Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Sliding
The sliding processes along the escarpment are closely
linked to the genesis of hanging valleys. Buoyant water
forces, weathering and possible seismic activity seem to
be important trigger mechanisms for the examined sliding
events but the principal reason for reducing slope stability
lies within the intersection of tension cracks. Close to the
triangular facets and hanging valleys north‐ and
southwards dipping discontinuities often build a line of
intersection that is inclined out of the slope (Fig. 7).
Thereby it forms a sliding axis and causes wedge failures,
a special type of movement for which Norrish and Wyllie
(1996) declare the following necessary condition:
slope > line of intersection > friction
Fig. 7: Wedge failure on two intersecting discontinuities with
a line of intersection which “daylights” the slope
Even if statistical evaluations indicate that most of the
intersection linears dip gently, the friction angle can be
exceeded because of water in cracks, that clearly reduces
shear strength. A wedge failure also might be triggered by
seismic energy. Recent tectonic is related to the ground
failures and architectural disturbances, which are
recorded at the nearby ancient Roman city of Baelo
Claudia (Silva et al., 2009).
All mentioned internal and external influences cause the
formation of hanging valleys which work as channels,
guiding the debris and boulders down the slope.
RUNOUT DISTANCES AND ROCK DEPOSITS
According to the steep slope below the range front falling
blocks reach distances of up to 600 m. In the course of
this study these long runout rockfalls as well as boulders
and debris up to a size of 2 m³ close to the range front
have been analysed with the software ROCKFALL 6.1.
Considering to selected slope and rock properties,
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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Fig. 8: Simulated rockfall along a section shape showing the
steepest path from the source zone
rockfalls have been simulated along several section
shapes showing the steepest pathes from the source zone
on. Exemplary, one of the calculated runout distances is
shown in Fig. 8 that also illustrates the energy and the
amplitude of a block versus distance to the source. Most
of the results concur with observed deposits being
mapped along the slope during the field work. It shows
that Rockfall 6.1 can offer a realistic simulation, however
only approximated values are used for parameter input.
This is helpful for classifying rockfalls and creating hazard
maps.
References
Dikau, R., Schrott, L. & Dehn, M. (1996). Topple. In: R. Dikau, D.
Brunsden, L. Schrott & M.‐L. Ibsen [Hrsg.], Landslide
Recognition, Identification, Movement and Courses, Wiley,
Chinchester, 29‐41.
Durand‐Delga, M. (2006). Geological adventures and
misadventures of the Gibraltar Arc. Z. dt. Ges. Geowiss., 157,
687‐716.
Norrish, N.I. & Wyllie, D.C. (1996). Rock slope stability analysis.
In: A.K. Turner & R.L. Schuster [Hrsg.], Spezial Report 247:
Landslides, Investigation and Mitigation, TRB, National
Research Council, National Academy Press, Washington, D.C.,
391‐425.
Silva P.G., Reicherter K., Grützner C., Bardají T., Lario J., Goy J.L.,
Zazo C., & Becker‐Heidmann P., (2009). Surface and
subsurface palaeoseismic records at the ancient Roman city of
Baelo Claudia and the Bolonia Bay area, Cádiz (South Spain).
Geological Society, London, Special Publications 2009; v. 316:
Palaeoseismology: Historical and prehistorical records of
earthquake ground effects for seismic hazard assessment.
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SE Spain. Earth‐Sci. Rev., 31, 153‐236.