Réf. Prager & al. 2008 - A

Référence bibliographique complète

PRAGER, C., ZANGERL, C., PATZELT, G.,  BRANDNER, R. 2008. Age distribution of fossil landslides in the Tyrol (Austria) and its surrounding areas. Natural Hazards and Earth System Sciences, 8, 377–407.

Abstract: Some of the largest mass movements in the Alps cluster spatially in the Tyrol (Austria). Fault-related valley deepening and coalescence of brittle discontinuities structurally controlled the progressive failure and the kinematics of several slopes. To evaluate the spatial and temporal landslide distribution, a first comprehensive compilation of dated mass movements in the Eastern Alps has been made. At present, more than 480 different landslides in the Tyrol and its surrounding areas, including some 120 fossil events, are recorded in a GIS-linked geodatabase. These compiled data show a rather continuous temporal distribution of landslide activities, with (i) some peaks of activity in the early Holocene at about 10 500–9400 cal BP and (ii) in the Tyrol a significant increase of deep-seated rockslides in the Subboreal at about 4200–3000 cal BP. The majority of Holocene mass movements were not directly triggered by deglaciation processes, but clearly took a preparation of some 1000 years, after ice withdrawal, until slopes collapsed. In view of this, several processes that may promote rock strength degradation are discussed. After the Late-Glacial, slope stabilities were affected by stress redistribution and by subcritical crack growth. Fracture propagating processes may have been favoured by glacial loading and unloading, by earthquakes and by pore pressure fluctuations. Repeated dynamic loading, even if at subcritical energy levels, initiates brittle fracture propagation and thus substantially promotes slope instabilities. Compiled age dating shows that several landslides in the Tyrol coincide temporally with the progradation of some larger debris flows in the nearby main valleys and, partially, with glacier advances in the Austrian Central Alps, indicating climatic phases of increased water supply. This gives evidence of elevated pore pressures within the intensely fractured rock masses. As a result, deep-seated gravitational slope deformations are induced by complex and polyphase interactions of lithological and structural parameters, morphological changes, subcritical fracture propagation, variable seismic activity and climatically controlled groundwater flows.

Mots-clés
 

Organismes / Contact

• alpS Centre for Natural Hazard Management, Innsbruck, Austria - prager@alps-gmbh.com
• ILF Consulting Engineers, Rum b. Innsbruck, Austria
• University of Innsbruck, former Institute for High Mountain Research, Austria
• University of Innsbruck, Institute for Geology and Paleontology, Austria


(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
       

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Austria Tyrol       ~ Holocene

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions

Central Greenland ice cores and Lake Ammersee (Bavaria, Germany) isotope records show rapid climate shifts between 15 and 5 ky, with significant warming at about 14.5 ky (transition Oldest Dryas–Bølling) and 11.5 ky (transition Younger Dryas–Preboreal; Grafenstein et al., 1999). After the Younger Dryas cold period, glaciers in the Central Eastern Alps rapidly retreated to modern extents. Subsequently, several smaller but nevertheless significant glacierand forest-line fluctuations indicate considerable changes of the Holocene climate. Glaciers varied in size around modern extents, yet were smaller for longer periods in the middle and early Holocene (Patzelt, 1977, 2005). The Lake Ammersee records clearly show the cold incursion of the 8.2 ky event and a plateau-like temperature trend with only minor fluctuations in the middle to younger Holocene (Grafenstein et al., 1998, 1999).

Glacier studies in the Central Eastern Alps show that long periods in the Holocene were characterised by favourable climatic conditions with average summer temperatures slightly higher than at present. These periods were repeatedly interrupted by relatively short, but pronounced deteriorations with multiple glacier advances, like the Löbben advance at about 3750–3250 cal BP (Patzelt and Bortenschlager, 1973). In line with this, Austria’s largest glaciers, the Pasterze and the Gepatschferner, were on several occasions and even for longer phases between 10 450 and 3650 cal BP smaller than at present and experienced several minor fluctuations between 3650 cal BP and the waning Roman age, until they reached their modern dimensions (Nicolussi and Patzelt, 2000, 2001).

Also in the Central Swiss Alps, unstable Holocene climatic conditions are indicated by glacier fluctuations featuring at least eight phases of significant glacier recession with several cold-wet periods in between. In the middle Holocene, in the period between 5290–3870 and 3640–3360 cal BP, glaciers were smaller than at present due to moderate climatic conditions (Hormes et al., 2001). Except for the non-correlating, but significantly high lake levels at 3500–3100 cal BP, younger Holocene lake level maxima in the Swiss Alps coincide with glacier advances. Thus these periods were characterised by a general drop in winter temperatures and an increase in summer moisture, controlled by fluctuations in solar activity (Holzhauser et al., 2005).

Observations
 
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

Bibliographic synthesis


(3) - Effets du changement climatique sur l'aléa
Reconstitutions

The Quaternary valley evolution in the Tyrolean Alps (Austria) is characterised by the occurrence of several deep-seated mass movements, the velocities of which range from slowly creeping landslides to catastrophic rockslides and rockfalls. In the majority of cases, radiometric dating of landslides in the Alps clearly yielded Holocene ages of failure, indicating that slope failures were not directly controlled by deglaciation processes. A dependency of Holocene landslide-activities on climatic fluctuations was assumed (e.g. Raetzo-Brülhart, 1997; Matthews et al., 1997; Dapples et al., 2003; Soldati et al., 2004).


Synopsis of data compilation

Dating data of about 60 rock slope failures and about 60 debris flows in the Tyrol and its surroundings have been compiled in a geodatabase and visualised in maps and graphs. As a first result, these data show a rather continuous temporal distribution of events in the Holocene, without longer time gaps. However, there is no evidence for increased landslide activities due to deglaciation processes during the Late-Glacial and early Holocene.

As a second result, slope collapses in the early Post-Glacial, at about 10 500–9400 cal BP, are only indicated by a few dates, but they comprise some of the largest failure events in the Alps. Among these are the deep-seated rockslides at Flims (volume at least 8000 million m3; Poschinger et al., 2006) and Kandertal (approx. 800 million m3; Tinner et al., 2005) in Switzerland as well as at Köfels (approx. 3200 million m3; Brückl et al., 2001) and Hochmais (approx. 30 million m3; Zangerl et al., 2007) in the Tyrol (Austria). Before and after these events, the compiled data show a lower frequency of dated landslides. Subsequent to 7500 cal BP, several smaller events, but also the huge Wildalpen rockslide (Styria, Austria; volume approx. 1400 million m3; Van Husen and Fritsch, 2007b) took place.

As a third result, numerous landslides were found to cumulate in the middle Holocene, with a significant emphasis in the Subboreal at about 4200–3000 cal BP (Prager et al., 2007). This temporal cluster, in graphs indicated by a less steep trend line of dated events, comprises some of the largest rockslides in the Tyrol. Among these are the deep-seated events at the Fernpass, Eibsee, Tschirgant and Tumpen, which also cluster spatially (“Fernpass cluster”), as well as those at the Hintersee and Pletzachkogel.

Periods of increased slope deformations in the early and middle Holocene were also established in adjacent regions of Austria. In central Switzerland two clusters of raised landslide activity were observed at about 10 000–9000 and 5200–1500 cal BP (Raetzo-Brülhart, 1997). In eastern Switzerland, five temporal pulses of slope instabilities were detected between 11 500–10 250, 6250–4800, 3500–2100, 1700–1150 and 750–300 cal BP (Dapples et al., 2003). In the Italian Dolomites two striking age-clusters were identified, one early Post-Glacial at about 13 000–9000 cal BP and the other one in the Subboreal at about 6500–2300 cal BP (Soldati et al., 2004). In the Trentino (Italy), different phases of rock slope failures have been determined at about 6500, 4700, 2200 and 1000 cal BP (Bassetti and Borsato, 2007). Data from outer-Alpine regions in Europe have not been compiled in this study, but they also point to a temporal clustering of slope instabilities e.g. in the early and middle Holocene (e.g. Matthews et al., 1997; Bertolini, 2007).

As a fourth result, dated debris flows, ranging from smaller local events to larger alluvial fans in the main valleys, also indicate periods of fluctuating accumulation activity. With respect to the Tyrolean Inn valley and its tributary rivers, Patzelt (1987) established phases of raised accumulation, firstly at about 9400 14C yrs BP (approx. 10 630 cal BP), secondly between 7500–6000 (approx. 8350–6840 cal BP) and thirdly at about 3500 14C yrs BP (approx. 3780 cal BP). According to this, some of the largest alluvial fans in Northern and Southern Tyrol, e.g. those of the rivers Gadria and Weißenbach, show significantly increased activity at about 7900–7100 cal BP. Other major debris flows accumulated considerably in the Subboreal, e.g. the rivers Sill and Melach at about 3700–3600 cal BP. In between these periods, at about 6000–4500 14C yrs BP (approx. 6840–5170 cal BP) the Inn valley was affected by a distinctive phase of fluvial erosion (Patzelt, 1987).

In the Lower Inn valley, two major phases of increased fluvial debris accumulation have recently been determined (Weber, 2003): within the first phase, the main rivers Inn and Wildsch¨onauer Ache show significantly increased accumulation between approx. 17 000–14 000 cal BP and mainly between approx. 7600–6000 cal BP; after a period of fluvial erosion, a second phase of increased debris accumulation occurred between approx. 4500(?) cal BP and the 19th century. Some minor debris flows in this region are characterised by fluctuating activity between approx. 12500 and 2050 cal BP (Weber, 2003).

Increased fluvial dynamics and debris accumulations in the middle to younger Holocene have also been established at other sites in the Tyrol and have recently been incorporated into a geodatabase. These records comprise the large Farchant debris flow (Fernpass region, dated 4340 and 3180 cal BP), the main river Isel (dated 5700– 2600 cal BP) and local events at Gepatsch/Kaunertal (dated 3990 cal BP), as well as data about sedimentological changes from fine to coarse deposits. The latter indicate phases of increased fluvial dynamics, which were observed e.g. in the northern Ötz valley at the Längenfeld basin (starting at approx. 3400 14C yrs BP, i.e. approx. 3650 cal BP), in the Stubai valley (prior to 1275 cal. BP; Blättler et al., 1995) and at some nearby torrents in the period between 3560–2590 cal BP (Mignon, 1971; Patzelt, 1987, 1999; Geitner, 1999; Veit, 2002).

As a fifth result, some radiometric data, which are not spatially attributable when depicted in graphs, prove polyphase reactivations of predisposed vulnerabilities and multiphase slope failures in different geological settings, e.g. at:
– Fernpass: a main event dating into the Subboreal, associated with a secondary rockslide and an unstable slope (Prager et al., 2006a, b),
– Tschirgant, Haiming and Pletzachkogel: multiple, but clearly differentiable rockslide events, with intense slope activities in the Subboreal (Patzelt, 2004a, b),
– Tumpen: several failure events, at least two of them being roughly dated, occurring within a relative small area (Poscher and Patzelt, 2000),
– Köfels: one well established main event (Heuberger, 1966), succeeded by a secondary rockslide (Ivy-Ochs et al., 1998; Hermanns et al., 2006),
– Several presently active landslides: pre-historical and/or historical precursory events documented at several active creeping slopes, e.g. at Gepatsch-Hochmais (Austria; Schmidegg, 1966), Heinzenberg (Switzerland; Weidner, 2000), La Clapière (France; Bigot-Cormier et al., 2005) and some catastrophic events, e.g. at Vajont (Italy; e.g. Hendron and Patton, 1987; Kilburn and Petley, 2003), Val Pola (Italy; e.g. Costa, 1991; Azzoni et al., 1992) and Randa (Switzerland; e.g. Sartori et al., 2003; Eberhardt et al., 2004).


Discussion

Compiled data of several sub-regions in Europe and of a few deep-seated landslides in the Alps suggest an increase in all types of rapid mass movements (i.e. landslides, debris flows, snow avalanches) in the middle and late Holocene, i.e. approx. after 5000 cal BP. Since this positively correlates with major phases of solifluction and glacier advances, landslides may contain proxy data on longer term variations of paleo-precipitation (Berrisford and Matthews, 1997; Matthews et al, 1997).

In Switzerland, Raetzo-Brülhart (1997) attributes two distinct clusters of raised landslide activity, at about 10 000– 9000 and 5200–1500 cal BP, to warmer and/or more humid paleoclimatic conditions. Dapples et al. (2003) correlate five late-glacial to Holocene pulses of increased landslide dynamics with glacier advances, increased solifluction and sedimentary changes within lacustrine deposits. Based on this, climatic deteriorations such as colder and especially more humid conditions controlled slope instabilities at about 11 500–10 250, 6250–4800, 3500–2100, 1700–1150 and 750–300 cal BP. Major landslide activities in the latest Pleistocene were succeeded by fluctuating activities due to variable climatic conditions, until approx. around 3800–3400 cal BP a climatic shift towards colder and wetter conditions led to another significant rise in slope activities (Dapples et al., 2003; Raetzo and Lateltin, 2003). Regionally, this climatic influence may have been further intensified by anthropogenic influences on vegetation (e.g. by forest clearing), which were indicated by significantly increased landslide activities in the Western Swiss Alps from 3650 cal BP onwards (Dapples et al., 2002).

In the Italian Dolomites, Soldati et al. (2004) also differentiated between two striking age clusters of landslides: an early post-glacial one at about 13 000–9000 cal BP, which is due to deglaciation processes and was probably favoured by increased precipitation and/or permafrost meltdown, and a younger one, at about 6500–2300 cal BP in the Subboreal, which is again assumed to correlate with an increase in precipitation. In the Northern Apennines (Italy), the majority of dated landslides yielded ages younger than 5000 cal BP and were assumed to correlate with climatic deteriorations (Bertolini, 2007; Bertolini et al., 2004).

However, several rockslides in higher Alpine environments occurred in periods of above-average temperature, when slope stabilities have been decreased by glacial debuttressing and/or thawing of permafrost (e.g. Geertsema, 2007). On according slopes, permafrost degradation may generally contribute to slope instabilities (Davies et al., 2001; Ballantyne, 2002; Gude and Barsch, 2005). Warming permafrost may be attributed to heat conduction and water percolation in fractures, i.e. crucial factors promoting failure in steep bedrock slopes (Gruber and Haberli, 2007). In view of these findings, thawing permafrost might have played a role during warmer periods in the Holocene, especially in the early Postglacial. Based on the radiometric dating of the Kandertal rockslide (Switzerland), Tinner et al. (2005) supposed that increased slope instabilities during the early Holocene were climatically controlled by a rise in precipitation and mainly by above-average (summer-)temperatures, which might have caused a withdrawal of permafrost due to a post-glacial climatic optimum. Temporally, the failures of some deep-seated rockslides in the early Holocene (e.g. at Köfels, Kandertal, Flims and Hochmais) coincide with an early phase of increased, precipitation-controlled, raised debris flow activities in the Tyrolean Inn valley, occurring at about 9400 14C yrs BP (Patzelt, 1987), i.e. approx. 10 630 cal BP.

Compiled dating data also indicate a significantly heightened landslide activity in the Subboreal at about 4200–3000 BP, which is clearly not directly linked to deglaciation processes. Several of these events, amongst them some of the largest rockslides in the Alps, are encountered in the Tyrol. They cluster both temporally and spatially (“Fernpass cluster”) and correlate with the activities of several large-scale debris flows in nearby major valleys. A phase of increased alluvial accumulation in the Inn valley, at about 3500 14C yrs BP (approx. 3780 cal BP), was established for some main tributaries such as the rivers Sill and Melach (Patzelt, 1987). This and the activities of local torrents and debris flows as well as glacier advances in the Austrian Central Alps indicate periods of greater water supply in the catchment areas.

Another coherent proxy for paleo-precipitation in the Eastern Alps has not been established yet. In the Mediterranean area, speleothem isotope records from central Italy coincide with a time of significant sapropel deposition and suggest enhanced regional rainfall between ca 8.9 and 7.3 ky, with a maximum between 7.9 and 7.4 ky (Zanchetta et al., 2007).

However, historically documented case studies show that increased precipitation is generally the dominant landslide trigger (Eisbacher and Clague, 1984; Gruner, 2006). Higher pore pressures favour large slope movements by increasing seepage forces and lowering the effective stresses respectively (e.g. Bonzanigo et al., 2000). They also accelerate the velocity of subcritical crack growth (Atkinson and Meredith, 1987) and reduce the friction angle of weathered and watersaturated rock surfaces, which is generally lower than that of dry and unweathered ones (e.g. Barton and Choubey, 1977).

Recent field studies in the Central Swiss Alps suggest that natural variations in groundwater pressure directly control seasonal slope deformations. The collected geodetic monitoring results show significant valley closures in the springtime, which are characterised by horizontal deformations of about 10–16mm and by vertical uplifts of about 10 mm. These elastic, reversible deformations normal to the valley axes correlate positively with groundwater recharge rates (Löw et al., 2007). In the long run, such annual openings and closures of valleys may, comparable to dynamic loadings by subcritical earthquakes, promote material fatigue due to brittle fracture propagation and may thus favour progressive failure of predisposed slopes.

Structural field investigations and subsurface data obtained at the basal Tschirgant massif (Tyrol, Austria), which is characterised by polyphase rockslide events, also yielded evidence of coupled hydro-mechanical destabilising processes. Here a test drill, at the base of the Haiming scarp, penetrated approx. 670m subhorizontally to the north and proved the existence of an effective water table. In the heavily fractured and thus highly permeable dolomites, dammed to the south by low permeable siliciclastics of the Raibl Group, steeply inclined hydraulic gradients and high pore pressures of up to 43 bar were measured (personal communication, Intergeo Consultants, Salzburg, Austria, 2005). These data suggest that here deep-seated slope deformations could have been favoured by climatically controlled groundwater level fluctuations.

Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

Deep-seated gravitational slope deformation is controlled by lithological, structural and morphological predisposition, by different time-depended long-term rock strength degrading processes and by shorter termed variable triggering factors. Detailed field studies of fossil and active landslides in the Tyrol indicate that these failures can basically be attributed to the complex intersection and coalescence of bedding and schistosity planes and brittle fault and joint systems, resulting in a substantial rock strength reduction extending to considerable depths (Brückl et al., 2004; Prager et al., 2006b, 2006c; Zangerl et al., 2007, 2008). Several of these deepseated gravitational slope deformations are encountered in seismically active regions and coincide temporally with periods of increased debris flow accumulations in the nearby main valleys. In view of these findings, rock strength degrading processes and parameters, which control slope instabilities in the Holocene, are discussed in the study: (1) Glacial loading and unloading, (2) Subcritical fracture propagation, (3) Dynamic loading, (4) Fault healing and fracture cementation, and (5) Climate changes.

This paper deals with the temporal distribution of dated mass movements in the Tyrol (Austria) and its surrounding areas. Here, several deep-seated landslides rank among the largest events in the Alps and show a close spatial distribution. A first comprehensive compilation of dated mass movements in the Eastern Alps has been made. It provides insights into potential causes and rock strength-degrading mechanisms that may have favoured slope failures during the Holocene.

To evaluate the spatial and temporal distribution of landslides in the Eastern Alps, a GIS-linked geodatabase has been set up. At present this includes various data of more than 480 different mass movements in the Tyrol and its surroundings, ranging from late-glacial to modern failure ages. Out of these, approx. 220 events feature unknown ages of failures and/or unknown activities. About 140 post-medieval to recently active landslides were compiled for the Tyrol but not considered in this study. Dated fossil (i.e. pre-historic and ancient historic) mass movements from adjacent areas such as southern Germany, northern Italy and eastern Switzerland were also included and presently comprise about 120 events. These are about 60 debris flows and about 60 rock slope failures, which are mainly rapid events such as rockfalls and rockslides.

Age determination of landslides is, most commonly, carried out by 14C-dating of organic remnants that are present in sediments overridden by the mass movement (maximum age of the event), and/or are entrapped within the landslide debris (proxy for event age), and/or accumulated in landslidedammed backwater deposits or lakes situated atop the mass movement (minimum age of the event). In the majority of cases, not the rock slope failure itself has been used for dating but material underlying and/or overlying the failed rock masses. Thereby, the time-lag between failure event and accumulation of the dated material can hardly be quantified. The available radiocarbon laboratory dates of 14C-dated mass movements were calibrated to calendar dates (quoted 0 BP=1950 AD) using the OxCal software version 3.10 (Bronk Ramsey, 2005) and its implemented calibration curve IntCal04. The ranges of the arithmetic mean ages are based on the statistical 2-σ standard deviation (corresponding to 95.4% probability). For mass movements featuring more than one dated sample, a mean and its standard deviation was calculated by applying the Gaussian error propagation law for linear cases on to the individual sample dating.


(4) - Remarques générales
 

(5) - Syntèses et préconisations

Conclusions

In the Tyrolean Eastern Alps, several well-exposed scarp areas show that slope failures were clearly structurally controlled by fracture propagation and the coalescence of brittle fault and joint systems. Morphological changes, due to fluvio-glacial valley deepening in the Pleistocene, uncovered preferentially orientated sliding planes and caused substantial stress redistributions in the undercut slopes. Since then, complex and time-dependent processes of subcritical fracture propagation have affected slope stabilities.

In order to identify potential causes and triggers of landslides, a first comprehensive compilation of dated mass movements in the Tyrol and its surroundings has been made. It reveals that the majority of Holocene mass movements were evidently not directly triggered by deglaciation processes, but needed a preparation time of some 1000 years, after the ice withdrawal, until the slopes collapsed. Some of the largest landslides in the Alps occurred in the early Holocene, at about 10 500–9400 cal BP. Remarkably, several deep-seated rockslides in the Tyrol were found to cluster temporally, at about 4200–3000 cal BP, and some of them also cluster spatially. This indicates striking environmental changes in this region in the middle Holocene.

In the Tyrol, several large rockslides are encountered near seismically active fault systems. Regional earthquake data record seismic events with comparatively high magnitudes M ≤ 5.3 and epicentral intensities I0 ≤ 7.5° as well as others, which are characterised by lower intensities but feature shallow-seated hypocentres located at depths of only 3-4 km. Active faults can not only directly trigger mass movements, but they can also produce intensely fractured and uncemented rock masses. Thus, repeated dynamic loading, even if at subcritical energy levels, initiates brittle fracture propagation and promotes slope instabilities.

Temporally, quite a few rock slope failures coincide with (i) dated landslides in the surrounding regions, (ii) increased debris flow activities and, partially, with (iii) glacier fluctuations in the Austrian Central Alps. In combination, these data may be proxy of paleoclimatic conditions and may indicate periods of raised precipitation and groundwater flows. This in turn, controls the pore pressure within fractured rock masses and favours progressive failure. Thus, structurally and morphologically predisposed mass movements were prepared and triggered not just by a single cause, but by the complex and polyphase interactions of several rock strength degrading processes. Deep-seated slope deformations may be attributed to critical fracture densities due to the propagation and coalescence of brittle discontinuities. This is favoured by different time-dependent and interacting processes which comprise (a) stress redistributions due to glacial loading and unloading, (b) subcritical crack growth, (c) seismic activity and (d) climatically controlled pore pressure changes. Any of these destabilising mechanisms, even if only at subcritical thresholds, can trigger a failure event if slope stability is already close to its limit equilibrium.

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