AMQ abs Rossato et al TEPRO 239-242.pub Available online http://amq.aiqua.it ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, Abstracts, AIQA 13-14/06/2018 Florence, 239 - 242 POST-LGM CATASTROPHIC LANDSLIDES IN THE DOLOMITES: WHEN, WHERE AND WHY Sandro Rossato1, Silvana Martin1, Susan Ivy-Ochs2, Alfio Viganò3, Christof Vockenhuber2, Manuel Rigo1, Nicola Surian1, Paolo Mozzi1 1 University of Padova - Department of Geosciences, Padova, Italy 2 Laboratory of Ion Beam Physics, ETH-Honggerberg, Zürich, Switzerland 3 Servizio Geologico della Provincia Autonoma di Trento, Trento, Italy Corresponding author: S. Rossato ABSTRACT: This paper aims to present the state-of-the-art about large and catastrophic landslides that have occurred in the Dolomites since the Last Glacial Maximum (LGM). Such events have been proved to happen multiple times in the same area, in correspondence of prolonged rainfall events, ice/snow melting and seismic shakings. Predisposing factors, such as rock fracturation and vertical bedding strata, may accumulate rock fatigue and induce the formation/weathering of shear planes, weakening the rock mass and allowing low- magnitude events to trigger the movement. KEYWORDS: Cosmogenic 36Cl, geological risk and hazard, geomorphic hazard, Dolomites 1. INTRODUCTION Landslides are phenomena that can deeply affect both natural and anthropogenic landscapes, and they can represent a serious risk (e.g. Petley, 2012). Under- standing their predisposing factors and potential triggers is of utmost importance to reduce their potential im- pacts, especially in the Alps where narrow valleys are increasingly inhabited (e.g. Huggel et al., 2012). In the last three decades, it has been estimated that the costli- est natural event occurred in Italy was the notorious Val Pola rock avalanche, occurred on 28th July 1987. It has been estimated that the total amount of damages reached US$ 400 million (Crosta et al., 2004). In the Alps, the largest landslides were presumed to have occurred during the latest phases of the Last Gla- cial Maximum (LGM), or within a few millennia after deglaciation as a response to glacial unloading of valley slopes. Recently, some of them, such as “Lavini di Marco”, (Martin et al., 2014), “Marocche di Dro” (Ivy- Ochs et al., 2017a), “Castelpietra” (Ivy-Ochs et al., 2017b) and “Mt. Peron” (Rossato et al., submitted), have been isotopically dated, proving to be Holocene in age, thus suggesting that such large events may occur without major changes in the equilibrium of the slopes. In the Dolomites several landslides occurred, such as the Fadalto, Antelao and Vajont events, causing damages and casualties, even when the moving masses were small. The Vajont event hit the headlines all over Europe for its disruptiveness, being the most catastrophic events in the study area, but other large landslides occurred in the past. Here we summarize the largest and most catastrophic landslides in the Dolo- mites, focusing on their type, timing and impact on hu- man life/activities, discussing their possible predisposing and triggering factors. We will not discuss Deep-seated Gravitational Slope Deformations and creep deforma- tions, since they are characterized by relatively low de- formation rates. 2. CATASTROPHIC DOLOMITES LANDSLIDES All landslides occurred in the dolomitic area that are relevant in terms of volume and/or impact on human activities are here presented (Tab. 1) and shortly de- scribed based on the papers listed. 2.1. Alta Badia (Borgatti & Soldati, 2010) Numerous events took place near Corvara, since 10 ka cal BP to present. Earth/mud/debris flows constitute most of the events, along with rotational rock slides. Wa- ter availability, due to permafrost melting and/or rainfall events, is believed to be the potential trigger for these events and climatic conditions are believed to be the main cause of widespread landsliding. 2.2. Alleghe (Ermini & Casagli, 2003) The Alleghe event was a large (10 Mm3) transla- tional rock fall, possibly evolved into a rock avalanche. The debris mass reached a runup elevation of about 150 m above the valley bottom, on the opposite flank. A sec- ond, smaller (3 Mm3) event took place some months after. Both events have been linked to the high amount of rain and snow occurred during that winter. 2.3. Antelao (Montandon, 1933) On 21st April 1814, a large debris mass fall from the Mt. Antelao, split into two tongues and inundated the localities of Taolen and Marceana, causing 260 casual- ties. A temporary lake formed in the Boite Valley. This event was the most damaging of a sequence of land- slides falling at the same place (e.g events are recorded also on 1348, 1729 and 1737 AD). 2.4. Col Mandro (Montandon, 1933) In December 1933, a debris mass, detached from the Col Mandro, dammed the Vanoi Valley, forming a temporary lake. This event was the biggest of a se- quence of mass movements in the area. It is believed https://doi.org/10.26382/AIQUA.2018.AIQUAconference that such events were due to deforestation and poor management of irrigation. 2.5. Cortina D’Ampezzo (Borgatti & Soldati, 2010) Since 14 ka cal BP, in this area several events (earth flows and translational rock slides) have oc- curred. As for the Alta Badia case, climate is invoked as the possible driving factor, due to their clustering in spe- cific time intervals. It is noteworthy that a single earth- flow was triggered by the 15th September shock of the 1976 Friuli earthquake sequence. 2.6. Fadalto (Pellegrini et al., 2006) This event is a large (135 Mm3) and complex land- slide involving various types of movements (rock slide, rock avalanche, debris slump, etc). It blocked the whole Valley, forming the S. Croce Lake. The initial trigger is believed to be the relaxation of the slopes owing to gla- cier retreat, but the main scarp has been repeatedly reactivated since then. 2.7. La Valle (Montandon, 1933) In April 1701, a “rock avalanche” (this term may not correspond to its modern scientific meaning) detached from the Mt. Moschesin, causing 48 casualties. Other minor events occurred later at the same place. 2.8. Marziai (Pellegrini et al., 2006) The rocky mass detached from Mt. Miesna, where rock layers are lightly dipping towards the slope and pervasive foliation is present. The landslide was sug- gested to have occurred in the first phase of deglacia- tion (i.e. 17-15 ka cal BP) by means of palynological data obtained from the remains of the temporary lake formed by the landslide itself. As for the Fadalto land- slide, the reaction of the slopes to glacier retreat is evoked as possible trigger. 2.9. Monte Salta (Marcato et al., 2007) This rock avalanche, which occurred in 1674 AD, is strictly connected to the presence of major faulting, fold- ing and fracturing of the rock mass, that dips steeply towards the slope. Such an unstable setting makes the slopes prone to failure and the authors suggested that seismicity may be, or may have been, the trigger. 2.10. Pecol (Montandon, 1933) This event occurred in 1841 AD, evolved by steps during an entire week, causing various casualties and destroying an entire part of the village of Pecol. 2.11. Mt. Peron (Rossato et al., submitted) This enormous rock avalanche (170 Mm3) fell from the top of Mt Peron and spread in the area where the villages of Peron, Ponte Mas, Vignole and Roe are cur- rently located. The event was believed to be Lateglacial in age, but recent cosmogenic dates and historical data proved that it occurred during the Roman Era, prior to the 2nd century AD. 2.12. Siror (Montandon, 1933) This event was one of the many mass movements related to the occurrence of an earthquake on 25th Janu- ary 1348. It detached from Mt. Belvedere and destroyed 240 Table 1 - Summary table of the largest and most damaging (in terms of human lives) landslides in the Dolomites. Rossato S. et al. 241 the village of Pubiaco, causing tens of casualties. 2.13. Vajont (Borgatti et al., 2004) The infamous, and extremely large, Vajont event was due to the reactivation of an old landslide, after nearly three years of progressive creeping. On 9th Octo- ber 1963, 270 Mm3 of debris and rock mass fell into an artificial reservoir, forming a flood wave that wiped out the village of Longarone, causing nearly 2000 casual- ties. 2.14. Val Cia (Montandon, 1933) The rocky mass detached from the left flank of the Cia Valley in the 1882 AD, blocked the valley, and formed a temporary lake that breached soon after. The resulting flood, 4 Mm3 large, affected the whole valley and propagated down to the Vanoi Valley. 2.15. Valle San Lucano (Aldighieri et al., 2016) On 3rd December 1908 a landslide detached from the fourth Pala di San Lucano and buried Pra and La- gunàz villages, causing 28 casualties. Unstable rock masses are still visible in the main scarp area. 3. PREDISPOSING AND TRIGGERING FACTORS The history of the Dolomites documented several landslides that caused damages and casualties, even when the moving masses were small (e.g. Valle San Lucano). The Vajont event hit the headlines all over Europe for its disruptiveness, being the most catastro- phic events occurred in the study area (Borgatti et al., 2004). However, other similar events occurred in the past, but are less known due to their “limited” amount of losses (e.g. Antelao) or old age (e.g. Mt. Peron). In some occasions, such landslides caused a radical change in the landscape, as the Alleghe landslide: the debris mass blocked the Cordevole River and induced the formation of the Alleghe Lake. Such natural barrier has been artificially stabilized, and the lake is still pre- sent (Draganits et al., 2014). Catastrophic slope failures are often related to the presence of discontinuities in the rock mass, as faults, shear zones and joints (Jaboyedoff et al., 2013; Stead and Wolter, 2015). Favorable bedding of strata (e.g. Mt. Peron) and the presence of relict landslides (e.g. Vajont) can concur to reduce the internal strength of the rock mass, contributing to the progressive accumulation of rock fatigue. The lower the energy required to break the equilibrium, the weaker the magnitude of the trigger event and the more probable the occurrence of forerun- ner events (Loew et al., 2017). Triggering events may be geological, hydrological, structural or even anthropo- genic (Cruden and Varnes, 1996; Wieczorek, 1996). Prolonged and intense rainfalls are recognized to initiate movements, along with progressive failure, seismic shaking/volcanic activity, human activity or accelerating creep (Keefer, 1993; Di Crescenzo and Santo, 2005; Adushkin, 2006; Loew et al., 2017). Catastrophic events may happen with (e.g. Loew et al., 2017), or without a clear trigger or precursory events (e.g. Dunning et al., 2006). The Dolomites are disposed to earthquake activity, as the instrumental record testifies (up to ~Mw=6.5; Viganò et al., 2015), and seismicity has been classified as a possible trigger for numerous Holocene large land- slides in the Alps (Ivy-Ochs et al., 2017a). Only one of the considered events has been surely related to an earthquake, but there is a long record of seismicity- related minor rockfalls and shallow landslides (Borgatti & Soldati, 2010). Conversely, some authors highlighted a good correspondence between large dolomitic land- slides (Borgatti & Soldati, 2010, Rossato et al., submit- ted) and rainfall climatic events/phases occurred at vari- ous scales (e.g. Tinner and Kaltenrieder, 2005; Benito et al., 2015; Rossato et al., 2015). 4. CONCLUSIONS As the predisposing factors (rock fracturation, verti- cal bedding strata, fracture planes cutting the strati- graphic sequence) and triggers (seismicity and/or rain- fall events) are often still present after the landslide oc- cur, there is a high likelihood of generating rock failures at the same place that may reach high magnitude. The dolomitic landslide record suggests that no exceptional event is required for large mass movements to happen, the continuing accumulation of predisposing factors lowering the energy needed to trigger them. Low- magnitude shakings, especially in combination with an increase of pore-pressure due to prolonged and intense rainfalls, may be enough to overcome the stability threshold. REFERENCES Adushkin V.V. (2006) - Mobility of rock avalanches trig- gered by underground nuclear explosions. Land- slides from Massive Rock Slope Failure, 267-284. Aldighieri B., Testa B., Bertini A. (2016) - 3D exploration of the San Lucano Valley: virtual geo-routes for everyone who would like to understand the land- scape of the Dolomites. Geoheritage, 8(1), 77-90. Benito G., Macklin M.G., Panin A., Rossato S., Fontana A., Jones A.F., Machado M.J., Matlakhova E., Mozzi P., Zielhofer C. (2015) - Recurring flood distribution patterns related to short-term Holocene climatic variability. Scientific Reports, 5, 16398. Borgatti L., Soldati M., Carton A., Corsini A., Galuppo A., Ghinoi A., Marchetti M., Oddone E., Panizza M., Pasuto A., Pellegrini G.B., Schiavon E., Sior- paes C., Surian N., Tagliavini F. (2004) - Geomor- phology and slope instability in the Dolomites (Northern Italy): from Lateglacial to recent geomor- phological evidence and engineering geological applications. Memorie descrittive della Carta Ge- ologica d’Italia 63(4). Borgatti L., Soldati M. (2010) - Landslides as a geomor- phological proxy for climate change: a record from the Dolomites (northern Italy). Geomorphology, 120(1), 56-64. Crosta G.B., Chen H., Lee C.F. (2004) - Replay of the 1987 Val Pola landslide, Italian alps. Geomorphol- ogy, 60(1-2), 127-146. Cruden D.M., Varnes D.J. (1996) - Landslides: investi- Catastrophic landslides in the Dolomites gation and mitigation. Chapter 3-Landslide types and processes. Transportation research board special report, (247). Di Crescenzo G., Santo A. (2005) - Debris slides–rapid earth flows in the carbonate massifs of the Cam- pania region (Southern Italy): morphological and morphometric data for evaluating triggering sus- ceptibility. Geomorphology, 66(1), 255-276. Draganits E., Grasemann B., Janda C., Hager C., Preh A. (2014) - 300MW Baspa II-India's largest private hydroelectric facility on top of a rock avalanche- dammed palaeo-lake (NW Himalaya): Regional geology, tectonic setting and seismicity. Engineer- ing Geology, 169, 14-29. Dunning S.A., Rosser N.J., Petley D.N., Massey C.R. (2006) - Formation and failure of the Tsatichhu landslide dam, Bhutan. Landslides, 3(2), 107-113. Ermini L., Casagli N. (2003) - Prediction of the behav- iour of landslide dams using a geomorphological dimensionless index. Earth Surface Processes and Landforms, 28(1), 31-47. Huggel C., Clague J.J., Korup O. (2012) - Is climate change responsible for changing landslide activity in high mountains?. Earth Surface Processes and Landforms, 37(1), 77-91. Ivy-Ochs S., Martin S., Campedel P., Hippe K., Alfimov V., Vockenhuber C., Andreotti E., Carugati G., Pasqual D., Rigo M., Viganò A. (2017a) - Geomor- phology and age of the Marocche di Dro rock ava- lanches (Trentino, Italy). Quaternary Science Re- views, 169, 188-205. Ivy-Ochs S., Martin S., Campedel P., Hippe K., Vocken- huber C., Carugati G., Rigo M., Pasqual D., Viganò A. (2017b) - Geomorphology and Age of Large Rock Avalanches in Trentino (Italy): Castelpietra. In Workshop on World Landslide Forum, 347-353. Springer, Cham. Jaboyedoff M., Penna I., Pedrazzini A., Baroň I., Crosta G.B. (2013) - An introductory review on gravita- tional-deformation induced structures, fabrics and modeling. Tectonophysics, 605, 1-12. Keefer D.K. 1993) - The susceptibility of rock slopes to earthquake-induced failure. Bulletin of the Associa- tion of Engineering Geologists, 30(3), 353-361. Loew S., Gschwind S., Gischig V., Keller-Signer A., Valenti G. (2017) - Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure. Landslides, 14(1), 141-154. Marcato G., Fujisawa K., Mantovani M., Pasuto A., Sil- vano S., Tagliavini F., Zabuski L. (2007) - Evalua- tion of seismic effects on the landslide deposits of Monte Salta (Eastern Italian Alps) using distinct element method. Natural Hazards and Earth Sys- tem Sciences, 7(6), 695-701. Martin S., Campedel P., Ivy-Ochs S., Viganò A., Alfimov V., Vockenhuber C., Andreotti E., Carugati G., Pasqual D., Rigo M. (2014) - Lavini di Marco (Trentino, Italy): 36 Cl exposure dating of a poly- phase rock avalanche. Quaternary Geochronology, 19, 106-116. Montandon F. (1933) - Chronologie des grands éboule- ments alpins du début de l’ère chrétienne à nos jours. Matériaux pour l’étude des calamités, 32, 271-340. Pellegrini G.B., Surian N., Albanese D. (2006) - Land- slide activity in response to alpine deglaciation: the case of the Belluno Prealps (Italy). Geografia Fisica e Dinamica Quaternaria, 29, 185-196. Petley D. (2012) - Global patterns of loss of life from landslides. Geology, 40(10), 927-930. Rossato S., Fontana A., Mozzi P. (2015) - Meta-analysis of a Holocene 14 C database for the detection of paleohydrological crisis in the Venetian–Friulian Plain (NE Italy). Catena, 130, 34-45. Rossato S., Martin S., Ivy-Ochs S., Viganò A., Vocken- huber C., Rigo M., De Zorzi M., Surian N., Mozzi P. (submitted) - The Mt. Peron rock avalanche (Eastern Alps): Roman age not Lateglacial. Geo- morphology. Stead D., Wolter A. (2015) - A critical review of rock slope failure mechanisms: the importance of struc- tural geology. Journal of Structural Geology, 74, 1- 23. Tinner W., Kaltenrieder P. (2005) - Rapid responses of high‐mountain vegetation to early Holocene envi- ronmental changes in the Swiss Alps. Journal of Ecology, 93(5), 936-947. Viganò A., Scafidi D., Ranalli G., Martin S., Della Ve- dova B., Spallarossa D. (2015) - Earthquake relo- cations, crustal rheology, and active deformation in the central-eastern Alps (N Italy). Tectonophysics 661, 81-98. Wieczorek G.F. (1996) - Landslide triggering mecha- nisms. Landslides: Investigation and mitigation, 247, 76-90. 242 Rossato S. et al. Ms. received: May 11, 2018 Final text received: May 28, 2018