Alessia Errera and Stefano Priano
On January 22, 2006 a dry-snow avalanche occurred in the Parsennfurgga (Kreuzweg) west face, in the Parsenn ski area (Davos). The avalanche covered an area of 0.9 ha and it was 240 m long (projected length) with a vertical drop of 130 m. The site is partly channelled with a mostly curved track. The release area was 53 m wide with a crown height of about 60 cm.
The avalanche featured a fluidized layer that extended to the sides of the main dense flow. This avalanche occurred after a minor snowfall of 10 cm in all the Parsenn area on a snow cover that contained weak strata.
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Fig. 1: Kreuzweg avalanche. The dense flow is coloured in green
while the fluidized layer is in dark blue. The location of the trenches
are indicated by the yellow triangles. |
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Fig. 3: The release area. |
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Fig. 2: In this image the deposit is clearly visible with a series of normal faults in the right part. Orange: The fluidized layer overtopped the moraine ridge whereas the dense part flowed around it. |
In particular at the side of the deposit we found some normal faults with a plunge (inclination of the failure plane with respect to the horizontal) that ranged from 40° to 50° and a set of fractures with a 90° plunge in the central part.
A normal fault is a type of fault in wich the hanging wall moves down relative to the footwall, the fault surface dips steeply, commonly from 40 to 60°.
A specific analysis through stereoplots in order to evidence the probable presence of conjugate sets couples was carried out. These couples feature the same direction and dip but an opposite azimuth. These structures are discussed in the famous “Anderson Theory”. The theory says that in a conjugate set of couples the intersection of the planes is parallel to the medium principal stress, the acute angle bisector coincides with the main principal stress, the obtuse angle bisector with the lower principal stress.
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Fig. 4: Inside the trench. The deposit is marked by the red line. D: dense flow; F: fluidized layer; S: snow stratification. |
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Fig. 5: Normal faults with 85° direction. Fault planes are indicated in yellow. |
Fig. 6: The entire snow profile (2.2 m). |
In our data we did not find clear evidence of these structures. There was something very similar to them but the topography was a complication factor that did not permit clear and incontestable statements.
It was nevertheless possible to conclude that the presence of normal faults would be good for better understanding snow motion. In addition the presence of these shapes, typical of structural geology, could explain some interesting features of the material involved. For example, the structures found indicate brittle behaviour of the material.
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Fig. 7: Planimetric view of the avalanche. The different sets of discontinuites are shown with the same colours in the map and in the relative boxes that contain the stereoplot of each set. In the legend set directions are specified. |
In particular the orange set is due to traction at the beginning of the motion on an open slope, the dark blue and the green were to the gravitative motion after the avalanche ascent on a ridge. At the end the violet set was due to the obvious avalanche extension after a channelized path.
Another useful statement is that all
the normal fault sets probably
present a transcurrent component. This means that the motion is not
only along the maximum dip line but also laterally.
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Fig. 8: Panoramic view of the Kreuzweg avalanche (composite image). |
