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appear only with degree 8 (not good), are not considered explicitly for degree 10, but
then are once more emphasized for higher degrees. Rivers are dammed with degrees
8, 10 and 12, but not explicitly for degrees 9 and 11. On the other hand, crevices of
different width, centimetres, several centimetres, 10 cm. and more, 1 m, are used as
criteria for degree 6, 8, 9, 10, sometimes under peculiar conditions. On the whole,
degree 7 is neglected. Liquefaction does not appear clearly (see degree 9).
Of course, these remarks are formal. Enlightened users would adapt criteria to
different cases with their specific backgrounds and extrapolate or interpolate when
necessary, but others, proceeding mechanically, uncritically, would make huge mis-
takes, with a lot of consequences (in the appraisal of seismic hazard, etc.) that we
cannot afford. Some cases can be given.
Rockfalls, frequent, and landslides, less frequent, are considered together with
a large extent, for two reasons. There are not always easily discriminated in old
sources, and give rise to similar methodological problems. While liquefaction may
be an important factor in landslides, it will be considered separately in the narrow
sense of liquefaction occurring in plains, clearly described by a wealth of sources.
For rockfalls and landslides let's consider some pitfalls.
First it should be stressed that most classical catalogues actually list as earth-
quakes some rockfalls and landslides that were not at all triggered by quakes. Such
confusion can result from:
the indiscriminate use of the word “earthquake” (or terraemotus) itself, by many
sources, ancient and modern;
an uncritical handling of such sources;
an inability to master the geological background of an event.
While classical catalogues allow a discrimination of more or less genuine effects
of earthquakes, sometimes at the price of painful retrieval of sources, misleading
statements resulting from hasty listing are not easily detected. Part of these problems
have been discussed by Albini and Vogt (1992).
A few significant examples are the following:
From France: since Perrey (1845) the otherwise well-known Pardines rockfall
(1733) is listed as an earthquake, latterly with an epicentre located by Rothe (1946).
From Spain: the 1885 Boltana landslide (1885) is wonderfully described by a lo-
cal newspaper (Diario de Huesca, 1885), which unfortunately uses in its title the
misleading word “Terremoto”, the reason why this event is listed as a quake in the
Ibero-Maghrebian catalogue (Mezcua and Martınez Solares, 1983).
From Morocco: it seems that a rockfall or a landslide in the northern Riff, where
they are common (Vogt, 1984), is responsible for a 1909 “earthquake” listed with
intensity 9 (!) by the same catalogue and 6 by Cherkaoui's (1988) Moroccan cata-
logue.
(Note: this has become, in modern databases, an earthquake of magnitude
6.1, the maximum event for the sensitive area around any projected crossing of the
Straits of Gibraltar).
In these cases (among many others) the fact that only one piece of information is
available, and from one location only, should have been a sufficient reason (besides
others) for some critical thinking
(Note: how should such a strong earthquake in
1909 not been felt more widely
?) Let's turn to more complex cases.
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