Figure 4. b-value curves for the study region. (a) Discrete occurrence rates, where n is the discrete number of earthquakes in the magnitude range M ±0.25. Error bars show the uncertainty range determined using the method illustrated in Figure 3. A different estimate for the uncertainty in the number of events per year can be inferred from the plot because earthquake occurrences are approximately a Poisson process (the approximation is better for M > 6), and for a Poisson process the variance is equal to the mean. (b) Cumulative earthquake occurrence rates, where N is the total number of earthquake events of magnitude M or greater.