Cook Strait Research Articles

Optimization of a Grid-Connected Microgrid Using Tidal and Wind Energy in Cook Strait

The Cook Strait in New Zealand is an ideal location for wind and tidal renewable sources of energy due to its strong winds and tidal currents. The integration of both technologies can help to avoid the detrimental effects of fossil fuels and to reduce the cost of electricity. Although tidal renewable sources have not been used for electricity generation in New Zealand, a recent investigation, using the MetOcean model, has identified Terawhiti in Cook Strait as a superior location for generating tidal power. This paper investigates three different configurations of wind, tidal, and wind plus tidal sources to evaluate tidal potential. Several simulations have been conducted to design a DC-linked microgrid for electricity generation in Cook Strait using HOMER Pro, RETScreen, and WRPLOT software. The results show that Terawhiti, in Cook Strait, is suitable for an offshore wind farm to supply electricity to the grid, considering the higher renewable fraction and the lower net present cost in comparison with those using only tidal turbines or using both wind and tidal turbines.

Illuminating the Pre‐, Co‐, and Post‐Seismic Phases of the 2016 M7.8 Kaikōura Earthquake With 10 Years of Seismicity

AbstractThe 2016 M7.8 Kaikōura earthquake is one of the most complex earthquakes in recorded history, with significant rupture of at least 21 crustal faults. Using a matched‐filter detection routine, precise cross‐correlation pick corrections, and accurate location and relocation techniques, we construct a catalog of 33,328 earthquakes between 2009 and 2020 on and adjacent to the faults that ruptured in the Kaikōura earthquake. We also compute focal mechanisms for 1,755 of the earthquakes used as templates. Using this catalog we reassess the rupture pathway of the Kaikōura earthquake. In particular we show that: (a) the earthquake nucleated on the Humps Fault; (b) there is a likely linking offshore reverse fault between the southern fault system and the Papatea Fault, which could explain the anomalously high slip on the Papatea Fault; (c) the faults that ruptured in the 2013 Cook Strait sequence were reactivated by the Kaikōura earthquake and may have played a role in the termination of the earthquake; and (d) no seismicity on an underlying subduction interface is observed beneath almost all of the ruptured region suggesting that if deformation did occur on the plate interface then it occurred aseismically and did not play a significant role in generating co‐seismic ground motion.

Heterogeneous material properties—as inferred from seismic attenuation—influenced multiple fault rupture and ductile creep of the KaikouraMw 7.8 earthquake, New Zealand

SUMMARYThe 2016 Mw 7.8 Kaikoura, New Zealand, earthquake occurred along the eastern margin of the transition region between active subduction in the North Island and oblique collision in the South Island. To infer crustal properties, we imaged Q (1/seismic attenuation) by combining selected M > 3.5 aftershocks with data from previous Q models. For 158 distributed aftershocks, we fit spectral decay on temporary stations and all Geonet stations, providing 6194 t*p and 19 497 t*s. Considering the varied rheology and faults, we also used 2.5-D numerical models to study ductile strain development. The complex earthquake ruptured an ∼180-km-long zone of multiple faults, which involved jumping around the complicated eastern end of the Hope fault, without significant slip on the Hope fault. The Qs and Qp results show features in the upper and lower crust which correlate to the distribution and types of fault rupture. This earthquake involved numerous faults over a region of greywacke crust, where the underlying high Q Cretaceous slab is about 30-km depth. It initiated with ∼5-m slip on the Humps fault in a region of background seismicity and low Q lower crust, adjacent to the Hope fault. The central region near Kaikoura shows a high Q crustal block, which appears to have inhibited rupture; as the rupture progressed over several small faults to jump offshore of the apparently strong block. Underlying the Kaikoura greywacke crust, below 20-km depth, there is a 40-km-long region of increased Vp, Qs and gravity, which likely represents an intraplate plutonic complex emplaced into the Hikurangi Plateau, forming an elevated section which influences deformation. In the northern section, in a region with relatively uniform moderate to low Q, the earthquake evolved into the relatively continuous ∼80-km-long major rupture along the Jordan, Kekerengu and Needles faults, with ∼6–20 m dextral slip at depth and surface displacements of ∼10 m dextral and ∼2 m vertical. The northern progression of the rupture stopped when it approached an abrupt change to high Q crust across Cook Strait. At 20–30-km depth northwest of the rupture, deeper zones with low Q are consistent with regions of distributed ductile shear and creep where the observed afterslip may have occurred, where the underlying slab is 25–40 km deep. The numerical model shows that ductile deformation localizes in this area of lower crust above the relatively strong slab, connecting outer faults (Kekerengu) to inland faults (Clarence, Awatere, Wairau), and demonstrates that no subduction thrust is required under the Marlborough region.

Fin whale acoustic populations present in New Zealand waters: Description of song types, occurrence and seasonality using passive acoustic monitoring.

Southern fin whales (Balaenoptera physalus) are known to migrate from the Antarctic to mid-latitudes during winter for breeding, but the occurrence and distribution of this species is not well known in the waters around New Zealand. The ‘doublet’ calls are one of the main calls emitted specifically by fin whales and repeated in a regular pattern, which make the acoustic detection of these calls relevant to detect the presence of fin whales. Using a signal processing algorithm to detect ‘doublet’ calls emitted by fin whales, we studied the occurrence, characteristics and seasonality of these ‘doublet’ calls in two regions around New Zealand; Cook Strait in 2016/2017 and offshore Gisborne in 2014/2015. The call detection procedure consisted of binarization of the spectrogram and a cross-correlation between the binarized spectrogram and a template of binarized ‘doublet’ calls spectrogram. A binarization threshold for the data spectrograms and a cross correlation threshold were then determined through multiple trials on a training dataset and a Receiver Operating Characteristics (ROC) curve. Fin whale ‘doublet’ calls occurred on the east side of New Zealand’s Cook Strait during austral winter, specifically in June 2017 and offshore Gisborne in June-August 2014. No ‘doublet’ calls were detected on the west side of Cook Strait. The ‘doublet’ calls’ Inter-Note Interval (INI) was similar in both datasets. However, there was a difference in alternation of the mean frequency for both HF components of ‘doublet’ calls in Cook Strait and Gisborne. As the song types were compared with those previously described in the literature, our findings suggest that some fin whales wintering in New Zealand waters may be part of a broader ‘acoustic population’ whose range extends west to southern Australia and south to Antarctica.

Earning your stripes: a second species of striped gecko in the New Zealand gecko genus Toropuku (Gekkota: Diplodactylidae).

The New Zealand diplodactylid gecko genus Toropuku is currently monotypic, but the sole member of the genus, T. stephensi, is distributed in two disjunct, geographically distant regions of New Zealand - the islands of Cook Strait (which includes the type locality, Stephens Island), between New Zealand's North and South Islands, and the Coromandel Peninsula, in the northeastern North Island. Previously published phylogenetic results, based on three total individuals, recognized substantial-possibly species-level-diversity between these disparate localities, although no taxonomic decisions were made at that time. More recently, additional animals have been found on the Coromandel Peninsula. We here present phylogenetic and morphological evidence based on this expanded dataset to formally describe the populations on the Coromandel Peninsula as a new species, Toropuku inexpectatus sp. nov. The specific epithet refers to the species' surprise discovery in a herpetologically well-surveyed area. The recognition of T. inexpectatus sp. nov. as a distinct species has implications for the conservation status of T. stephensi, which is now considered restricted to three islands in Cook Strait.

Concordant geographic and genetic structure revealed by genotyping-by-sequencing in a New Zealand marine isopod.

Population genetic structure in the marine environment can be influenced by life‐history traits such as developmental mode (biphasic, with distinct adult and larval morphology, and direct development, in which larvae resemble adults) or habitat specificity, as well as geography and selection. Developmental mode is thought to significantly influence dispersal, with direct developers expected to have much lower dispersal potential. However, this prediction can be complicated by the presence of geophysical barriers to dispersal. In this study, we use a panel of 8,020 SNPs to investigate population structure and biogeography over multiple spatial scales for a direct‐developing species, the New Zealand endemic marine isopod Isocladus armatus. Because our sampling range is intersected by two well‐known biogeographic barriers (the East Cape and the Cook Strait), our study provides an opportunity to understand how such barriers influence dispersal in direct developers. On a small spatial scale (20 km), gene flow between locations is extremely high, suggestive of an island model of migration. However, over larger spatial scales (600 km), populations exhibit a clear pattern of isolation‐by‐distance. Our results indicate that I. armatus exhibits significant migration across the hypothesized barriers and suggest that large‐scale ocean currents associated with these locations do not present a barrier to dispersal. Interestingly, we find evidence of a north‐south population genetic break occurring between Māhia and Wellington. While no known geophysical barrier is apparent in this area, it coincides with the location of a proposed border between bioregions. Analysis of loci under selection revealed that both isolation‐by‐distance and adaption may be contributing to the degree of population structure we have observed here. We conclude that developmental life history largely predicts dispersal in the intertidal isopod I. armatus. However, localized biogeographic processes can disrupt this expectation, and this may explain the potential meta‐population detected in the Auckland region.

Migratory insights from singing humpback whales recorded around central New Zealand.

The migration routes of wide-ranging species can be difficult to study, particularly at sea. In the western South Pacific, migratory routes of humpback whales between breeding and feeding areas are unclear. Male humpback whales sing a population-specific song, which can be used to match singers on migration to a breeding population. To investigate migratory routes and breeding area connections, passive acoustic recorders were deployed in the central New Zealand migratory corridor (2016); recorded humpback whale song was compared to song from the closest breeding populations of East Australia and New Caledonia (2015–2017). Singing northbound whales migrated past New Zealand from June to August via the east coast of the South Island and Cook Strait. Few song detections were made along the east coast of the North Island. New Zealand song matched New Caledonia song, suggesting a migratory destination, but connectivity to East Australia could not be ruled out. Two song types were present in New Zealand, illustrating the potential for easterly song transmission from East Australia to New Caledonia in this shared migratory corridor. This study enhances our understanding of western South Pacific humpback whale breeding population connectivity, and provides novel insights into the dynamic transmission of song culture.

Spatial Structure of Low Salinity Submesoscale Features and Their Interactions With a Coastal Current

Submesoscale features, characterised by a low salinity layer originating from river discharges, enhance water column stability in a New Zealand shelf sea - the Greater Cook Strait. Using a combination of data from multiple ocean glider surveys and regional modelling, we show that low salinity submesoscale features (LSMFs) can cause increased stratification on the order of 10^-4. Modelled oceanographic conditions compared well to observations, especially in austral spring. Stably stratified LSMFs can replace the previously well mixed layer in the water column up to a distance of 100 km offshore before getting entrained by the regional barotropic current in Greater Cook Strait. LSMFs observed from glider surveys and reproduced from modelled results generate strong vertical and horizontal salinity differences of ~0.45 psu. These salinity differences define density fronts and stratification in the upper ~30 m. Temperature differences of up to ~1.4°C associated with LSMFs were not large enough to entirely cancel the density effect of salinity. The offshore advection reach of LSMFs is partly constrained by the variability of the barotropic d'Urville Current. Its presence and strong winds inhibit the propagation of LSMFs offshore in Greater Cook Strait, leaving the water column unstable while enhancing mixing and deepening the mixed layer depth. In contrast, moderate winds and weak current enable the propagation of LSMFs furthest offshore in Greater Cook Strait, where the water column becomes stably stratified. A stably stratified regime inhibits vertical mixing of nutrients and phytoplankton, which may lead to enhanced primary production in the region.

Rapid initial assessment of the number of turbines required for large-scale power generation by tidal currents

Abstract Large turbine farms in strong tidal flows could contribute significantly to the global demand for renewable energy. Key to developing large scale power generation is determining how many turbines are required to deliver a given amount of power from proposed sites. Answering this question is computationally difficult, as large-scale power extraction changes the strength of the currents driving the turbines. As a consequence, the large hydrodynamic models used to assess the tidal current resource must be run many times to optimize power output for every potential site and farm size. This paper outlines an approach that can rapidly assess farm power output using an existing hydrodynamic model. This approach is aimed at rapidly determining the most promising farm sites, sizes and shapes within a region, enabling work with more detailed, realistic and slower models to focus on a smaller number of farms. The approach is used to assess how much of Cook Strait, New Zealand’s 15,000 MW potential could be realized with current generation turbines. A basic economic analysis suggests that a 90 MW farm with 95 20 m-diameter turbines might be viable in Cook Strait, if turbine manufacturing costs fall, or energy prices increase, by around 25%.

What We Do in the Shallows: Natural and Anthropogenic Seafloor Geomorphologies in a Drowned River Valley, New Zealand

The shallow marine environment represents a region of high biological productivity, ecological diversity, and complex oceanographic conditions, and often supports various human activities and industries. Mapping of the seafloor in shallow marine environments reveals seafloor features in detail, shedding light on a range of natural and anthropogenic processes. We present a high-resolution (2-m) multibeam dataset, combined with geologic samples that reveals a complete map of the seafloor from the land-water interface to ~350 m water depth within Queen Charlotte Sound/Tōtaranui (QCS) and Tory Channel/Kura Te Au (TC), Marlborough Sounds, New Zealand. These data reveal that the seafloor geomorphology and distribution of natural and anthropogenic features varies spatially from the inner QCS to the Cook Strait. Tidal currents play a large role in the erosion, transport, and deposition of sediments in QCS and TC. The distribution and depth of seafloor scouring suggests that tidal flow is locally intensified by coastal geometry and bathymetric barriers, resulting in concentrated scouring where tidal flow is restricted or redirected. In addition, superimposed bedforms reflect localised variations in flow direction that have likely developed across a range of spatial and temporal scales. Evidence for extensive seafloor fluid expulsion is preserved in >8500 pockmarks mainly located within the inner and central QCS. The size and spatial distribution of pockmarks suggest multiple fluid sources in the region. The cumulative anthropogenic footprint on the seafloor within QCS represents 6.4 km2 (~1.5%) of the total seafloor area and is predominantly related to maritime activities including anchor dragging (47.5%) and mooring blocks (24%). This study provides a unique example of the information that can be revealed by a comprehensive survey programme that mapped from the land-water interface to the subtidal zone. Results presented in this study form a robust basis upon which to develop improved hydrodynamic models and benthic habitat maps and to assess the full extent of anthropogenic activities in the shallow marine realm.

Spatio-temporal analysis of seismic anisotropy associated with the Cook Strait and Kaikōura earthquake sequences in New Zealand

SUMMARYLarge earthquakes can diminish and redistribute stress, which can change the stress field in the Earth’s crust. Seismic anisotropy, measured through shear wave splitting (SWS), is often considered to be an indicator of stress in the crust because the closure of cracks due to differential stress leads to waves polarized parallel to the cracks travelling faster than in the orthogonal direction. We examine spatial and temporal variations in SWS measurements and the Vp/Vs ratio associated with the 2013 Cook Strait (Seddon, Grassmere) and 2016 Kaikōura earthquakes in New Zealand. These earthquake sequences provide a unique data set, where clusters of closely spaced earthquakes occurred. We use an automatic, objective splitting analysis algorithm and automatic local S-phase pickers to expedite the processing and to minimize observer bias. We present SWS and Vp/Vs measurements for over 40 000 crustal earthquakes across 36 stations spanning close to $5\frac{1}{2}$ yr between 2013 and 2018. We obtain a total of 102 260 (out of 398 169) high-quality measurements. We observe significant spatial variations in the fast polarization orientation, ϕ. The orientation of gravitational stresses are consistent with most of the observed anisotropy. However, multiple mechanisms (such as structural, tectonic stresses and gravitational stresses) may control some of the observed crustal anisotropy in the study area. Systematic analysis of SWS parameters and Vp/Vs ratios revealed that apparent temporal variations are caused by variation in earthquake path through spatially varying media.

A modelling synthesis of the volume flux through Cook Strait

ABSTRACT We present hindcasts of currents in Cook Strait from a baroclinic, tide-resolving ocean model. Three hindcasts are run, forced respectively at the lateral boundaries with data from the Bluelink Reanalysis (BRAN), the global Hybrid Coordinate Ocean Model (HYCOM) and a New Zealand Shelf Seas baroclinic hindcast (NZROMS). The hindcast forced by BRAN is evaluated against measured currents at locations in the Cook Strait Narrows and South Taranaki Bight. The model reproduces the observed tidal, subtidal and mean currents well. The three hindcasts produce a 3-year-mean volume flux southward through the Narrows of 0.360, 0.734 and 0.230 Sv, respectively. When corrected for the observed discrepancies between model and measurements in the Narrows, the estimates are 0.430, 0.498 and 0.337 Sv. We conclude that true 3-year mean volume flux through Cook Strait is 0.42 ± 0.08 Sv. The tidal volume flux amplitude is 4.68 Sv (M2). The subtidal fluctuations in volume flux have a standard deviation of 0.62 Sv. The effect of wind is investigated with a wind-forced barotropic model, which produces volume flux fluctuations that are highly correlated (r = 0.93) with the subtidal fluctuations from the baroclinic model, implying that the latter are largely wind-generated.

Environmental context shapes the long‐term role of nutrients in driving producer community trajectories in a top–down dominated marine ecosystem

Abstract Two predominant anthropogenic impacts on ecosystems, nutrient enrichment and the removal of consumers, are predicted to interact in their effects on producer diversity. Yet, measures of diversity alone may not capture changes occurring in the underlying mechanisms structuring communities. Furthermore, evidence for these interactions in rocky intertidal systems is mixed and may be confounded by variable baseline productivity or short experimental durations that do not capture seasonality, environmental heterogeneity or successional processes. We conducted a 2‐year experiment examining the main and interactive effects of nutrients and herbivores in a low‐productivity environment on the northern shore of Cook Strait near Wellington, New Zealand. We explicitly accounted for the small‐scale spatial environmental heterogeneity characteristic of rocky shores. Rapid and dramatic shifts in diversity metrics and cover of algal communities in the first year were driven by removal of herbivores. Within 2 months of herbivore exclusion, open space decreased from >90% to <15%, regardless of nutrient addition, and both algal diversity and evenness plummeted while species richness rose. In contrast to theoretical predictions, strong interactions between top–down and bottom–up forces on diversity were rare; rather, over time, the addition of nutrients negatively impacted diversity. Further, where herbivores were removed, nutrients inhibited succession beyond ephemeral forms, contrasting with the tolerance model of succession observed under ambient nutrients. The effects of nutrients were only elucidated by accounting for variation in habitat complexity and seasonality, demonstrating that environmental heterogeneity can mask bottom–up processes. Moreover, shifts in relative species abundances revealed that after 10 months, nutrient addition caused increased community turnover regardless of herbivore presence or absence, indicating reduced stability not apparent in simple measures of diversity. Synthesis. Our results suggest that nutrient addition and herbivore reduction exert strong but mostly independent control on community biodiversity, where context dependency in habitat complexity, seasonality and successional stage played key roles in determining community outcomes. These findings demonstrate the importance of longer‐term field studies incorporating environmental context and multiple metrics of diversity to reveal the mechanisms underlying effects of nutrient enrichment and herbivore loss on producer community structure.

Interpretations of aftershock advice and probabilities after the 2013 Cook Strait earthquakes, Aotearoa New Zealand

Probabilistic statements can be a valuable tool for natural hazard risk communication, including forecasts. However, individuals often have a poor understanding of such probabilistic forecasts caused by them distorting their interpretations of event likelihoods towards the end of the time window and discounting the risk today. We investigated the use of an ‘anchoring’ time statement to mitigate this bias via an opportunistic survey run after the 2013 Cook Strait earthquakes, near Wellington, New Zealand. Participants rated their interpretations of likelihoods for an immediate aftershock forecast, and for an earthquake in the future. We explored the influence of aftershock and information concern, emotions and felt shaking, gender and education, as well as preparedness actions.The anchoring time window statement mitigated the skew in interpretations for the short (24 h to within 1 week) aftershock forecast statement. However, the skew still existed for the longer future earthquake forecast (7 days to within 1 year). We also found that heightened sensory experience (felt shaking) or emotional reactions (nervousness, fear, alertness) during the earthquakes was associated with an increase in the perceived likelihoods of future events. Gender was found to significantly influence results, with females rating higher levels of information concern and anxiety, and recording higher perceived likelihoods for the immediate aftershock forecast. Findings, including the importance of ‘anchoring’ time windows within a forecast to encourage immediate preparedness actions, support recommendations for effective crafting of these forecasts and warnings.

First Report of the Dinoflagellate Genus Effrenium in the East Sea of Korea: Morphological, Genetic, and Fatty Acid Characteristics

Most species in the family Symbiodiniaceae are symbiotic partners to invertebrate and protist hosts, but a few live freely in water columns. Here, a free-living dinoflagellate was isolated from the waters off the Dokdo Islands in the East Sea of Korea. Morphological and molecular analyses show this isolate belongs to Effrenium voratum. Prior to the present study, E. voratum had been reported to live in the waters in the temperate latitudes in the western North Pacific, the southwest Western Pacific, the eastern North Pacific, the eastern Atlantic, and the Mediterranean Sea. To our knowledge, this is the highest latitude in the western North Pacific, where E. voratum has been reported. This report extends the known range of this dinoflagellate to the temperate waters of the western North Pacific Ocean. The sequence of the D1/D2 region of the large subunit ribosomal DNA (LSU rDNA) was identical to E. voratum found in Jeju Island, Korea, Tsushima Island, Japan, and Cook Strait, New Zealand, suggesting this species is cosmopolitan. However, it was different by 1 bp from those found in Blanes, Spain and Santa Barbara, USA. In the phylogenetic tree built on the basis of the LSU (D1-D2) rDNA region sequences, this dinoflagellate was clustered within a clade, including all the other E. voratum strains. Morphological characteristics were like those of strains found in the waters of Jeju Island. This is the first report conducted on the fatty acid profile of fully characterized E. voratum. Importantly, the isolate possessed a high ratio of long-chain omega-3 polyunsaturated fatty acids (PUFAs) such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) relative to total lipid. This dinoflagellate could be a candidate for commercial applications, such as aquaculture feed and essential omega-3 PUFA productions.

Spatio-temporal variation in species composition of New Zealand's whitebait fishery

ABSTRACT Whitebait comprise a culturally, commercially and recreationally important fishery in New Zealand, where post-larvae are netted while returning from their marine phase. In this study, we expanded an historical (1964) sampling programme to gain a contemporary understanding of the species composition of the whitebait fishery; 87 rivers were sampled over six months in 2015. Over the entire country, >12 species were found in samples and 87.6% of these were īnanga (Galaxias maculatus). Kōaro (G. brevipinnis) and banded kōkopu (G. fasciatus) were abundant in some rivers and regions at particular times of the year. Buller was the most variable region, spatially and temporally, for species composition; Canterbury was the least variable. Banded kōkopu whitebait migrated one month earlier north of Cook Strait than in the south. There was a positive association between the abundance of kōaro and banded kōkopu in samples and the level of indigenous forest cover in catchments. Compared to samples from 50 years ago, there was a greater proportion of kōaro and banded kōkopu whitebait throughout the country. This spatio-temporal variability requires fishery regulations to be more tailored and flexible if they are to conserve the diversity of life-histories present in the catch and sustain the whitebait fishery.

LIQUEFACTION ASSESSMENT OF RECLAIMED LAND AT CENTREPORT, WELLINGTON

Observations of liquefaction-induced damage at the port of Wellington (CentrePort) provide an opportunity to evaluate the applicability of state-of-the-practice liquefaction evaluation methodologies on reclaimed land. This study focuses on the application of widely used simplified liquefaction assessment methods on the end-dumped gravelly fills and hydraulically-placed silty and sandy fills at CentrePort for the 2013 Cook Strait, 2013 Lake Grassmere, and 2016 Kaikōura earthquakes. Liquefaction assessment of the gravel reclamation poses several challenges due to its large percentage of gravel-sized particles making it difficult to obtain high-quality in situ data. The hydraulic fills at CentrePort are also of significant interest as they relate to a range of issues in the simplified engineering assessment around effects of fines and their plasticity on the liquefaction resistance. Following the 2016 Kaikōura earthquake, subsurface explorations were performed which included 121 Cone Penetration Tests (CPTs). Results of CPT-based liquefaction triggering and post-liquefaction reconsolidation settlement assessments using state-of-the-practice procedures are discussed and compared with observed liquefaction manifestation and settlements.

Upper Plate Heterogeneity Along the Southern Hikurangi Margin, New Zealand

AbstractControlled and natural source seismic data are used to build a 3‐D P wave model for southern North Island, New Zealand, where the Pacific Plate subducts beneath the Australian Plate at a rate of ~41 mm/year. Our analysis reveals an abrupt along‐strike transition in overthrusting plate structure within Cook Strait. Contrasts in properties (Vp, Vp/Vs, and Qs) likely reflects the degree of deformation in the Australian Plate, where the Alpine‐Wairau and Awatere Faults mark the northern boundary of a terrane that has undergone >50° of clockwise vertical‐axis rotation since the early Miocene. Heterogeneity of the crustal transition is likely associated with changes in frictional and elastic properties that may impact elastic stress accumulation and inhibit southward propagation of megathrust earthquakes. Low connectivity of faults in Cook Strait is consistent with the heterogeneity we observe and may promote complex earthquake triggering by lateral stress loading during earthquakes or slow slip events.

Circulation in Tasman-Golden bays and Greater Cook Strait, New Zealand

ABSTRACT Analyses are presented of model simulations and Lagrangian drifter experiments designed to quantify circulation and dispersal in Tasman and Golden Bays, and their connection to Cook Strait, New Zealand. Observations made in autumn 2017 and summer 2018 are compared with numerical model results. The drifter experiments were focussed on three repeat experiments in each bay designed to measure the local horizontal eddy diffusivity. An additional experiment investigated the connection with, and flow through, Greater Cook Strait. Point by point velocities extracted from the model show low correlation with, but on average are in the same direction as observed velocities. Eddy diffusivities computed from the drifter experiments are highly variable, but the observations support the modelling. We find that the mean velocity in Tasman and Golden Bays is weak and the circulation is dominated by wind and tidal flows. The mean depth-averaged velocity shows anticyclonic flows in the south of both Tasman and Golden Bays, with cyclonic flow in the north of Golden Bay. The mean near-surface velocity is generally out of each bay and is compensated for by inward circulation at depth.

Use of High-Resolution Numerical Models and Statistical Approaches to Understand New Zealand Historical Wind Speed and Gust Climatologies

AbstractThis paper describes how a combination of high-resolution numerical modeling, a robust homogenization algorithm, and local pressure observations have been used to understand and reconcile time series of daily, seasonal, and annual peak wind gusts recorded at observing sites in the Cook Strait region of New Zealand. The homogenization algorithm consists of corrections for the relocation of masts, changes in instrumentations, data acquisition and signal processing, surrounding surface roughness, and measurement heights. In addition, a statistical method, penalized maximal F test (PMFT), was used to assess the homogeneity of the wind speed time series and detect and eliminate all remaining, undocumented, artificial (i.e., nonclimatic) breakpoints. A three-dimensional time-dependent computational fluid dynamics (CFD) simulation was carried out using the Gerris model to characterize the turbulence environment at the mast sites and to estimate topographic speedup effects. Trends in magnitudes and frequencies of the homogenized seasonal and annual peak wind gusts are evaluated and presented for the Cook Strait region. The pressure gradients between pairs of stations were used to study the correlation between the gust wind speeds and the pressure field. A high-resolution convection-resolving numerical weather prediction model [the New Zealand Convective-Scale Model (NZCSM)] was employed to aid the interpretation of results and analyze wind trends. The trend in gust speeds is also shown to be consistent with larger-scale NCEP–NCAR reanalysis pressure trends. The homogenization algorithm showed promising results in eliminating the artificial breakpoints and trends. Overall, strong correlations were found between the homogenized gust speeds, the pressure field across the region, and NZCSM predictions.