CRITICAL-BEHAVIOR AND THE EVOLUTION OF FAULT STRENGTH DURING EARTHQUAKE CYCLES

The problem of how fault rheology and heterogeneity interact to produc e the observed scaling of earthquakes (such as the power-law moment-fr equency relationship) remains largely unsolved. Rock friction experime nts have elucidated the properties of smooth faults(1-3), but seem ins ufficient to explain the observed complexity of real fault dynamics(4, 5). The recognition of a connection between fault-related processes an d critical phenomena in other physical systems, together with numerica l models of repeated earthquakes, have resulted in significant progres s in the theoretical interpretation of earthquake scaling(4-14). But f ault rheology and heterogeneity have so far been treated separately. H ere I attempt to unify the requirements of fault rheology and heteroge neity using numerical calculations of quantized slip in an elastic con tinuum, I show that cyclical fault strength evolves naturally by means of a statistical selection for high-strength fault patches (asperitie s), resulting in the accumulation and eventual failure of those asperi ties, The applicability of these results to real fault systems is supp orted by a recent analysis of time-dependent earthquake statistics(15) . These results imply that self-similarity and criticality on a fault emerge during an earthquake cycle, and suggest that the character of l ocal seismicity can be useful in earthquake forecasting by revealing h ow advanced a fault is within its cycle.