Multiplicative earthquake likelihood models incorporating strain rates

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We examine the potential for strain-rate variables to improve long-term earthquake likelihood models. We derive a set of multiplicative hybrid earthquake likelihood models in which cell rates in a spatially uniform baseline model are scaled using combinations of covariates derived from earthquake catalogue data, fault data and strain rates for the New Zealand region. Three components of the strain rate estimated from Global Positioning System data over the period 1991–2011 are considered: the shear (SSR), rotational (RSR) and dilatational strain rates (DSR). The hybrid model parameters are optimized for earthquakes of M5 and greater over the period 1987–2006 and tested on earthquakes from the period 2012–2015, which is independent of the strain-rate estimates. The SSR is overall the most informative individual covariate, as indicated by Molchan error diagrams as well as multiplicative modelling. Most models including strain rates are significantly more informative than the best models excluding strain rates in both the fitting and testing period. A hybrid that combines the SSR and the DSR with a smoothed seismicity covariate is the most informative model in the fitting period, and a simpler model without the DSR is the most informative in the testing period. These results have implications for probabilistic seismic hazard analysis and can be used to improve the background model component of medium-term and short-term earthquake forecasting models.