Geoscience Reference
In-Depth Information
1 Introduction
A first draft on geological effects has been prepared for the 1991 meeting of the ESC
WG on macroseismic scales, held in Munich. Later I received one contribution from
T. Zsiros and a paper by L. Serva written in 1990 in another context, but serving our
purpose (Serva, 1994); then I prepared a second draft at the last minute, in a haste.
I was in the unhappy situation of using mainly my own files and experience and
submitting mainly personal opinions. Nevertheless I tried:
to develop a more thorough and logical discussion, repeating myself to some
extent;
to give more examples of methodological interest, among a wealth of cases;
to formulate some guidelines, here and there.
This paper stems from the material prepared for that purpose.
2 Problems with Seismogeological Effects of Earthquakes
It is commonplace to state that the geological effects of earthquakes are often more
important than the direct ones. Such a statement was made, for instance, by Solo-
nenko (1976), at the 25th International Geological Congress: “. . . sometimes during
earthquakes most of the damage (up to 80-90%) is not due to the earthquake itself,
but to accompanying seismogravitational phenomena”. Such effects, are dealt with
by a wealth of papers, by seismologists, engineers, geologists, geomorphologists
and even historians, usually without, at my knowledge, any considered methodolog-
ical and interdisciplinary discussion. Although they are sometimes a main feature of
the interpretations of major events, geological effects are far from being mastered by
classical intensity scales, with criteria seemingly formulated by seismologists in a
kind of “splendid isolation”, while interdisciplinary discussion is needed even more
than for other criteria. Such a situation is rather paradoxical.
Of course, such remarks led to some misunderstandings in the past. Criticism
by people with geological backgrounds has sometimes been interpreted by seis-
mologists as a wish to discard geological effects from intensity scales. Doing so,
geologists would dig a pit for themselves. It would have been better, while dis-
cussing their use by scales, to emphasise geological effects and consider them more
systematically.
At this point, a discussion of the geological criteria used by the original MSK
(Medvedev-Sponheuer-Karnık) scale is useful. Some logical flaws can be pointed
out. In a general way, these criteria show a sharp contrast of major discontinuities
and utter precision given for one or another degree. Sometimes conditions are given
(one case of waterlogged soil, some cases of lithology, several cases of relief, etc.),
sometimes not. So river-banks, particularly sensitive, appear only with degree 9 in
a specific way. While the possibility of landslides in mountains is considered in a
general way for degree 6 (which good), only small slides under peculiar conditions
are used as criteria (along with others, of course) for degree 7 and even 8. Rockfalls
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