title: Consistent magnitudes over time for the revision of the New Zealand National Seismic Hazard Model
Creators: name: Christophersen, Annemarie
name: Bourguignon, Sandra
name: Rhoades, David A
name: Allen, T. I.
name: Salichon, Jerome
name: Ristau, John
name: Rollins, Chris
Publisher: name: GNS Science
Year: 2022

description: The revision of the National Seismic Hazard Model (NSHM) requires an earthquake catalogue to derive seismicity rate models. Earthquake catalogues are a record of past earthquakes and thus can inform the likelihood of future earthquakes. They include information about the time, location and size of earthquakes. Due to uneven seismic station coverage in space and changes in the seismic network and seismic data processing over time, earthquake catalogues are heterogeneous in space and time. Location methods and magnitude estimates have changed over time as instrumentation and methods have improved. In this report, we focus on deriving earthquake size estimates that are as homogeneous over as long of a time as possible. For seismic hazard modelling, the earthquake size is ideally expressed in moment magnitude to be consistent with modern ground motion characterisation models, which are calibrated to use moment magnitude. Routine moment magnitude calculations were introduced to New Zealand in 2007, although some earlier estimates for larger earthquakes are also available. The most prevalent magnitude in the New Zealand earthquake catalogue is a New Zealand-specific local magnitude, MLNZ77. This has been consistently calculated for the earliest instrumental records in 1931 until 2011, when a new seismic-processing software was introduced that allows for different magnitude-type calculations. The new types include a local magnitude MLSC3 and a summary magnitude M, which is a mix of MLSC3 and a broadband magnitude mB. The MLNZ77 scale is known to estimate larger values compared to moment magnitude for moderate-sized earthquakes in the range 4.5–6.0. A regression relation already exists to convert moment magnitude from the New Zealand local magnitude for earthquakes of magnitude 4.55 and above. Recently, a new local-type magnitude has been derived, MLNZ20, which is calibrated against New Zealand moment magnitude and is applicable for earthquakes of magnitude 5.5 and less. Here, we calculate MLNZ20 for more than 250,000 earthquakes using more than 2 million individual waveforms with sufficient quality from before and after the introduction of the new processing software. We derive regressions of MLNZ20 on the existing local magnitudes, MLNZ77 and MLSC3. These regressions allow us to derive moment-magnitude-consistent size estimates for more than 99% of earthquakes in the catalogue. The project has produced a curated version of the catalogue with these new magnitudes, which was available for the NSHM project in June 2021 and is now published as a stand-alone earthquake dataset. During the revision of this report, we have modified the data selection for the regression; thus, the revised magnitudes have slightly changed as described in this report. As the dataset is used, we expect further suggestions for improvement that we will capture in the metadata catalogue and address as appropriate in future updates. The report starts with some background on the seismic instrumental network development to better understand catalogue changes over time, including different location methods and velocity models used over time. The challenging task of relocating past earthquake origins was outside the scope of the current project but would be helpful in achieving even better consistency over time. Going forward, it will be important for GeoNet to provide a magnitude consistent with moment magnitude, such as including MLNZ20, in routine earthquake magnitude calculations. (The authors)