What controls an earthquake's size? Results from a heterogeneous cellular automaton

The controls on an earthquake's size are examined in a heterogeneous cellular automaton that includes stress concentrations which scale with rupture size. Large events only occur when stress is highly correlated with strength over the entire fault. Although the largest events occur when this correlation is the highest, the magnitude of the correlation has no predictive value as events of all magnitudes occur during times of high stress/strength correlation. Rather, the size of any particular event depends on the local stress heterogeneity encountered by the growing rupture. Patterns of energy release with time for individual ruptures reflect this heterogeneity and many show nucleation-type behaviour, although there is no relation between the duration of nucleation phase and the size of the event. These results support the view that earthquake size is determined by complex interactions between previous event history and dynamic stress concentrations and suggest that deterministic earthquake prediction based on monitoring nucleation zones will not be possible.