Geoscience Reference
In-Depth Information
Some are linked with chronological problems. Rockfalls and landslides, some-
times in distant places, are easily attributed by old sources to earthquakes occurring
more or less at the same time, maybe within weeks or months. The lack of chrono-
logical precision makes any interpretation a difficult task, for instance for various
phenomena reported here and there at the time of the famous Basel earthquake of
1356. Such amalgams (in areas of high intensity) and accretions (at a distance) crept
into catalogues, easily escaping even critical minds.
While a more or less precise chronology is of help, numerous problems of diag-
nostics nevertheless arise. Consider as an example the southern Tunisian sequence
of 1881. On one hand we have rather precise information (chronology, damage)
from the Gabes area. On the other a rockfall is reported vaguely to the west, in
a uninhabited area. While an intensity is easily assessed in the first case, it is
practically unassessable in the second with the available meagre information. As
a consequence, estimating the location of an epicentre would be hazardous. Special
attention is required by complex sequences of earthquakes as well as of rockfalls
and/or landslides. So a thorough discussion of the famous Yvorne events of 1584
(Vogt, 1981), would be welcome to ascertain if the geological effects are the best
argument for the location of the epicentre (Alexander, 1983).
Even with the best possible knowledge of the chronology, the discussion of de-
layed geological effects is arduous. So De Quervain (1925) was tortured by ma-
jor rockfalls occurring on the shores of Walensee 34 h after the earthquake of 7
November 1924, with an epicentral intensity of V-VI. Disagreeing to some extent
with geologists, quoting a similar example from Central Alps in 1917, considering
theoretical accelerations, he insists on the earthquake as the main triggering factor,
despite the rather low intensity.
Actually, many seemingly clear cases are not clear at all unless the background
is known.
Among whereabouts, ground acceleration is of course fundamental with a wealth
of theoretical discussions, during the last years. Meteorological conditions are also
critical. At the time of the 1716 Algerian earthquake, a landslide occurred in the
Chelif basin, South of Algiers, after a period of heavy rains. This may be essential
for the understanding of the earthquake, but its interrelation is not an easy task
(Ambraseys and Vogt, 1988). A major rockfall in Southern Jura, 1791, is consid-
ered by Riboud (1817) a consequence of both rains and earthquake, the shaking
being actually slight. We would easily misinterpret geological effects of the 1887
Ligurian earthquake if we did not know that rain brought down rocks loosened by
the earthquake in the hinterland of Menton.
Backgrounds should be considered in the widest sense of the word. In many cases
they are not discussed at all or in a very unsatisfactory way even if information is
available.
In most cases, geological maps are hardly usable for such purposes for reasons
either of scale or of mapping doctrine. Many maps do not provide reasonable infor-
mation on surface formations, weathering, joints. Specific maps (maps of surface
formations, geotechnical maps) are seldom available. So field-work, possibly with
specialists, is needed in many cases to identify and weigh factors, a condition
sine
qua non
for the assessment of intensities from such data.
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