HR: 15:10h
AN: S12C-04 INVITED     [PDF]
TI: Partitioning between P and S energies in the long-period seismic coda
AU: * Shapiro, N
EM: nshapiro@fignon.colorado.edu
AF: Department of Physics, University of Colorado, Campus Box 390, Boulder, CO 80309-0390 United States
AU: Campillo, M
EM: michel.campillo@obs.ujf-grenoble.fr
AF: Laboratoire de G$\'e$ophysique Interne et Tectonophysique, BP53, Grenoble, Cedex 9, 38041 France
AB: Long-period seismic signals excited by large (M$>8$) earthquakes remain above the noise level for tens of hours. This long-period seismic coda is composed of surface and body waves that circle or cross the Earth many times and, therefore, can be used to study its average anelastic and scattering properties. Because both direct and scattered waves remain trapped in the Earth, the scattering does not change the average energy balance of the long-period coda. As a consequence, traditional methods based on average time decay cannot be applied to the long-period coda to extract information about the Earth's scattering. Therefore, we propose here another approach that is based on recent theoretical developments of the radiative transfer theory and uses a partitioning between P and S energies as a marker for presence of multiple scattering. Without scattering, the P-to-S energy ratio and, as a consequence, the vertical-to-horizontal energy ratio (VHR) would grow infinitely with time because the quality factor inside the Earth is much higher for P waves than for S waves. This behavior is well predicted by synthetic seismograms computed in an average spherically symmetrical model (PREM). However, VHR calculated using the observed seismograms tend to stabilize after certain time (typically 40000 s at 100 s period) at a value that is independent of earthquake and station locations. We interpret the observed stabilization as an indication of the presence of the multiple scattering and the diffuse waves. This motivates further investigations of a possibility to use the long-period seismic coda, and specifically the cross-correlations between records at two stations, to extract a coherent information about the elastic response of the Earth.
DE: 5144 Wave attenuation
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting