HR: 0830h
AN: S11E-0341    [PDF]
TI: Flux Anisotropy, Equipartition and Their Role in Field-Field Seismic Correlations: Theoretical and Numerical Investigations.
AU: Van Tiggelen, B
EM: Bart.Van-Tiggelen@grenoble.cnrs.fr
AF: Laboratoire de Physique et Modelisation de la Matiere Condensee, Maison des MagistŠres/UJF Polygone Scientifique Louis N‚el B.P. 166, Grenoble, 38042 France
AU: * Margerin, L
EM: Ludovic.Margerin@ujf-grenoble.fr
AF: Laboratoire de Geophysique Interne et Tectonophysique, Maison des Geosciences BP 53, Grenoble, 38041 France
AU: Campillo, M
EM: Michel.Campillo@ujf-grenoble.fr
AF: Laboratoire de Geophysique Interne et Tectonophysique, Maison des Geosciences BP 53, Grenoble, 38041 France
AB: Recent seismic experiments have reported the emergence of coherent propagating waves in averaged cross-correlations of diffuse seismic waves. The remarkable result is that coda records of several distant earthquakes at stations A and B yield the Green function $G_{AB}$ between the two stations. While existing theories -relying on the presence of equipartition of modes- predict that the averaged cross-correlations should be symmetric in time, -i.e. both the retarded and the advanced Green functions should emerge- the experiments have reported a clear asymmetry. We argue that this property can be explained by the persistence of an angular anisotropy in the diffuse field of seismic waves, caused by a nonzero energy current. We show theoretically that the existence of a current that transports the energy released by seismic sources to the stations entails the appearence of an antisymmetric term to the field-field correlation. Numerical solutions of the elastic radiative transfer equation reveal that while a stabilization of total $P$ and $S$ energies - known as equipartition - rapidly occurs in the seismic coda, the angular distribution of the energy flow remains strongly anisotropic even after a large number of scattering events. Although the flow must become isotropic at very late times, in the coda window of most seismic experiments the energy is mostly transported by waves coming from the source. Hence, an anisotropic source coverage is a very good candidate to explain the absence of symmetry in the observations.
DE: 3210 Modeling
DE: 7203 Body wave propagation
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting