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

Abstract

The 2022 revision of the New Zealand National Seismic Hazard Model—Te Tauira Matapae Pūmate Rū 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 MwNZ, 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 ML. In New Zealand, ML is systematically larger than MwNZ 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 MwNZ. A new magnitude, MLNZ20, has an attenuation relation and station corrections consistent on average with MwNZ. We have calculated MLNZ20 for nearly 250,000 earthquakes between 2000 and 2020. MLNZ20 is a reasonable proxy for MwNZ for earthquakes with ML<5.5. For earthquakes with ML>4.6, MwNZ is reliably available. We have applied ordinary least squares (OLS) regression for MwNZ and MLNZ20 on ML before and after 2012. We argue that OLS is the most appropriate method to calculate a proxy for MwNZ from individual ML 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 MwNZ a standardized magnitude Mstd, which is Mw when available, MLNZ20 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.