1. Accelerating Precursory Activity in Statistical Fractal Automata Dion Weatherley and Peter Mora QUAKES, Univ. of Qld., Australia.

2. Outline Model Description Results from a parameter space study Event size scaling Evolution of mean stress Stress field evolution Accelerating precursory activity Summary of results Effect of varying the fractal dimension Conclusions and questions

3. Model Description Uniform rectangular grid of cells Each cell has two properties Constant, scalar Strength Variable, scale Stress Uniform loading of all cells Cell failure when a cell’s stress exceeds its strength Nearest Neighbour stress redistribution Periodic boundary conditions

4. Event size scaling

5. Mean Stress Evolution models with a char. event distribution (A) relatively low time-averaged mean stress large, saw-tooth fluctuations models with a roll-over distribution (D) high time-averaged mean stress small, irregular fluctuations models with high dissipation (C) near-constant mean stress models with GR-scaling (D) correspond with drop in time-averaged mean stress mark a transition from large to small fluctuations

6. Snapshots of Stress Evolution

7. Accelerating Precursory Activity 1.Select the 100 largest events in each simulation for analysis. 2.Define Tf as the time of the large event. 3.Vary To and m within a range. 4.Compute A,B via least squares fit. 5.Compare power-law with a linear fit. 6.Select best power-law fit for each event. 7.Count number of good power-law fits for each simulation = fit probability.

8. Summary of Results Accelerating Precursory Activity in models with: overabundance of larger events relatively low time-averaged mean stress large, saw-tooth fluctuations in mean stress relatively smooth stress field over broad regions Constant rate of activity in models with: an under-abundance of large events high, near-constant mean stress stress field dominated by small-scale heterogeneity Evidence for a distinct transition between two modes of activity GR-scaling only for simulations along a line through parameter space change in character of mean stress fluctuations across this line change in stress field heterogeneity across this line probability of accelerating precursory activity drops to near-zero at this line

9. Effect of varying the Fractal Dimension of cell Strengths D=1.875 D=2.250 D=2.625

10. Conclusions and some Questions Accelerating Precursory Activity occurs in models in which : there is an overabundance of larger events these large events perturb the system from a state of high stress stress redistribution smoothes the stress field within failed region long-range clustering of cell strengths (smaller fractal dimension) Stress field heterogeneity apparently a factor determining the mode of activity relatively smooth stress field corresponds with accelerating activity a stress field dominated by small-scale heterogeneity corresponds with a constant rate of activity Some questions to motivate discussions Is the transition line a SPINODAL i.e. a line of Critical Points? How might one formulate a mathematical description for these models? Are these models any more or less appropriate as earthquake analogues than pseudo-spinodal or SOC models?