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Researchers from the CNRS "Laboratoire de géophysique interne
et tectonophysique" (Laboratory of internal geophysics and tectonophysics,
CNRS – Université Joseph Fourier, Grenoble) have developed
a method that may ultimately improve the analysis of the composition of
the inner Earth using late waves following an earthquake. Through the
study of these so-called "coda" waves, the geophysicists have
shown that by stacking their pulses, they obtain a direct wave signal
(P and S waves). This new method, the details of which appear in the January
24, 2003, issue of the journal Science, is based on studying this
signal.
"Seismic codas" are the last visible waves after an earthquake.
They spread through the Earth for several minutes. They are the result
of a combination of direct P and S waves and undergo a number of heterogeneities
in the inner Earth. As they travel, they cross media with different properties,
causing them to diffract. Bit by bit, they rapidly lose their energy by
dividing; certain waves are reflected while others are refracted at the
media interfaces. This random and complex wave propagation therefore offers
evidence of the Earth's inner structure.
In Grenoble, a team led by Michel Campillo and Anne Paul(*)
, has studied the recordings taken at seismological stations based in
Mexico, where 101 earthquakes of a magnitude of over 4 were recorded in
1998. Their findings show that, after a few seconds, or even a few minutes,
the "coda" waves do indeed arrive at the seismological observatories.
These "codas" have undergone enough diffractions in the Earth's
crust to belong to what is known as a "diffuse field." The waves
move in all directions, thereby filling all the available space surrounding
them.
After having checked the nature of the waves, the geophysicists computed
the "codas" measured at two recording stations, A and B, located
several dozen kilometers apart. Using mathematical calculations, they
observed that the average correlation between signals was none other than
the seismic response that would have been recorded at station B if a strong,
brief pulse had been generated at station A. Moreover, this response between
stations corresponds to sources that are perfectly located in space and
time, which is not the case in the event of an earthquake.
Such findings could therefore enable scientists to describe the properties
of the inner Earth using these "codas." Geologists have always
limited their analysis to the local structure of geographical zones based
on the velocity of direct waves recorded at stations located near the
earthquake. Regions must be sufficiently seismic for this to work. Today,
researchers hope to be able to define the heterogeneities of the Earth's
superficial envelopes in geographical zones with very low seismicity,
thanks to these late waves coming from distant earthquakes.
(*)
"Laboratoire de géophysique interne et tectonophysique"
(Laboratory of internal geophysics and tectonophysics), in collaboration
with the "Laboratoire de physique et modélisation des milieux
condensés" (Laboratory of physics and modeling of condensed
media, CNRS – Université Joseph Fourier, Grenoble) and in
the framework of the research group entitled "Propagation des ondes
et imagerie en milieu aléatoire" (PRIMA, Wave propagation
and imaging in random media – CNRS, Paris).
Researcher
contact:
Michel Campillo
Tel: +33 4 76 82 80 36
e-mail: michel.campillo@ujf-grenoble.fr
Press contact :
Martine Hasler
Tel : +33 1 44 96 46 35
e-mail : martine.hasler@cnrs-dir.fr
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