Energy partition of seismic coda waves in layered media: theory and application to Pinyon Flats Observatory

Ludovic Margerin (1) , Michel Campillo (2) , B. van Tiggelen (3) , Renaud Hennino (2)

1 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement
2 LGIT - Laboratoire de Géophysique Interne et Tectonophysique
3 LPM2C - Laboratoire de physique et modélisation des milieux condensés

We have studied the partition of shear, compressional and kinetic energies in the coda of ten earthquakes recorded on a dense array, located at Pinyon Flats Observatory (PFO), California. We observe a clear stabilization of the shear to compressional ($W^s/W^p$) energy ratio in the coda, with an average value of about 2.8. The ratio between the vertical and horizontal kinetic energies ($V^2/H^2$) can be measured from 5 to 25Hz and shows an abrupt transitionfrom 0.1 in the 5-10Hz band, to about 0.8 in the 15-25Hz band. These measured values are in sharp contrast with the theoretical prediction for equipartitioned elastic waves in a homogeneous half-space. To explain these observations, we have developed a theory of equipartition in a layered elastic half-space. Using a rigorous spectral decomposition of the elastic wave equation, we define equipartition as a white noise distributed over the complete set of eigenfunctions. The theory predicts that close to the resonance frequency of a low-velocity layer, the ratio between shear and compressional energies strongly decreases. Using a detailed model of the subsurface at PFO, this conterintuitive result is found to be in good qualitative and quantitative agreement with the observations.

Domaines: Géophysique [physics.geo-ph]

Ludovic Margerin, Michel Campillo, B. van Tiggelen, Renaud Hennino. Energy partition of seismic coda waves in layered media: theory and application to Pinyon Flats Observatory. Geophysical Journal International, 2009, 177 (2), pp.571-585. ⟨10.1111/j.1365-246X.2008.04068.x⟩. ⟨hal-00261432⟩