HR: 1340h
AN: S13C-1072    [Abstracts]
TI: Correlation of oceanic microseisms at Californian seismic stations
AU: * Stehly, L
EM: laurent.stehly@obs.ujf-grenoble.fr
AF: Laboratoire de G‚ophysique Interne et Techtonophysique, 1381, rue de la Piscine, Grenoble, 38041 France
AU: Shapiro, N M
EM: nshapiro@ciei.colorado.edu
AF: Center for Imaging the Earth's Interior, University of Clorado at Boulder Campus Box 390, Boulder, CO 80303 United States
AU: Campillo, M
EM: michel.campillo@obs.ujf-grenoble.fr
AF: Laboratoire de G‚ophysique Interne et Techtonophysique, 1381, rue de la Piscine, Grenoble, 38041 France
AU: Ritzwoller, M H
EM: ritzwoll@ciei.colorado.edu
AF: Center for Imaging the Earth's Interior, University of Clorado at Boulder Campus Box 390, Boulder, CO 80303 United States
AB: We attempt to reconstruct Green functions between pairs of stations by cross-correlating records of the ambient seismic noise at those stations. We compute cross-correlations between vertical component records for several days of ambient seismic noise observed at various station-pairs located in California and separated by distances of a few hundreds kilometers. Emerging coherent waveforms are dominated by fundamental mode Rayleigh waves with travel times similar to those measured for the same paths from earthquake excited ballistic surface waves. This reconstruction is expected to work perfectly when the correlated wavefield is completely random and isotropic. Therefore, results of the cross-correlations provide also information about the degree of randomness and isotropy of the target ambient seismic noise.At periods corresponding to the oceanic microseisms (around 7-8 s), the amplitude of the emerging Rayleigh wave is larger for paths nearly perpendicular to the coastal line than for those that are nearly parallel to the coastal line. For paths perpendicular to the coastal line, where the ocean-solid Earth coupling occurs, the resulting cross-correlation are strongly asymmetric demonstrating that more energy is propagating from the coast than in the opposite direction. However, coherent Rayleigh waves also emerge from cross-correlations for inter-station paths that are almost parallel to the coastal line suggesting that, in addition to direct waves excited at the coast, oceanic microseisms contain a non-negligible amount of surface waves that were scattered on inhomogeneities within the Earth. We observe that the azimuthal distribution of energy in the noise is changing with the period band considered.Finally we show that meaningful geological information can be obtained by extracting surface waves from cross-correlations of the seismic noise, providing a way for a passive imaging of the Earths structure.
DE: 7255 Surface waves and free oscillations
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting