Upper Plate and Subduction Interface Deformation Models in the 2022 Revision of the Aotearoa New Zealand National Seismic Hazard Model

Abstract

As part of the 2022 revision of the Aotearoa New Zealand National Seismic Hazard Model (NZ NSHM 2022), deformation models were constructed for the upper plate faults and subduction interfaces that impact ground-shaking hazard in New Zealand. These models provide the locations, geometries, and slip rates of the earthquake-producing faults in the NZ NSHM 2022. For upper plate faults, two deformation models were developed: a geologic model derived directly from the fault geometries and geologic slip rates in the NZ Community Fault Model version 1.0 (NZ CFM v.1.0); and a geodetic model that uses the same faults and fault geometries and derives fault slip-deficit rates by inverting geodetic strain rates for back slip on those specified faults. The two upper plate deformation models have similar total moment rates, but the geodetic model has higher slip rates on low-slip-rate faults, and the geologic model has higher slip rates on higher-slip-rate faults. Two deformation models are developed for the Hikurangi–Kermadec subduction interface. The Hikurangi–Kermadec geometry is a linear blend of the previously published interface models. Slip-deficit rates on the Hikurangi portion of the deformation model are updated from the previously published block models, and two end member models are developed to represent the alternate hypotheses that the interface is either frictionally locked or creeping at the trench. The locking state in the Kermadec portion is less well constrained, and a single slip-deficit rate model is developed based on plate convergence rate and coupling considerations. This single Kermadec realization is blended with each of the two Hikurangi slip-deficit rate models to yield two overall Hikurangi–Kermadec deformation models. The Puysegur subduction interface deformation model is based on geometry taken directly from the NZ CFM v.1.0, and a slip-deficit rate derived from published geodetic plate convergence rate and interface coupling estimates.

Upper Plate Faults May Contribute to the Paleoseismic Subsidence Record Along the Central Hikurangi Subduction Zone, Aotearoa New Zealand

“…The extent of subduction interface slip drastically affects expected earthquake size, deformation, and hazard potential. The NZ NSHM incorporates the subduction interface coupling distribution for estimating shaking hazard from the subduction zone (Van Dissen et al., 2022). Because the paleoseismic record in Hawke's Bay indicates past large earthquakes, but the modern coupling distribution shows almost no slip deficit (i.e., no coupling), the hazard model manually imposes a slip deficit of 20% to the central Hikurangi margin.…”

“…The discrepancy between the inferred earthquakes in the Ahuriri Lagoon record and the modern low coupling along the central margin is difficult to reconcile. The current New Zealand National Seismic Hazard Model (NZ NSHM) takes a conservative approach to this problem and incorporates a higher coupling coefficient for the central Hikurangi interface than the geodetically derived values (Van Dissen et al., 2022; Wallace, 2020). Therefore, the Ahuriri Lagoon paleoseismic record currently underpins models that ultimately inform building codes, and improvements to fault source characterizations will have a direct impact on future mitigation.…”

“…Large‐magnitude subduction earthquakes have not been observed on the Hikurangi margin since European colonization in 1840 CE. Therefore, much of the subduction zone earthquake characterization, expected slip behavior, and hazard forecasts are heavily informed by interface coupling models (e.g., Van Dissen et al., 2022). Coupling estimates use modern geodetic motions to infer the degree of interface locking across the plate margin (e.g., Johnson et al., 2022; Wallace et al., 2004).…”

Robust Imaging of Fault Slip Rates in the Walker Lane and Western Great Basin From GPS Data Using a Multi‐Block Model Approach

“…(2022b) and Johnson et al. (2024). They point out that while the slip rate estimates exhibit broad similarity, they differ in some important measures owing to multiple factors that sometimes result in high variability.…”

“…The degree of independence that the estimated geodetic rates have from geologic rates is an issue with the NSHM deformation models identified in recommendations for future efforts (Johnson et al., 2024). In our method described here geologic data from the NSHM (Hatem et al., 2022a) are used to define the location, geometry, and style of fault segments, but not their slip rates.…”