The Magnitude–Frequency Distributions of Earthquakes in Aotearoa New Zealand and on Adjoining Subduction Zones, Using a New Integrated Earthquake Catalog

Abstract

ABSTRACT Using a new integrated earthquake catalog for Aotearoa New Zealand (described in a companion article), we estimate the magnitude–frequency distributions (MFDs) of earthquakes in the greater New Zealand region and along the Hikurangi–Kermadec and Puysegur subduction zones. These are key inputs into the seismicity rate model (SRM) component of the 2022 New Zealand National Seismic Hazard Model. The MFDs are parameterized by a b-value (describing the relative rates of small and large earthquakes) with its epistemic uncertainty expressed by three logic tree branches (low, central, and high), and by the annual rate of M ≥ 5 earthquakes, here called the N-value, which has a separate value conditioned on each b-value branch. The N-value has its own epistemic uncertainty besides the dependence on the b-value, and this is also estimated here and propagated through the SRM by scaling all event rates up and down by a “low” and a “high” scalar value on either side of 1.0, called “N scaling.” Adapting an approach used previously in California, we estimate these MFD parameters in the onshore and near-shore region incorporating data back to 1843, balanced with the better data in the more recent part of the instrumental catalog. We estimate the MFD parameters on the Hikurangi–Kermadec and Puysegur subduction zones using a slightly simplified version of this approach and more recent data. We then use a globally-based method to estimate the potential earthquake rate uncertainty on the Hikurangi–Kermadec subduction zone and an SRM-specific moment-rate-related argument to construct an appropriately wide rate uncertainty for the Puysegur subduction zone.