The Energy Partitioning and the Diffusive Character of the Seismic Coda

Following recent theoretical developments of the radiative transfer the- ory of elastic waves, we propose to use the ratio R of energies of curl and divergence part of the ground displacement to distinguish between the different possible domi- nant scattering regimes in the lithosphere. A consequence of the diffusion regime is the partitioning of the energy between different vibrational modes, which is inde- pendent of time. It results in the stabilization of R. This behavior is not expected if low-order diffraction (such as single scattering) is dominant. We apply our technique to seismograms recorded by a small-aperture seismic array operated during June- August 1997 in Guerrero (Mexico). We estimate the energy ratio R in the coda of 13 earthquakes recorded by the array. We find it to be very stable in the coda window, while the energy level itself changes by several orders of magnitude. The value of R is 7 1, independent of the earthquake location and the magnitude. The observed stabilization of R is a strong indication of the diffusive regime of the seismic coda.