Reassessing Geologic Fault-Slip Rates and Their Uncertainties in Aotearoa New Zealand

ABSTRACT Geological slip rates on faults are crucial inputs to modern seismic hazard models. This use often involves large compilations of slip rates spanning different data types, qualities, and methods, which may lead to incomplete handling of uncertainties and biases in slip rates. First, seismic hazard models need a slip rate for every potentially hazardous fault, but typically only a fraction of faults will have had offsets directly measured, and slip rates on the remaining faults are inferred qualitatively. Second, when a slip rate is measured from a cumulative fault offset over a small number of earthquakes, natural variability in the earthquake process creates biases and uncertainties in the estimates. Finally, the inclusion of open intervals on either end of the paleoseismic record can bias slip rates lower. These effects are not typically captured in the large slip rate compilations and workflows used by seismic hazard models. Here, we explore how handling these factors more fully impacts the slip rates and uncertainties used in the Aotearoa New Zealand National Seismic Hazard Model. We find that correcting for limited observational timespan has the largest effect on slip rate uncertainties, which on average increase by up to 2.9 times. This effect is largest on faults that have slip rates calculated over short timeframes and low activity rates. Faults with inferred (rather than measured) slip rates also experience a large change in uncertainty, becoming on average 2.7 times wider. For most faults, correcting for open intervals does not change slip rate substantially. However, open intervals remain important to consider on faults with long recurrence intervals and short slip rate timeframes. The work presented here is an initial exploration into the uniform treatment of slip rate in large fault datasets and illustrates the impact these additional sources of uncertainty have on slip rate variability.