Probability of rupture of the Alpine Fault: allowing for uncertainties

Executive Summary

The time-varying hazard of rupture on the Alpine fault is estimated using a technique that takes account of uncertainties in data and parameter values. The north-east (Karangarua-Haupiri) and south-west (Haast) sections of the fault are considered separately, i.e., it is not assumed that they will necessarily rupture together. Data inputs are based on geological estimates of the long-term slip rate and previous studies of fault traces and forest ages and times of disturbance. The geological average strike-slip rate is taken to be 26±5 mm/yr and the average single-event displacements to be 5.0±1.4m and 8.0±2.6m on the north-east and south-west sections, respectively. The last four events on the northwest section are dated at 1717, 1620±10, 1445±20 and 1150±50, and the last three events on the south-west section at 1717, 1450±100 and 1150±50 A.D. Using these data and associated uncertainties, the current hazard of rupture on the north-east section of the fault is estimated to be 0.0049, 0.0092, 0.0104 and 0.0074 events per year under the exponential, lognormal, Weibull and inverse Gaussian models respectively. The corresponding probabilities of rupture in the next 20 years are 9%, 17%, 19% and 14%, respectively. The current hazard on the south-west section of the fault is estimated to be  0.0032,0.0072,0.0064 and 0.0052 events per year for the four models, and the 20 year  probabilities 6%, 14%, 13%, and 10%, respectively. Increased precision in the date of the
second to last event on the south-west section of the fault would result in only small  changes to these rates and probabilities. The hazard under the lognormal model is about double the long-term average (exponential) rate but less than half of that previously estimated without taking account of uncertainties in the data and parameter values.