Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

especially in the zones of hanging valleys. Fig. 3 indicates
the orientation of discontinuities in terms of their strike
direction. The dip angel is shown next to the joint
diagram. Apart from some exceptions, the measured
planes are striking nearly perpendicular to the
escarpment from east to west. With an average angle of
70°‐80° they dip uniformly into northern and southern
directions. The distinctive topographic front shown in
Fig. 1 evidence a “X‐formation” of two big crossing joints,
which represent the described inclination of tension
cracks in a two‐dimensional way.
Fig. 3: Joint diagrams presenting the orientation of tension
cracks in two of the hanging valleys
The La Laja slope face represents the orientation of
bedding planes. Due to this fact these discontinuities also
dip with approximately 70° into eastern direction. They
separate abundant, plan‐parallel columns with a width of
2 m on average. This is illustrated in Fig. 4, which shows
the structure of the rock formation using a 3D‐model
developed by a light detection and ranging method
(LIDAR).
Fig. 4: 3D‐LIDAR shot of the steeply inclined sandstone strata
LIDAR sends a wide‐ranged laser beam and is able to
detect the scattered light. The distance to an object is
determined by the measured time delay between
transmission of a pulse and detection of the reflected
signal. In contrast to RADAR, the wavelengths of LIDAR
are much. In order to reach every gap, two shoots were
taken from different angles and put together with a
special software. By using this method the range front
was measured with millimeter precision.
KINEMATICS AND FORMATION OF HANGING VALLEYS
Due to the special orientation of bedding planes and
tension cracks toppling failures of columns as well as
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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wedge sliding failures on two intersecting planes can be
observed along the range front. According to Dikau
(1996), mathematical constraints shown in Table 1 depict
the criteria whether a toppling or a sliding process occurs.
Table 1: Stability criteria of a cubic block
<
b/h > tan A block is stable
> b/h > tan
< b/h < tan
> b/h < tan
A block will slide but
not topple
A block will not slide
but topple
A block will both slide
and topple
The geometrical data is based on a single block model,
where is the slope angle, is the friction angle, and b/h
is the ration width versus height (Fig. 5).
Fig. 5: Marginal case of a block between a stable position, a
toppling and a sliding process
Toppling
Apart from these basics joint and bedding plane water
pressures can trigger or accelerate rock movement. Such
influences can be observed along the northern part of the
range front, where potholes give evidences for water
circulation between sandstone strata. Hence the columns
lose contact to in‐situ formations and topple eastwards.
Fig. 6 clarifies this process and points out the
sedimentation of lose masses between displaced strata.
The sediments weight acts like a wedge and accelerates
the toppling. The progression of weathering leads to
rockfalls with its source zone at the peak of the exposed
columns.
Fig. 6: E‐Toppling of thick columns along the escarpment