HR: 09:10h
AN: S51D-05    [Abstracts]
TI: Diffuse Waves and Energy Densities Near Boundaries
AU: * Sanchez-Sesma, F J
EM: sesma@servidor.unam.mx
AF: Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Cd. Universitaria, Circ. Escolar s/n, Coyoacan, DF 04510, Mexico
AU: Rodriguez-Castellanos, A
EM: arcastel@imp.mx
AF: Instituto Mexicano del Petroleo, Eje Central L. Cardenas 152, Mexio, DF 07730, Mexico
AU: Campillo, M
EM: Michel.Campillo@obs.ujf-grenoble.fr
AF: LGIT ; Observatoire de Grenoble, Universite J. Fourier, BP 53, Grenoble, 38041, France
AU: Perton, M
EM: MPerton@iingen.unam.mx
AF: Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Cd. Universitaria, Circ. Escolar s/n, Coyoacan, DF 04510, Mexico
AU: Luzon, F
EM: fluzon@ual.es
AF: Departamento de Fisica Aplicada; Universidad de Almeria, Canada de San Urbano s/n, Almeria, 04120, Spain
AU: Perez-Ruiz, J A
AF: Departamento de Fisica Aplicada; Universidad de Almeria, Canada de San Urbano s/n, Almeria, 04120, Spain
AB: Green function can be retrieved from averaging cross correlations of motions within a diffuse field. In fact, it has been shown that for an elastic inhomogeneous, anisotropic medium under equipartitioned, isotropic illumination, the average cross correlations are proportional to the imaginary part of Green function. For instance coda waves are due to multiple scattering and their intensities follow diffusive regimes. Coda waves and the noise sample the medium and effectively carry information along their paths. In this work we explore the consequences of assuming both source and receiver at the same point. From the observable side, the autocorrelation is proportional to the energy density at a given point. On the other hand, the imaginary part of the Green function at the source itself is finite because the singularity of Green function is restricted to the real part. The energy density at a point is proportional with the trace of the imaginary part of Green function tensor at the source itself. The Green function availability may allow establishing the theoretical energy density of a seismic diffuse field generated by a background equipartitioned excitation. We study an elastic layer with free surface and overlaying a half space and compute the imaginary part of the Green function for various depths. We show that the resulting spectrum is indeed closely related to the layer dynamic response and the corresponding resonant frequencies are revealed. One implication of present findings lies in the fact that spatial variations may be useful in detecting the presence of a target by its signature in the distribution of diffuse energy. These results may be useful in assessing the seismic response of a given site if strong ground motions are scarce. It suffices having a reasonable illumination from micro earthquakes and noise. We consider that the imaginary part of Green function at the source is a spectral signature of the site. The relative importance of the peaks of this energy spectrum, ruling out non linear effects, may influence the seismic response for future earthquakes. Partial supports from DGAPA-UNAM, Project IN114706, Mexico; from Proyect MCyT CGL2005-05500-C02/BTE, Spain; from project DyETI of INSU-CNRS, France, and from the Instituto Mexicano del Petróleo are greatly appreciated.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7260 Theory
DE: 7290 Computational seismology
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2007 Fall Meeting