Geoscience Reference
In-Depth Information
Several peculiar cases, easily leading to exaggerated intensities, should be
mentioned. First, let's consider coastal cliffs, with very different backgrounds.
While a small cape fell into the sea near Vostizza during the strong 1817 earthquake
(Pouqueville, 1820), two huge rocks fell from the cliffs of Yeu Island in 1808,
yet no damage to buildings was reported (Le Moniteur Universel, 1808; Journal
de l'Empire, 1808). Alluvial banks are most sensitive and prone to failure even
with low intensities. Sinkholes (
karst collapse
) occurring during quakes must be
considered with the utmost care.
Mastering whereabouts and background and, of course, common sense are
needed to solve many cases of more or less discordant evidence obtained at some
distance from geological effects. While damage in the town of Carpentras in 1738
(Daleman, 1740) suggests a degree
7 MSK, deep and large crevices in the coun-
tryside would mean, strictly, an intensity of 9 MSK, clearly incompatible. Simi-
lar problems arise in 1763 in Provence, with rockfalls in the Luberon mountains
(Anonymous, 18th cent.; Vogt, 1991) and in 1765 in the Pyrenees, with rocks rolling
in the valley of Couserans (Vogt, 1987). Such apparent discordances may remain
controversial for long years, a fine example, being the 1564 Nissart (hinterland of
Nice) sequence (Vogt, 1992; Moroni and Stucchi, 1993), with widely different inter-
pretations of rockfalls and landslides by catastrophist and anti-catastrophist minds
raising even a sort of political strife.
In a more general way, inquiries about the relative decrease of geological effects
with distance can be rewarding. Consider, for instance, the major 25 January 1946
Valais earthquake. While huge rockfalls occurred the epicentral area, minor ones
were reported from Savoie, for instance, of rocks of several m
3
on the banks of the
Rhone. Further away, on the shores of Lake Evian, fishermen's nets were buried by
sliding masses of mud, with intensity 5 (Lambert and Vogt, n.d.). Such a discussion
for the 1755 Lisbon quake would doubtless be enlightening, for instance about the
case of rockfall at Gibraltar.
Looking for coherence of intensities from geological effects and from other cri-
teria, investigators should also consider to what extent the same effects could also
occur with lower intensities, an important step for a sound assessment of risks at a
regional scale.
Such remarks could be developed at great length. While it is often suggested
that ground effects are particularly important for establishing the highest intensity
degrees, at the top of the intensity scale where damage effects on buildings have
probably reached saturation, these effects really have to be studied in the context of
the whole range of intensities.
It should be emphasised once more that rockfalls and landslides on the whole
occur within a wide range of intensities. A world-wide sample of forty events
(named “landslides” but including rockfalls) of twelve types has been discussed by
Keefer (1984), mostly from literature but, in the cases from the USA, from macro-
seismic enquiries. Looking for “the smallest earthquake that causes landslides”,
Keefer (1984) correlates his sample with magnitude (in a rather abstract way) and
intensities. We learn that “few landslides are caused by events smaller than ML 4.0”
and, for given types, the predominant minimum intensity was 6 MM, or in some
cases, 4 MM.
≤
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