be uncovered in a larger spatial context. Scientists working with Michael Dietze from the German Research Center for Geosciences in Potsdam are therefore dedicated to researching clues to the movements in rock masses. The high bird offered itself as an ideal research object. To find out when and why the rock was moving on the summit, they set up a network of six seismometers on the mountain in 2018.
As Dietze and his colleagues explain, it is known that tensions build up in the rock on steep slopes, for example due to its own weight or temperature fluctuations, which then discharge in the decay processes: At some point the material becomes so unstable that it breaks apart completely. In order to uncover possible signs of this process, the researchers recorded over a period of three months the frequency with which the high bird’s summit vibrates back and forth. As they explain, even solid rock is set in vibration by external stimuli such as wind and shocks in the earth’s crust – similar to an instrument. The frequency of the rock depends on the tension, which in turn is influenced by factors such as temperature, material stress and the degree of disruption of the rock.
Characteristic vibration behavior
Through their investigations, the scientists were able to demonstrate a recurring sawtooth-like course of the frequency at the peak of the high bird: it rose again and again over a period of five to seven days from 26 to 29 Hertz, and then fell to the original value in less than two days. The increase in frequency was coupled with an increase in tension in the rock. As the frequency dropped, the researchers found an increasing number of seismic signals associated with failed rock contacts, such as those created when rock cracks break open. This cyclical build-up and release of tension through jerky movements, also known as “stick slip motion”, is a typical harbinger of threatening mass disruptions. The decisive factor here is: the closer this event comes, the shorter the observed cycles, so they are an important indicator of danger. “With the help of our seismic approach, we can now for the first time record and process this cyclical phenomenon continuously and almost in real time,” says Dietze.
In the course of their measurements, the researchers also made another interesting discovery: While the sawtooth-like build-up and release of stresses was clearly visible in the first months after the snow melt, the pattern disappeared in the late summer of the drought year 2018. The researchers attribute this to the absence of the Back smear effect of the water. However, there was still an ups and downs in the frequencies over the course of the day. This effect arises from the fact that the rock contracts in the cold hours of the night. As a result of the heat of the sun, the rock expands, closes small gaps, and thus generates a higher vibration frequency. Using their method, the researchers now want to investigate more closely how these daily and longer-term cycles are connected and what influence the water in deep rock crevices has on the Hochvogel summit.
However, it will take some time before it can be widely used, says Dietze: “We have currently provided the ‘proof of concept’, so to speak, now we have to repeat the tests elsewhere.” Dietze believes that it will be too difficult. “And with the increased activity on the many three-thousand-meter peaks in the Alps, there are probably also plenty of areas of application,” says the scientist.
Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences, specialist article: Earth Surface Processes and Landforms, doi: 10.1002 / esp.5034