Estimates of the time-varying hazard of rupture of the Alpine Fault, New Zealand, allowing for uncertainties

The time-varying hazard of rupture of the Alpine Fault is estimated using a renewal process model and a statistical method that takes account of uncertainties in data and parameter values. Four different recurrence-time distributions are considered. The central and southern sections of the fault are treated separately. Data inputs are based on estimates of the long-term slip rate, the average single-event displacement, and the dates of earthquakes that have occurred in the last 1000 yr from previous studies of fault traces, landslide and terrace records, and forest ages and times of disturbance. Using these data and associated uncertainties, the current hazard of rupture on the central section of the fault is estimated to be 0.0051, 0.010, 0.012, and 0.0073 events per year under the exponential, lognormal, Weibull, and inverse Gaussian recurrence-time distributions, respectively. The corresponding probabilities of rupture in the next 20 yr are 10, 18, 21, and 14%, respectively. The current hazard on the southern section of the fault is estimated to be 0.0033, 0.0075, 0.0070, and 0.0053 events per year for the four models, and the 20 yr probabilities 6, 14, 13, and 10%, respectively. Increased precision in the date of the second to last event on the southern section of the fault would result in only small changes to these rates and probabilities. The indicated hazard under the lognormal model is about double the long-term average rate but less than half of that estimated in previous studies that did not take account of all the uncertainties. Dating additional prehistoric ruptures is likely to have a greater effect on the hazard estimates than improved precision in the existing data.