HR: 09:00h
AN: S31A-03 INVITED     [Abstracts]
TI: Imaging the Earth Structure From Long Distance Time Correlations of the Seismic Noise
AU: * Campillo, M
EM: Michel.Campillo@ujf-grenoble.fr
AF: LGIT Universite Joseph Fourier and CNRS, BP 53, Grenoble, 38041 France
AU: Shapiro, N
EM: nshapiro@fignon.colorado.edu
AF: Center for Imaging the Earth's Interior, Department of Physics, University of Colorado, Campus Box 390, Boulder, CO 80309 United States
AU: Stehly, L
EM: Laurent.Stehly@ujf-grenoble.fr
AF: LGIT Universite Joseph Fourier and CNRS, BP 53, Grenoble, 38041 France
AU: Ritzwoller, M
EM: ritzwoll@merckx.colorado.edu
AF: Center for Imaging the Earth's Interior, Department of Physics, University of Colorado, Campus Box 390, Boulder, CO 80309 United States
AB: The field to field correlation of diffuse waves has been shown to allow the reconstruction of the Green function between the two points of measure. This property is related to the property of equipartition of diffuse waves. An averaging over a set of distant sources is nevertheless required when dealing with data of finite duration as seismograms. The emergence of Green function from correlation of coda waves from large earthquakes can be observed for periods up to 50 s. At longer period, the scattering is less efficient to produce isotropic, equipartitioned wave-field. In absence of scattering, an almost perfect azimuthal coverage of source is required to reconstruct the Green function by correlation. These properties are illustrated by numerical, seismological and ultrasound experiments. Consequently, the possibility of using seismic ambient noise to reconstruct Green functions depends on the actual sources of the noise, which spatial distribution is still an open problem, and on the presence of scattering. When averaged over long time series, we expect at the first order the distribution of noise sources to randomize. This assumption does not need to be strictly fulfilled when scattering in the earth participate in randomizing the wave field. We present examples showing that long period surface waves can be reconstructed from seismic noise. At period between 5 and 100 seconds, the expected time symmetry of the correlation is often broken, an effect that we interpret as a mark of a net flux of energy resulting from a non-isotropic distribution of sources and an incomplete equipartition. The cross-correlation of records from a series of couples of stations can be used simply to determine the back-azimuth of the dominant source of noise without the restrictive conditions of array processing. We apply this approach with data from Europe and North America to investigate the main source regions. Finally, we present examples of tomographic images produced from surface wave measurements obtained from noise correlations.
DE: 7260 Theory and modeling
DE: 7294 Instruments and techniques
SC: Seismology [S]
MN: 2005 Joint Assembly