A Geomechanical Evaluation of the Rock Mass Involved in the 1963 Vajont Landslide (NE Italy)

Abstract. The “Masiere di Vedana” rock avalanche, located in the Belluno Dolomites (NE Italy) at the foot of Mt. Peron, is reinterpreted as historic on the base of archeological information and cosmogenic 36Cl exposure dates. The deposit is 9 km2 wide, has a volume of ∼170 Mm3 corresponding to a pre-detachment rock mass of ∼130 Mm3, and has a maximum runout distance of 6 km and an H∕L ratio of ∼0.2. Differential velocities of the rock avalanche moving radially over different topography and path material lead to the formation of specific landforms (tomas and compressional ridges). In the Mt. Peron crown the bedding is subvertical and includes carbonate lithologies from Lower Jurassic (Calcari Grigi Group) to Cretaceous (Maiolica) in age. The stratigraphic sequence is preserved in the deposit with the formations represented in the boulders becoming younger with distance from the source area. In the release area the bedding, the SSE-verging frontal thrust planes, the NW-verging backthrust planes, the NW–SE fracture planes, and the N–S Jurassic fault planes controlled the failure and enhanced the rock mass fragmentation. The present Mt. Peron crown still shows hundreds-of-metres-high rock prisms bounded by backwall trenches. Cosmogenic 36Cl exposure ages, mean 1.90±0.45 ka, indicate failure occurred between 340 BCE and 560 CE. Although abundant Roman remains were found in sites surrounding the rock avalanche deposit, none were found within the deposit, and this is consistent with a late Roman or early Middle Ages failure. Seismic and climatic conditions as landslide predisposing factors are discussed. Over the last few hundred years, earthquakes up to Mw=6.3, including that at 365 CE, have affected the Belluno area. Early in the first millennium, periods of climate worsening with increasing rainfall occurred in the NE Alps. The combination of climate and earthquakes induced progressive long-term damage to the rock until a critical threshold was reached and the Masiere di Vedana rock avalanche occurred.

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Structural and climate drivers of the historic Masiere di Vedana rock avalanche (Belluno Dolomites, NE Italy)

10.5194/nhess-2019-413 ◽

2020 ◽

Author(s):

Sandro Rossato ◽

Susan Ivy-Ochs ◽

Silvana Martin ◽

Alfio Viganò ◽

Christof Vockenhuber ◽

...

Keyword(s):

Rock Mass ◽

Structural Characteristics ◽

Rock Avalanche ◽

Upper Jurassic ◽

Lower Jurassic ◽

Runout Distance ◽

Ne Italy ◽

First Millennium ◽

Climatic Drivers ◽

Exposure Ages

Abstract. The “Masiere di Vedana” rock avalanche, located in the Belluno Dolomites (NE Italy) at the foot of the Mt. Peron, is re-interpreted as Historic on the base of archaeological information and cosmogenic 36Cl exposure dates. The deposit is 9 km2 wide, has a volume of ~ 170 Mm3 correspondings to a pre-detachment rock mass of ~ 130 Mm3, and a maximum runout distance of 6 km and an H / L ratio of ~ 0.2. Differential velocities of the rock avalanche moving radially over different topography and path-material lead to the formation of specific landforms (tomas and compressional ridges). In the Mt. Peron crown the bedding is subvertical and includes carbonate lithologies from lower Jurassic (Calcari Grigi Group) to Cretaceous (Maiolica) in age. The proximal deposit is made of Calcari Grigi Group limestone, the distal deposit comprises upper Jurassic limestones (Fonzaso Formation, Rosso Ammonitico, and Maiolica), while the middle Jurassic Vajont Limestone dominates the central sector of the deposit. In the release area the bedding, the SSE-vergent frontal thrust planes, the NW-vergent backthrust planes, the NW-SE fracture planes, and the N-S Jurassic fault planes controlled the failure and enhanced the rock mass fragmentation. Cosmogenic 36Cl exposure ages, mean 1.90 ± 0.45 ka, indicate failure occurred between 340 BC and 560 AD. Although abundant Roman remains were found in sites surrounding the rock avalanche deposit, none was found within the deposit, and this is consistent with a Late Roman or early Middle Age failure. Seismic and climatic drivers are discussed. Over the last few hundred years, earthquakes up to Mw 6.3 including that at 365 AD, affected the Belluno area. Early in the first millennium, periods of climate worsening with increasing rainfall are known in the NE Alps. The combination of climate and earthquakes induced progressive long-term damage to the rock. The present Mt. Peron crown shows hundreds of meters-high rock prisms bound by backwall trenches, suggesting a potential landslide hazard for the whole mountain belt north of Belluno affected by the same structural characteristics.

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