The role of disorder and stress concentration in nonconservative fault systems

The aim of this paper is to understand the origin of the deviations from th e Gutenberg-Richter law observed for individual earthquake faults. The Gute nberg-Richter law can be reproduced by slider-block fault models showing in its quasi-static limit self-organized criticality. However, in this model limit the earthquake ruptures are described by propagating narrow slip puls es leading to unrealistic low stress concentrations at the rupture front. T o overcome such unrealistic rupture behavior, we introduce a new state-depe ndent stress distribution rule accounting for broader slip pulses up to cra ck-like behavior. Our systematic analysis of the generalized model shows th at the earthquake characteristics can be described in terms of critical poi nt behavior, resulting in subcritical, critical, and supercritical system s tates. We can explain the realized state of self-organized systems by the e ffect of individual ruptures on the stress field. This effect depends stron gly on the fault roughness. For spatially smooth systems, more realistic ru pture characteristics lead to supercritical behavior, equivalent to charact eristic earthquake distributions empirically observed for several individua l faults. For rough faults, earthquakes cannot rupture the whole system and seismic energy is released by small events only. The transition between bo th regimes occurs at an intermediate degree of heterogeneity, where the ear thquake activity is reminiscent of self-organized criticality. Thus, our re sults predict that for individual faults one should in general observe syst ematic deviations from the Gutenberg-Richter law for large earthquake sizes . (C) 2001 Elsevier Science B.V. All rights reserved.