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Structural geologists are well aware of the fact that isotropic
rocks are quite exceptional in nature. Whicheverorigin,
sedimentary, metamorphicormagmatic, rocks are shaped with a plane
of mineral flattening, the foliation in geologists' jargon, and
with a line ofmineral elongation, the lineation. Just like a good
quarryman, a trained structural geologistwill
detectapreferredorientationin an apparently isotropic granite.
Preferred mineral orientation and thus structural anisotropy are
the rule in nature. Consideringthe
largevariationsinelasticcoefficientsofrock-forming minerals,
itcould be predicted that, in turn, seismic anisotropy should exist
and be important, provided thatdomains withasimilarstructural
signatureare largeenough to affectseismic waves. This is why, in
1982 at a conference held in Frankfurt, which was oneofthe fIrst
meetings devoted to the subject of seismic anisotropy, I asked Don
Anderson the question of why seismologists had not considered
earlier in their models the obvious constraint of anisotropy. I
still remember Don's answer: "Adolphe, we knew that our isotropic
models were not very good but we had no other choice. It is simply
that, so far, computerswere not largeenough tointegrate the
anisotropy parameter." Changingisotropic glassesfor anisotropic
ones permits us to obtain betterand more realistic seismic
modelsofthe Earth's interior, but, maybe more importantly, it has,
for a seismologist, the farreaching consequenceofsteppinginto the
fIeld ofgeodynamics.
Structural geologists are well aware of the fact that isotropic
rocks are quite exceptional in nature. Whicheverorigin,
sedimentary, metamorphicormagmatic, rocks are shaped with a plane
of mineral flattening, the foliation in geologists' jargon, and
with a line ofmineral elongation, the lineation. Just like a good
quarryman, a trained structural geologistwill
detectapreferredorientationin an apparently isotropic granite.
Preferred mineral orientation and thus structural anisotropy are
the rule in nature. Consideringthe
largevariationsinelasticcoefficientsofrock-forming minerals,
itcould be predicted that, in turn, seismic anisotropy should exist
and be important, provided thatdomains withasimilarstructural
signatureare largeenough to affectseismic waves. This is why, in
1982 at a conference held in Frankfurt, which was oneofthe fIrst
meetings devoted to the subject of seismic anisotropy, I asked Don
Anderson the question of why seismologists had not considered
earlier in their models the obvious constraint of anisotropy. I
still remember Don's answer: "Adolphe, we knew that our isotropic
models were not very good but we had no other choice. It is simply
that, so far, computerswere not largeenough tointegrate the
anisotropy parameter." Changingisotropic glassesfor anisotropic
ones permits us to obtain betterand more realistic seismic
modelsofthe Earth's interior, but, maybe more importantly, it has,
for a seismologist, the farreaching consequenceofsteppinginto the
fIeld ofgeodynamics.
Proceedings of the Summer School organized in Strasbourg, France,
July 15-August 1, 1986, by European Mediterranean Seismological
Centre and Institute de Physique du Globe de Strasbourg
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