HR: 0830h
AN: NG41C-0081    [PDF]
TI: Search for Direct Empirical Spatial Correlation Signatures of the Critical Earthquake Model and a New Mechanism for Long-Range Interactions
AU: * Ouillon, G
EM: ouillon@free.fr
AF: Laboratoire de Physique de la Mati\`{e}re Condens\'{e}e, CNRS UMR 6622, Universit\'{e} de Nice-Sophia Antipolis, Parc Valrose, Nice, 06108 France
AU: Sornette, D
EM: sornette@moho.ess.ucla.edu
AF: Laboratoire de Physique de la Mati\`{e}re Condens\'{e}e, CNRS UMR 6622, Universit\'{e} de Nice-Sophia Antipolis, Parc Valrose, Nice, 06108 France
AU: Sornette, D
EM: sornette@moho.ess.ucla.edu
AF: Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, 3845 Slichter Hall, University of California, Los Angeles, CA 90095 United States
AB: We propose a new test of the critical earthquake model based on the hypothesis that precursory earthquakes are ``actors'' that create fluctuations in the stress field which exhibit an increasing correlation length as the critical large event becomes imminent. Our approach constitutes an attempt to build a more physically-based time-dependent indicator (cumulative scalar stress function), in the spirit of but, improving on the cumulative Benioff strain used in previous works documenting the phenomenon of accelerating seismicity. Using a simplified scalar space and time-dependent visco-elastic Green function of a two-layers model of the Earth lithosphere, we compute spatio-temporal pseudo-stress fluctuations induced by a series of events before four of the largest recent shocks in Southern California. Through an appropriate spatial wavelet transform, we then estimate the contribution of each event in the series to the correlation properties of the simplified scalar stress field around the location of the mainshock at different scales. This allows us to define a cumulative scalar stress function which shows neither an acceleration of stress storage at the epicenter of the mainshock nor an increase of the spatial stress-stress correlation length with time, in contradiction with those deduced previously from the cumulative Benioff strain. The earthquakes we studied are thus either simple ``witnesses'' of a large scale tectonic organization, or are simply unrelated, and/or the Green function describing interactions between earthquakes has a significantly longer range than predicted for standard visco-elastic media used here. We then propose a simple mechanism for these long-range interactions, based on seeing the Earth crust as crisscrossed by faults and cracks filled with fluid at close to lithostatic pressures. We develop a model in which its elastic modulii are different in net tension versus compression. In 2D, for a given strike-slip earthquake source, such nonlinear elasticity is observed to (i) rotate, widen or narrow the different lobes of stress transfer, (ii) to modify the $1/r^2$ 2D-decay of elastic Green functions into the generalized power law $1/r^{\gamma}$ where $\gamma$ varies with increasing tension-compression asymmetry and depends on the azimuth, and can reach values significantly lower than 1. Using reasonable estimates, this implies an enhancement of the range of interaction between earthquakes by a factor up to 5-10. This may explain certain long-range earthquake triggering and hydrological anomalies in wells and suggest to revisit the standard stress tranfer calculations which use linear elasticity.
DE: 3220 Nonlinear dynamics
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting