Standardizing Earthquake Magnitudes for the 2022 Revision of the Aotearoa New Zealand National Seismic Hazard Model

The 2022 revision of the New Zealand National Seismic Hazard Model-Te Tauira Matapae Pumate Ru i Aotearoa-requires an earthquake catalog that ideally measures earthquake size in moment magnitude. However, regional moment tensor solutions, which allow the calculation of moment magnitude M-wNZ, were introduced in New Zealand only in 2007. The most reported magnitude in the national New Zealand earthquake catalog is a variation of local magnitude M-L. In New Zealand, M-L is systematically larger than M-wNZ over a wide magnitude range. Furthermore, the introduction of the earthquake analysis system SeisComP in 2012 caused step changes in the catalog. We address the problems by converting magnitudes using regressions to define a standardized magnitude as a proxy for M-wNZ. A new magnitude, M-LNZ20, has an attenuation relation and station corrections consistent on average with M-wNZ. We have calculated M-LNZ20 for nearly 250,000 earthquakes between 2000 and 2020. M-LNZ20 is a reasonable proxy for M-wNZ for earthquakes with M-L< 5.5. For earthquakes with M-L > 4.6, M-wNZ is reliably available. We have applied ordinary least squares (OLS) regression for M-wNZ and M-LNZ20 on M-L before and after 2012. We argue that OLS is the most appropriate method to calculate a proxy for M-wNZ from individual M-L measurements. The slope of the OLS regression compares well to the slope from the method of moments, which accommodates equation error that is present when there is scatter beyond measurement error, as is the case for our magnitude data. We have defined as a proxy for M-wNZ a standardized magnitude M-std, which is M-w when available, M-LNZ20 with some restrictions as a second choice, and otherwise the magnitude derived from regression. Standardization of the magnitudes reduces the total number of earthquakes with a magnitude of >= 4.95 by more than half and corrects step changes in the spatial distribution of earthquakes between 2011 and 2012.