Time-varying earthquake occurrence in Canterbury following the Darfield earthquake

We present one year earthquake forecasts in the Canterbury region for the year 1 July 2011 to 30 June 2012, using three approaches. First we estimate the expected total number of aftershocks of the 4 September 2010 earthquake based on the Omori-Utsu law for aftershock decay and the Gutenberg-Richter frequency-magnitude relation. Secondly we produce forecasts using two well-studied models installed in the New Zealand earthquake forecast testing centre, namely the Short-Term Earthquake Probability (STEP) model and the Every Earthquake a Precursor According to Scale (EEPAS) model, on a spatial grid in steps of 0.1 magnitude units.

Under the Omori-Utsu law with our preferred parameter estimates, 23.2 aftershocks of the Darfield earthquake with magnitude 4.0 or greater are expected to occur during the year, with a 95% Poisson confidence range between 14 and 33 aftershocks. In a wider Canterbury region, 22.9 earthquakes with magnitude 4.0 or greater are expected according to the STEP model, and 2.3 earthquakes with magnitude 5.0 or greater are expected according to the EEPAS model.