Geoscience Reference
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While everybody should agree with these general conclusions, despite formal
problems met by Keefer's investigation, it seems that an European undertaking of
this kind, with its specific statistical repartition of earthquakes; with its wealth of
information over centuries, with the possibility of a detailed knowledge of back-
grounds, could lead to better founded and more differentiated conclusions, specially
for processes associated with low intensities. Such an enquiry combining the wealth
of available earthquake and other information with field-work, would be useful in
the Alps and the Pyrenees and their foreland, for instance.
One of the main aims of future research in this area should be the often diffi-
cult distinction between fundamentally different earthquake-linked processes, two
extremes of which are outlined.
On the one hand, effects of different types (geotechnically speaking) and scales,
often clearly disproportionate with the seismic factor, are triggered in more or less
sensitive sites. Such effects have sometimes been named “purges”, the “cleaning” of
slopes, cliffs of vulnerable blocks. In any post-seismic enquiry, the following ques-
tions should be a first step: could such rockfalls and landslides also occur without
triggering by earthquakes? Did such normal processes occur in the past? The scale
of events should be considered.
Of course, local processes in most sensitive sites should be easily understandable.
The 1959 Ubaye (French Alps) earthquake triggered numerous falls of stones in an
area clearly affected by rain and snow melting (Lettre, 1959). At another scale the
1855 Valais earthquake was considered responsible for the revival and enlargement
of a 38 year-old crevice in a moving mass in Glarus (Volger, 1858). Problems are
more complex with “normal” processes at a “geological scale”, occurring in the
Alps, the Caucasus, etc. perhaps only at the interval of every 100 years or so and.
Sometimes these phenomena are too easily explained by some earthquake. So con-
sideration of time is also essential, with a more developed question: could “normal”
processes, without triggering, occur tomorrow, in a month, in a year, in a century?
On the other hand, genuine earthquake-linked geological effects mostly at a “ge-
ological scale” should be defined, with an acontrario question: was no “normal”
factor of some importance capable of producing such effects? While rocks simply
drop during a “purge”, the may be thrown from a cliff by an earthquake with a
high acceleration. The clearest case is of course faulting, possibly inducing other
minor secondary geological effects. More complex are once more delayed effects,
described by Solonenko (1976) from Devochan (Stanovoy): “The recurrence of col-
lapse common to Devochan is conditioned by a higly stressed state of the rock
masses in the body of the seismogenetic structure: therefore, large collpses often
occur without visible cause, even on seemingly stable slopes. During the Khait
earthquake, some new fractures systems appeared in the zone of collapse. These
systems predetermined the subsequent large collapse.” At such a geological scale
arise of course tricky problems of convergence of a mainly seismic factor or mainly
“normal” factors.
Even without a discussion of complex intermediate cases, between plain “purge”
and genuine seismogenetic effects sensu stricto , not in Solonenko's wide sense, it is
clear that such problems requiere a high interdisciplinary expertise, a sense of scales
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