Using synthetic seismicity to evaluate seismic hazard in the Wellington region, New Zealand

A computer synthetic seismicity program for the Wellington region has been implemented and used to infer the effect of the 1848 Awatere and 1855 Wairarapa mega-earthquakes on the timing of the next characteristic (magnitude 7.35 or more) earthquake on the Wellington-Hutt Valley section of the Wellington Fault. We find that, on average over several hundred cases, a Wellington Fault event is delayed by 259 years. The synthetic catalogue of 500 000 events, magnitude 5.5 or more, also gives the average recurrence time of characteristic Wellington Fault events (960 yr) and their Coefficient of Variation (0.40). There is also a strong retarding effect on moderate magnitude Wellington Fault events, magnitude 6.5-7.3. Sensitivity tests indicate that these results are robust. The synthetic model contains 58 known major faults and 3000 randomly placed small faults to reproduce the rate and spatial distribution of background seismicity observed recently. From the synthetic catalogue it is possible to determine the average recurrence rate for all major faults. Also, there is strong evidence of the short-term (10 yr) temporal clustering of large events throughout the region, a factor that contributes to a highly variable moment release rate over time spans of a few hundred years.