Susan Ellis

Simulating present‐day solid Earth deformation and volatile cycling requires integrating diverse geophysical data sets and advanced numerical techniques to model complex multiphysics processes at high resolutions. Subduction zone modeling is particularly challenging due to the large geographic extent, localized deformation zones, and the strong feedbacks between reactive fluid transport and solid deformation. Here, we develop new workflows for simulating 3‐dimensional thermal‐mechanical subduction and patterns of volatile dehydration at convergent margins, adaptable to include reactive fluid transport. We apply these workflows to the Hikurangi margin, where recent geophysical investigations have offered unprecedented insight into the structure and processes coupling fluid transport and solid deformation across broad spatiotemporal scales. Geophysical data sets constraining the downgoing and overriding plate structure are collated with the Geodynamic World Builder, which provides the initial conditions for forward simulations using the open‐source geodynamic modeling software code ASPECT. We systematically examine how plate interface rheology and hydration of the downgoing plate and upper mantle influence Pacific–Australian convergence and seismic anisotropy. Models prescribing a plate boundary viscosity of Pa s best reproduce observed plate velocities, and changing the configuration of the Pacific–Australia plate boundary directly influences the modeled plate motions. Models considering hydrated olivine fabrics best reproduce observations of seismic anisotropy. Predicted patterns of slab dehydration and mantle melting correlate well with observations of seismic attenuation and arc volcanism. These results suggest that hydration‐related rheological heterogeneity and related fluid weakening may strongly influence slab dynamics. Future investigations integrating coupled fluid transport and global mantle flow will provide insight into the feedbacks between subduction dynamics, fluid pathways, and arc volcanism.

We compile an integrated earthquake catalog for Aotearoa New Zealand (NZ) by overwriting event parameters in the national operational seismic catalog (the most complete record of NZ’s seismicity) with refined estimates of event depths, focal mechanisms, locations, and magnitudes from other sources. This was required for several uses in the 2022 NZ National Seismic Hazard Model (NZ NSHM 2022), including distinguishing (classifying) upper-plate, subduction-interface, and intraslab earthquakes to guide the statistics of separate components of the NZ NSHM 2022’s Seismicity Rate Model. Starting from a branch of the operational catalog with standardized event magnitudes, we import revised parameters for 60% of the catalog (including 92% of all 2000–2020 events, 89% of 1951–2020 M ≥ 5.5 events and 84% of 1917–2020 M ≥ 6 events) from relocation studies, literature, the NZ Centroid Moment Tensor database and global catalogs. Next, we classify earthquakes as upper plate, subduction, or intraslab by comparing their depths, locations, and focal mechanisms to the Hikurangi–Kermadec and Puysegur subduction interface geometries and relative plate-motion directions. We show that this event classification would be either highly error-prone or effectively blind in subduction regions if the catalog had not been revised beforehand. Finally, we estimate the depth distribution of upper-plate earthquakes in multiple regions for use in the NZ NSHM 2022 and explore some post-2022 developments of this approach.

Simulating present-day solid Earth deformation and volatile cycling requires integrating diverse geophysical datasets and advanced numerical techniques to model complex multiphysics processes at high resolutions. Subduction zone modeling is particularly challenging due to the large geographic extent, localized deformation zones, and the strong feedbacks between reactive fluid transport and solid deformation. Here, we develop new workflows for simulating 3-dimensional thermal-mechanical subduction and patterns of volatile dehydration at convergent margins, adaptable to include reactive fluid transport. We apply these workflows to the Hikurangi margin, where recent geophysical investigations have offered unprecedented insight into the structure and processes coupling fluid transport and solid deformation across broad spatiotemporal scales. Geophysical data sets constraining the downgoing and overriding plate structure are collated with the Geodynamic World Builder, which provides the initial conditions for forward simulations using the open-source geodynamic modeling software ASPECT. We systematically examine how plate interface rheology and hydration of the downgoing plate and upper mantle influence Pacific–Australian convergence and seismic anisotropy. Models prescribing a dry rheology and a plate boundary viscosity of 5e20 Pa s best reproduce observed plate velocities. Conversely, models considering hydrated olivine fabrics best reproduce observations of seismic anisotropy. Predicted patterns of slab dehydration and mantle melting correlate well with observations of seismic attenuation and arc volcanism. These results suggest that hydration-related rheological heterogeneity and related fluid weakening may strongly influence slab dynamics. Future investigations integrating coupled fluid transport and global mantle flow will provide insight into the feedbacks between subduction dynamics, fluid pathways, and arc volcanism.

The 2022 revision of Aotearoa New Zealand National Seismic Hazard Model (NZ NSHM 2022) has involved significant revision of all datasets and model components. In this article, we present a subset of many results from the model as well as an overview of the governance, scientific, and review processes followed by the NZ NSHM team. The calculated hazard from the NZ NSHM 2022 has increased for most of New Zealand when compared with the previous models. The NZ NSHM 2022 models and results are available online.

We summarize estimates of the maximum rupture depth on New Zealand’s active faults (“New Zealand Fault-Rupture Depth Model v.1.0”), as used in the New Zealand Community Fault Model v1.0 and as a constraint for the latest revision of the New Zealand National Seismic Hazard Model (NZ NSHM 2022). Rupture depth estimates are based on a combination of two separate model approaches (using different methods and datasets). The first approach uses regional seismicity distribution from a relocated earthquake catalog to calculate the 90% seismicity cutoff depth (D90), representing the seismogenic depth limit. This is multiplied by an overshoot factor representing the dynamic propagation of rupture into the conditional stability zone, and accounting for the difference between regional seismicity depths and the frictional properties of a mature fault zone to arrive at a seismic estimate of the maximum rupture depth. The second approach uses surface heat flow and rock type to compute depths that correspond to the thermal limits of frictional instabilities on seismogenic faults. To arrive at a thermally-based maximum rupture depth, these thermal limits are also multiplied by an overshoot factor. Both the models have depth cutoffs at the Moho and/or subducting slabs. Results indicate the maximum rupture depths between 8 (Taupō volcanic zone) and >30 km (e.g., southwest North Island), strongly correlated with regional thermal gradients. The depths derived from the two methods show broad agreement for most of the North Island and some differences in the South Island. A combined model using weighting based on relative uncertainties is derived and validated using constraints from hypocenter and slip model depths from recent well-instrumented earthquakes. We discuss modifications to the maximum rupture depths estimated here that were undertaken for application within the NZ NSHM 2022. Our research demonstrates the utility of combining seismicity cutoff and thermal stability estimates to assess the down-dip dimensions of future earthquake ruptures.

We present surface and structural models of a swarm of dm‐scale subparallel faults exposed in a ∼2000 m ² glaciated outcrop near Franz Josef Glacier, in the Southern Alps of New Zealand. These structures are inferred to have slipped at ∼20 km depth in the hanging‐wall Alpine Schist of the Alpine Fault under conditions that were variably brittle to ductile as the Pacific Plate was tilted and uplifted. Using field mapping, real‐time kinematic GPS and digital images from a remotely piloted aircraft system, we have created a digital surface model of the outcrop and orthophotographs at a ground resolution of ∼1 cm to map compositional layering in the metasediments and the array of brittle‐ductile faults displacing them. In order of decreasing relative age (and average thickness), displaced markers in the schist include primary psammite and pelite beds, quartz veins, and a deformational foliation. The surface models have been used to create 2‐D transects and a 3‐D model where faults are projected down‐dip, to determine the connectivity, topology and intersection types of the fault swarm. Lithological variations, particularly the interface between pelitic and psammitic schist, were a primary control on the topology of the fault network and the spacing between faults.

The New Zealand Community Fault Model (NZ CFM) is a publicly available representation of New Zealand fault zones that have the potential to produce damaging earthquakes. Compiled through collaborative engagement between New Zealand earthquake‐science experts, this first edition (version 1.0) of the NZ CFM builds upon previous compilations of earthquake‐source active fault models with the addition of new and modified information. Developed primarily to support an update of the New Zealand National Seismic Hazard Model, the NZ CFM comprises two principal components. The first dataset is a two‐dimensional map representation of the surface traces of 880 generalised fault zones. Each fault zone is assigned specific geometric and kinematic attributes, including uncertainties, supplemented with a subjective quality ranking focused primarily on the confidence in assigned slip rates. The second component is a three‐dimensional representation of the fault zones as triangulated mesh surfaces that are projected down‐dip from the two‐dimensional mapped traces to a geophysically‐defined maximum fault rupture depth. This article summarises the compilation and parameterisation of the NZ CFM, along with background on its relation to predecessor datasets, and forward applications to probabilistic seismic hazard assessment and physics‐based earthquake models currently being developed for Aotearoa New Zealand.