Accounting for earthquake rates’ variability through Uniform Rate Zone forecasts. Applications to the New Zealand Seismic Hazard Model update.

Pablo C. Iturrieta, Matthew C. Gerstenberger, Chris Rollins, Russ J. Van Dissen, Ting Wang, & Danijel Schorlemmer

Published September 11, 2022, SCEC Contribution #12441, 2022 SCEC Annual Meeting Poster #210

The distribution of earthquakes in time and space is seldom stationary. In low-seismicity regions, non-stationarity and data scarcity may preclude a significant statistical analysis. We investigate the performance of stationary Poisson forecasts (such as smoothed-seismicity models - SSM) in terms of the available training data. We design bootstrap experiments for multiple pairs of consecutive training/forecast windows of a catalogue to: (i) analyse the lowest available training data in which SSM performs spatially better than the least-informative Uniform Rate Zone (URZ) model; (ii) characterise the rate temporal variability in terms of its over-dispersion and non-stationarity. The experiments rely on the assumption that catalogues from high-seismicity regions can be used as proxy of long-term low-seismicity. Formally, the assumption of stationarity is relaxed into locally-incremental stationarity and self-similarity described by a power-law. Results show rate variability up to 10 times higher as predicted by Poisson, and evidence the impact of non-stationarity in assuming a constant mean rate in training-forecast periods. The description of rate variability is translated into a reduction of spatial precision, by using URZs, whose impact on seismic hazard is evaluated. First, analytical distributions are used to describe rate variability, which are conditioned to the information available from a training period. Under the assumption that strain-rate is related to time scales of earthquake interactions, we devise a data-driven method based on strain rate maps to delineate spatially URZs. A rate distribution is inferred for each URZ only from the training events within. We provide a set of forecasts for the update of the New Zealand National Seismic Hazard model, which have increased mean rates up to 4 times higher in extensive low-seismicity regions, compared to optimised SSM. The impact of the forecasts is studied the seismic hazard space, by implementing a negative-binomial distribution in the hazard equations. For a 10% probability of exceedance in 50 years, using URZ with rate variability descriptions increases the expected PGA up to 0.16 g in low-seismicity regions (e.g. Auckland, Dunedin) compared to SSM, whereas the effect on high-seismicity is minimal. Our results highlight the relevance, as well as the feasibility, of including analytical formulations of seismicity that extend beyond the inadequate stationary-Poisson description of seismicity.

Citation
Iturrieta, P. C., Gerstenberger, M. C., Rollins, C., Van Dissen, R. J., Wang, T., & Schorlemmer, D. (2022, 09). Accounting for earthquake rates’ variability through Uniform Rate Zone forecasts. Applications to the New Zealand Seismic Hazard Model update.. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)