Recurrence analysis of the mass movement activity at Stambach (Austria) based on radiocarbon dating
Introduction
In mountain areas like the Alps in central Europe, mass movements are a very common cause of natural disaster and the consequences, especially for the inhabitants of the area, can often be very severe. Since the retreat of the large glaciers at the end of the last glaciation, the Alps have been an attractive area for ever increasing settlement and land use activities. In recent years, more and more houses and sensitive infrastructure (for example, roads, transmission lines, ski lifts) have been built in areas with a high risk of mass movements. Hazard analyses are therefore necessary to foster a deeper understanding of the causes and the recurrence intervals of mass movements. Earth flows are a specific type of mass movement (Cruden and Varnes, 1996, Hungr et al., 2001) which can affect large areas. In the alpine mountains, earth flows are common in the Flysch Zones and in the Northern Calcareous Alps, especially in the Hallstatt Zone. As the terminology employed by Cruden and Varnes (1996) is not used by all authors, the phenomenon is often described with the more general terms “landslide” or “mass movement” resulting in the underestimation of the frequency of earth flows. Short active phases of earth flows, lasting for a few weeks or months, often alternate with long inactive phases. Hence, it is extremely important to estimate the recurrence intervals of earth flows, as we do in the case study presented here.
To determine the age of mass movement events, different methods are applicable. An overview of dating methods applied to mass movements is given, for example, by Lang et al. (1999). Remnants of trees buried during single events are of special interest, as they can be radiocarbon dated providing a tool to estimate the reactivation frequency of mass movements.
Before numerical dating methods were available, the age of mass movements was derived from morphological observations in the field. Thus, it was assumed that most large Alpine mass movements were triggered by the retreat of the large glaciers during the late Pleistocene and early Holocene. In recent decades, more and more numerical dating methods have been applied to Alpine mass movements, especially to large rock slides and rock falls (Poschinger, 2002), but also to about 60 rock slope failures and about 60 debris flows in Tyrol (Austria) and its surroundings. Prager et al. (2008) show that there is no evidence for increased mass movement activity due to deglaciation processes during the late Pleistocene and early Holocene. Most of these mass movements provided Holocene ages, while only the oldest debris flows in the Tyrolean Inn Valley date back to around 13,400 ± 600 14C years BP (i.e. 14,200–17,800 cal BP) (Prager et al., 2008).
While earth flows in the Emilia-Romagna region (Northern Apennines, Italy) are already well dated (Bertolini et al., 2004, Bertolini et al., 2005), there are only very few numerical ages of Alpine earth flows. Bertolini et al. (2004) present 14C dates based on wood samples from 20 earth flows in the Emilia-Romagna, which allowed them to trace back the earth flow activity from 13,790–13,670 cal BP to 950–790 cal BP. In contrast, in our study area, only the oldest sediments of the Gschliefgraben earth flow, which will be discussed in detail below, have been dated to 16,450–15,650 cal BP (13175 ± 75 14C years BP; Eichkitz et al., 2009). Therefore, we here present new numerical data from an Alpine earth flow to enhance the understanding of mass movement activity in this densely inhabited and highly sensitive region.
The Stambach mass movement, which we discuss here, is similar to many others in the Alps and our study may thus serve as a representative example for adjacent regions. The last active phase occurred in 1982, after which monitoring of the area started. In the same region, at least three other large compound mass movements that were activated for the first time thousands of years ago have been reactivated several times since then: the Plassen mass movement, the Gschliefgraben mass movement, and the Sandling mass movement.
The Plassen mass movement covers an area of approximately 1 km2 between Mt. Plassen and the municipality of Hallstatt (Fig. 1; Rohn et al., 2005). Geotechnical, archaeological, and dendrochronological investigations indicate that earth flows were triggered by huge rock falls and/or rotational landslides in or shortly after 1245 cal BC (3195 cal BP), in the middle of the 4th century BC, and in 1985 (Ehret, 2009a, Ehret, 2009b).
The Gschliefgraben mass movement is located 3 km south of the town of Gmunden east of Lake Traunsee and covers an area of approximately 2 km2 (Fig. 1). After the last reactivation in winter 2007/2008, wood samples were obtained from sediment cores for 14C dating. They indicate that this mass movement was reactivated several times since at least 16,450–15,650 cal BP (Eichkitz et al., 2009).
In 1920, the collapse of a huge rock tower near the summit of Mt. Sandling, located 3.5 km east of our study area (Fig. 1), triggered an earth flow twice the size of the Stambach mass movement (Rohn et al., 2004). This was the first time that the initial formation of a large earth flow was observed by eyewitnesses in this area. Hence, no information about the recurrence interval of this earth flow is available so far.
There was no age information of previous active phases for the Stambach mass movement available. Only an old legend gives some evidence for the (re-)activation of the Stambach mass movement in historical times, possibly connected with damages and losses in the town of Bad Goisern. According to the legend, a “lindworm” (a snake-like dragon) was living in the mountains above the town of Bad Goisern. This lindworm destroyed the castle of King Cleonus and his family residing in Bad Goisern (Gebhart, 1862).
Section snippets
Study area
The study area is located a few kilometres northeast of the Upper Austrian town Bad Goisern (47°38′ N, 13°37′ E; Fig. 1). Geologically, this region belongs to the Northern Calcareous Alps (NCA; Frisch and Gawlick, 2003). The NCA are characterized by thick Mesozoic marine sedimentary formations. Triassic and Jurassic carbonates dominate, but Triassic and Jurassic clay- and marlstones are also common (Faupl and Wagreich, 2000). The main geotechnical features in the area are rigid slabs of
Geotechnical mapping and exploration drilling
In order to understand the geotechnical setting of the Stambach mass movement, an 8 km2 large area was mapped at 1:5000 scale. Fig. 2 shows a simplified version of this geotechnical map.
After the activity of the Stambach mass movement had slowed down to the order of centimetres per year, six exploration drillings (each between 18 m and 29 m deep) with double core barrel and 100 mm internal diameter were performed at different locations on the earth flow in the years 1988 and 1989 (B1–B6, Fig. 2,
Geotechnical investigations
Subsequent to the 1982 event, the Stambach mass movement system was intensely investigated and monitored. The regular inclinometric measurements show that there are continuous creep movements in the whole flow mass, but the velocity averages only 3 mm per year. Tape dilatometer measurements in fissures near the edge of the limestone scarp face yield opening rates of about 11 mm/yr on average (Fig. 3). The values have been nearly constant during the last 23 years of observation. Piezometric
Discussion
Early authors (Penck, 1882, Ampferer, 1904) thought that a destabilisation of the hillslopes caused postglacial mass collapses. However, Van Husen, 1977, Van Husen, 1987 later showed that even the external parts of the Dachstein area became free of ice during the Allerød Interstadial (ca. 13,900–12,650 cal BP). The latest ice re-advance in the area, the Egesen Stadial (ca. 12,400–12,300 cal BP; Kerschner et al., 2008), correlates with the early Younger Dryas, which is still more than 3000 years
Conclusions
We show that radiocarbon dating of wood remains buried by earth flow masses can be used – with some limitations – to reconstruct phases of activities of mass movements. For the Stambach mass movement we have been able provisionally to determine three active phases throughout the Holocene, before the most recent activity in 1982. The comparison of our data with landslide records from the Northern Apennines (Bertolini et al., 2004) and with the rapid climate change events during the Holocene as
Acknowledgements
We thank Bernd Kromer (Radiocarbon laboratory, Heidelberg Academy of Sciences) for facilitating the 14C measurements, Oliver Nelle (Graduate School “Human development in landscapes”, Kiel University) for determining the wood species, the German Research Foundation (DFG) for supporting the geotechnical investigations and the drillings during the research project “Mechanismus von Schuttströmen”, and the Austrian Service for Torrent and Avalanche Control for supporting the field works. We thank
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