Hikurangi Subduction Interface Research Articles

Probabilistic assessment of road accessibility under cascading hazards

Natural hazard events such as seismic events tend to trigger cascading hazards damaging the road networks which are multi-component and widespread systems. Road accessibility is vital for response activities including emergency services by critical facilities. For disaster response and recovery planning purposes, a good understanding of hazards in the region and their impact on road accessibility is essential. To achieve this objective the study suggests a comprehensive approach to model hazards and the response of different types of assets that comprise the road network.Due to the uncertain nature of hazard events and response of road network components, a probabilistic framework is proposed to evaluate road accessibility by systematically considering the effects of: (i) multi-hazards that are cascading; and (ii) damage of various components of the network. The method tracks the uncertainty in the hazard modelling to the fragility of the road assets through a Monte Carlo simulation that creates thousands of possible road damage realizations. In each realization, the road accessibility is calculated, and accessibility probability maps for post-event response are generated from that. The method has been demonstrated for the road infrastructure of Napier City, New Zealand for a Mw8.4 Hikurangi Subduction interface earthquake. The road accessibility maps are useful resources for local authorities for resilience planning purposes. The study also presents the benefits of a few resilience intervention options in improving the accessibility of affected suburbs in the study region.

A time-dependent seismic hazard model following the Kaikōura M7.8 earthquake

ABSTRACT Following the 2016 M7.8 Kaikōura earthquake, a time-varying seismic hazard model (KSHM) was developed to inform decision-making for the reinstatement of road and rail networks in the northern South Island. The source model is the sum of a gridded 100-year earthquake clustering model and an updated fault source model. The gridded model comprises long-term, medium-term and short-term components. The 100-year gridded model is constructed as the sum of 100 annual forecasts. A discounting method trades off expected earthquake occurrences of the distant future against those of the near future. The fault source model includes updates to account for newly revealed faults that ruptured in the Kaikōura earthquake and other recently obtained new information, and new time-varying probabilities of rupture for four fault segments. Two different characterisations of the Hikurangi subduction interface are incorporated via a logic tree, with weights determined by an expert panel. A suite of ground motion prediction equations contribute to a logic tree in order to account for epistemic uncertainties in source modelling for each of four tectonic region types. Here, we compare the resulting hazard estimates with the 2010 National Seismic hazard Model and recorded motions in past New Zealand and global earthquakes.

The 2019 East Coast Slow Slip Event, New Zealand: Spatiotemporal Evolution and Associated Seismicity

Slow slip events (SSEs) are interpreted as the transient quasi-static fault deformation in the deep transition zone from locked to freely slipping in many subduction zones. Using continuous Global Positioning System (cGPS) data collected in New Zealand, we estimate the spatiotemporal evolution model during the 2019 SSE and analyze the influence of subduction interface heterogeneity on seismicity during SSEs at the Hikurangi margin. The results reveal that the 2019 SSE extends from the northern (Gisborne) to the central (Hawke’s Bay) Hikurangi subduction interface and decays rapidly within approximately 3-4 weeks. It releases a total seismic moment of about 4.83 × 1019 N·m (Mw 6.8), with a significant slip in Gisborne and a secondary slip in Hawke’s Bay. The slip depths are similar, but peaks, durations, and rates differ slightly. By combining previous SSEs (2011-2019), diverse characteristics are concluded, i.e., shorter duration and more frequency in Gisborne and relatively longer duration and less frequency in Hawke’s Bay. The seismicity offshore and onshore indicates along-strike variations, which appear to be spatially correlated with the variations in topography, such as subduction seamounts. The heterogeneities on the subduction interface are related to the spatiotemporal distribution of SSEs and seismicity along the Hikurangi margin.

Late Holocene coseismic uplift of the Kaikōura coast, New Zealand

Abstract The complex 2016 MW 7.8 Kaikōura earthquake ruptured >20 faults and caused highly variable uplift and subsidence of an ~110 km stretch of coastline. The earthquake raised questions about fault interactions in regions of oblique convergence and especially subduction to strike-slip transition zones like the Kaikōura region. We integrate 2016 coastal vertical deformation observations with new mapping and dating of Holocene marine terraces to: (1) compare spatial patterns of 2016 coseismic and longer-term vertical motions, (2) investigate possible past multi-fault ruptures or temporal clusters of earthquakes around Kaikōura, and (3) assess the relative contributions of crustal faults and the Hikurangi subduction interface to late Holocene coastal uplift. We identify possible multi-fault ruptures or loose clusters of earthquakes at ca. 850–550 yr B.P. and ca. 350–100 yr B.P. Most (and possibly all) of the Kaikōura coast has been uplifted over the late Holocene; the 25-km-long Parikawa section of coast subsided coseismically in 2016 but appears to be uplifted through reverse slip on an offshore fault. Late Holocene uplift everywhere along the coastline of interest can be attributed to slip on known upper-plate faults; slip on a shallow-dipping (<20°) subduction interface cannot be ruled out but is not required to explain uplift.

A Snapshot of New Zealand's Dynamic Deformation Field From Envisat InSAR and GNSS Observations Between 2003 and 2011

AbstractMeasuring the deformation at the Earth's surface over a range of spatial and temporal scales is vital for understanding seismic hazard, detecting volcanic unrest, and assessing the effects of vertical land movements (VLMs) on sea level rise. Here, we combine ∼10 years of Interferometric Synthetic Aperture Radar (InSAR) observations from Envisat with interseismic campaign and continuous GNSS velocities to build a high‐resolution velocity field of New Zealand. Exploiting the horizontal GNSS observations, we estimate the vertical component of the deformation to provide the VLM for the entire 15,000‐km‐long coastline. The estimated vertical rates show large variability around the country as a result of volcanic, tectonic, and anthropogenic sources. Interseismic subsidence is observed in Kaikoura region supporting models of at least partial locking of the southern Hikurangi subduction interface. Despite data challenges in the mountainous regions from landslides, sediment compaction, and glaciers, InSAR data shows localized uplift of the Southern Alps.

Evidence for Deeply Subducted Lower‐Plate Seamounts at the Hikurangi Subduction Margin: Implications for Seismic and Aseismic Behavior

AbstractSeamounts are found at many subduction zones and act as seafloor heterogeneities that affect slip behavior on megathrusts. At the Hikurangi subduction zone offshore the North Island, New Zealand, seamounts have been identified on the incoming Pacific plate and below the accretionary prism, but there is little concrete evidence for seamounts subducted beyond the present‐day coastline. Using a high‐resolution, adjoint tomography‐derived velocity model of the North Island, we identify two high‐velocity anomalies below the East Coast and an intraslab low‐velocity zone up‐dip of one of these anomalies. We interpret the high‐velocity anomalies as previously unidentified, deeply subducted seamounts, and the low‐velocity zone as fluid in the subducting slab. The seamounts are inferred to be 10–30 km wide and on the plate interface at 12–15 km depth. Resolution analysis using point spread functions confirms that these are well‐resolved features. The locations of the two seamounts coincide with bathymetric features whose geometries are consistent with those predicted from analog experiments and numerical simulations of seamount subduction. The spatial characteristics of seismicity and slow slip events near the inferred seamounts agree well with previous numerical modeling predictions of the effects of seamount subduction on megathrust stress and slip. Anomalous geophysical signatures, magnetic anomalies, and swarm seismicity have also been observed previously at one or both seamount locations. We propose that permanent fracturing of the northern Hikurangi upper plate by repeated seamount subduction may be responsible for the dichotomous slow slip behavior observed geodetically, and partly responsible for along‐strike variations in plate coupling on the Hikurangi subduction interface.

Paleotsunamis on the Southern Hikurangi Subduction Zone, New Zealand, Show Regular Recurrence of Large Subduction Earthquakes

Abstract Geological records of subduction earthquakes, essential for seismic and tsunami hazard assessment, are difficult to obtain at transitional plate boundaries, because upper-plate fault earthquake deformation can mask the subduction zone signal. Here, we examine unusual shell layers within a paleolagoon at Lake Grassmere, at the transition zone between the Hikurangi subduction zone and the Marlborough fault system. Based on biostratigraphic and sedimentological analyses, we interpret the shell layers as tsunami deposits. These are dated at 2145–1837 and 1505–1283 yr B.P., and the most likely source of these tsunamis was ruptures of the southern Hikurangi subduction interface. Identification of these two large earthquakes brings the total record of southern Hikurangi subduction earthquakes to four in the past 2000 yr. For the first time, it is possible to obtain a geologically constrained recurrence interval for the southern Hikurangi subduction zone. We calculate a recurrence interval of 500 yr (335–655 yr, 95% confidence interval) and a coefficient of variation of 0.27 (0.0–0.47, 95% confidence interval). The probability of a large subduction earthquake on the southern Hikurangi subduction zone is 26% within the next 50 yr. We find no consistent temporal relationship between subduction earthquakes and large earthquakes on upper-plate faults.

Triggered afterslip on the southern Hikurangi subduction interface following the 2016 Kaikōura earthquake from InSAR time series with atmospheric corrections

The 2016 Mw 7.8 Kaikōura earthquake represents an extremely complex event involving over ten major crustal faults, altering conventional understanding of multi-fault ruptures. Although evidence for coseismic slip on the Hikurangi subduction interface is controversial, we present afterslip on the subduction zone beneath Marlborough using 13 months of Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) observations. The spatially and temporally correlated atmospheric errors in SAR interferograms are problematic, and hence a new InSAR time series approach, combining the Generic Atmospheric Correction Online Service (GACOS) with a spatial-temporal Atmospheric Phase Screen (APS) filter to facilitate the InSAR time series analysis, is developed. For interferograms with over 250 km spatial extent, we achieve a 0.77 cm displacement RMS difference against GPS, improving 61% from the conventional InSAR time series method (TS). Comparisons between the overlapping region of two independent tracks show an RMS difference of 1.1 cm for the TS-GACOS-APS combined method, improving 54% from the TS method and 27% from using TS with an APS filter only. The APS filter reduces the short wavelength residuals substantially, but fails to remove the long wavelength error even after the ramp removal, revealing that the GACOS correction has played a key role in mitigating long wavelength atmospheric effects. The resultant InSAR displacements, together with the GPS displacements, are used to recover the time-dependent afterslip distribution on the Hikurangi subduction interface, which provides insights for reviewing the co-seismic slip sources, the present status of the subduction plate boundary and future seismic hazards.

Coseismic and Early Postseismic Deformation of the 2016 Mw 7.8 Kaikōura Earthquake, New Zealand, from Continuous GPS Observations

The Mw 7.8 Kaikōura, North Canterbury, New Zealand earthquake, which occurred on 14 November 2016 (local time), was one of the most complex continental earthquakes ever documented and among the largest instrumentally recorded events in New Zealand history. The epicenter was located at the southern termination of the Hikurangi margin, where the subducting Pacific Plate transfers into the dextral Alpine transform fault. In this work, we precisely estimate three-dimensional coseismic and postseismic displacements caused by this event from continuous global navigation satellite systems (GNSS) stations in New Zealand. The Kaikōura earthquake activated significant and diverse coseismic and postseismic deformation on a large spatial scale, located mainly in the southern part of the North Island and the northern part of the South Island. Station CMBL had the largest coseismic offsets and the most remarkable postseismic displacements. The accumulated postseismic displacements at this station reached 13, 7 and 29% of the coseismic values on the east, north and vertical components, respectively, in the first 1.5 years after the mainshock. Integrating our estimated coseismic displacements with previously published coseismic displacements, we inverted for the spatial distribution of coseismic slip and spatiotemporal evolution of postseismic slip. Our optimal coseismic model suggests that rupture occurred both on shallow crustal faults, and to some extent at the southern Hikurangi subduction interface. The GPS-inverted coseismic moment release is equivalent to an Mw 7.9 event. The postseismic slip was not only significantly extended at the subduction interface, but also appeared on the Needles fault. The cumulative moment magnitude is Mw 7.35 in the first 1.5 years after the event, and Mw 7.35, Mw 6.95 and Mw 6.80 during the periods 0.0–0.5, 0.5–1.0 and 1.0–1.5 years, respectively, indicating rapid decay of the postseismic deformation. Comparing the spatial distribution of the postseismic to the coseismic slip, although their direction is similar, the discrepancy between their location is significant: the slip located along the shallow crustal faults activated the coseismic deformation, while the slip located on the deep subduction interface controlled the postseismic deformation.

Spatial-temporal evolution of slow slip movements triggered by the 2016 Mw 7.8 Kaikoura earthquake, New Zealand

The 2016 Mw 7.8 Kaikoura earthquake involved complex rupture across multiple faults and triggered large-scale slow slip movements on the Hikurangi subduction zone. The complex seismically triggered slow slip events (SSEs) contribute to the understanding of triggering mechanisms of SSEs in New Zealand. Here, we invert the continuous Global Positioning System (cGPS) observations to investigate the temporal evolution of the seismically triggered slow-slip behaviour on the Hikurangi subduction interface. Similar to previous studies, our optimal geodetic inversion model reproduces deep (30–50 km) Marlborough afterslip of over 60 cm, deep Kapiti slow slip of up to 31 cm and shallow (<15 km), moderate (>10 cm) east coast slow slip on the Hikurangi subduction interface. The slow accumulated seismic moment over the past eight months from the large-scale slow slip motion is ~3.12 × 1020 Nm, which is equivalent to a magnitude 7.6 earthquake. Both the Marlborough afterslip and Kapiti slow slip were considerably active during the first week after the mainshock and gradually slowed down in the following months. These deep ongoing fault motions may continue for at least several months and are expected to decay logarithmically with time. In agreement with previous studies, the east coast slow slip motion occurred rapidly within 2–3 weeks, which stretched from the central to northern Hikurangi subduction margin. The coseismic Coulomb failure stress change investigation suggests that large-scale stress increase (over 1–2 MPa) appears on the southern Hikurangi subduction interface, which encourages afterslip underlying the Marlborough fault system and slow slip beneath the Kapiti coast. Consistent with previous studies, the long-distance slow slip motion offshore the east coast was likely dynamically triggered by the Kaikoura earthquake, where the static Coulomb stress increase is insignificant (<5 kPa).

Coseismic and postseismic deformation associated with the 2016 Mw 7.8 Kaikoura earthquake, New Zealand: fault movement investigation and seismic hazard analysis

The 2016 moment magnitude (Mw) 7.8 Kaikoura earthquake demonstrated that multiple fault segments can undergo rupture during a single seismic event. Here, we employ Global Positioning System (GPS) observations and geodetic modeling methods to create detailed images of coseismic slip and postseismic afterslip associated with the Kaikoura earthquake. Our optimal geodetic coseismic model suggests that rupture not only occurred on shallow crustal faults but also to some extent at the Hikurangi subduction interface. The GPS-inverted moment release during the earthquake is equivalent to a Mw 7.9 event. The near-field postseismic deformation is mainly derived from right-lateral strike-slip motions on shallow crustal faults. The afterslip did not only significantly extend northeastward on the Needles fault but also appeared at the plate interface, slowly releasing energy over the past 6 months, equivalent to a Mw 7.3 earthquake. Coulomb stress changes induced by coseismic deformation exhibit complex patterns and diversity at different depths, undoubtedly reflecting multi-fault rupture complexity associated with the earthquake. The Coulomb stress can reach several MPa during coseismic deformation, which can explain the trigger mechanisms of afterslip in two high-slip regions and the majority of aftershocks. Based on the deformation characteristics of the Kaikoura earthquake, interseismic plate coverage, and historical earthquakes, we conclude that Wellington is under higher seismic threat after the earthquake and great attention should be paid to potential large earthquake disasters in the near future.

Mapping subduction interface coupling using magnetotellurics: Hikurangi margin, New Zealand

AbstractThe observation of slow‐slip, seismic tremor, and low‐frequency earthquakes at subduction margins has provided new insight into the mechanisms by which stress accumulates between large subduction (megathrust) earthquakes. However, the relationship between the physical properties of the subduction interface and the nature of the controls on interplate seismic coupling is not fully understood. Using magnetotelluric data, we show in situ that an electrically resistive patch on the Hikurangi subduction interface corresponds with an area of increased coupling inferred from geodetic data. This resistive patch must reflect a decrease in the fluid or sediment content of the interface shear zone. Together, the magnetotelluric and geodetic data suggest that the frictional coupling of this part on the Hikurangi margin may be controlled by the interface fluid and sediment content: the resistive patch marking a fluid‐ and sediment‐starved area with an increased density of small, seismogenic‐asperities, and therefore a greater likelihood of subduction earthquake nucleation.

Slow slip events and the 2016 Te AraroaMw7.1 earthquake interaction: Northern Hikurangi subduction, New Zealand

AbstractFollowing a sequence of three Slow Slip Events (SSEs) on the northern Hikurangi Margin, between June 2015 and August 2016, aMw7.1 earthquake struck ~30 km offshore of the East Cape region in the North Island of New Zealand on the 2 September 2016 (NZ local time). The earthquake was also followed by a transient deformation event (SSE or afterslip) northeast of the North Island, closer to the earthquake source area. We use data from New Zealand's continuous Global Positioning System networks to invert for the SSE slip distribution and evolution on the Hikurangi subduction interface. Our slip inversion results show an increasing amplitude of the slow slip toward the Te Araroa earthquake foreshock and main shock area, suggesting a possible triggering of theMw7.1 earthquake by the later stage of the slow slip sequence. We also show that the transient deformation following the Te Araroa earthquake ruptured a portion of the Hikurangi Trench northeast of the North Island, farther north than any previously observed Hikurangi margin SSEs. Our slip inversion and the coulomb stress calculation suggest that this transient may have been induced as a response to the increase in the static coulomb stress change downdip of the rupture plane on the megathrust. These observations show the importance of considering the interaction between slow slip events, seismic, and aseismic events, not only on the same megathrust interface but also on faults within the surrounding crust.

The 2016 Kaikōura, New Zealand, Earthquake: Preliminary Seismological Report

ABSTRACT The 2016 M w 7.8 Kaikōura earthquake continued a notable decade of damaging earthquake impacts in New Zealand. The effects were wide ranging across the upper South Island, and included two fatalities, tsunami, tens of thousands of landslides, the collapse of one residential building, and damage to numerous structures and infrastructure. We present a preliminary overview focused on the seismological aspects of this earthquake and the corresponding seismological response effort. The earthquake rupture was extremely complex, involving at least 13 separate faults extending over ∼150 km, from the epicenter in north Canterbury to near the Cook Strait. We use backprojection and slip inversion methods to derive preliminary insights into the rupture evolution, identifying south‐to‐north rupture, including at least three distinct southwest‐to‐northeast propagating phases. The last phase is associated with a strong second pulse of energy release in the northern half of the rupture zone ∼70 s after rupture initiation, which we associate with the Kekerengu–Needles faults where some of the largest surface displacements (dextral) were observed. The mechanism of the mainshock was oblique thrust and relocated aftershocks show a range of thrust and strike‐slip mechanisms across three dominant spatial clusters. GeoNet datasets collected during the Kaikōura earthquake will be crucial in further unraveling details of the complex earthquake rupture and its implications for seismic hazard. Ground motions during the earthquake exceeded 1 g at both ends of the rupture. Spectral accelerations exceeded 500‐year return period design level spectra in numerous towns in the upper South Island, as well as in parts of the capital city of Wellington at critical periods of 1–2 s, influenced by site/basin and directivity effects. Another important part of the response effort has been the provision of earthquake forecasts, as well as consideration of the implications of slow slip on the Hikurangi subduction interface triggered as a result of the Kaikōura earthquake.

The New Zealand Strong Motion Database

This article summarises work that has been undertaken to compile the New Zealand Strong Motion Database, which is intended to be a significant resource for both researchers and practitioners. The database contains 276 New Zealand earthquakes that were recorded by strong motion instruments from GeoNet and earlier network operators. The events have moment magnitudes ranging from 3.5 to 7.8. A total of 134 of these events (49%) have been classified as occurring in the overlying crust, with 33 events (12%) located on the Fiordland subduction interface and 7 on the Hikurangi subduction interface (3%). 8 events (3%) are deemed to have occurred within the subducting Australian Plate at the Fiordland subduction zone, and 94 events (34%) within the subducting Pacific Plate on the Hikurangi subduction zone. There are a total of 4,148 uniformly-processed recordings associated with these earthquakes, from which acceleration, velocity and displacement time-series, Fourier amplitude spectra of acceleration, and acceleration response spectra have been computed. 598 recordings from the New Zealand database are identified as being suitable for future use in time-domain analyses of structural response. All data are publicly available at http://info.geonet.org.nz/x/TQAdAQ.

Understanding the potential for tsunami generated by earthquakes on the southern Hikurangi subduction interface

ABSTRACTThe earthquake and tsunami potential of the southern Hikurangi subduction interface has been assessed by reviewing current research in paleoseismology, paleotsunami, geodesy and passive and active source seismology. In addition, new interpretations of seismic lines are presented demonstrating the location and influence of splay faults and a modelling study examines the sensitivity of tsunami in Cook Strait to the southern termination of Hikurangi earthquake ruptures. Key conclusions are: the southern portion of the Hikurangi interface, south of c. 40°S, is currently strongly coupled and experiences great megathrust earthquakes of Mw >8 (some of which may also rupture further north); the downdip limits of strong coupling at southern Hikurangi are outlined by slow slip events and non-volcanic tremor; and the behaviour at the updip limit and the southern termination will be strongly influenced by splay faults. The tsunami impact on Cook Strait is shown to be highly sensitive to how far ruptures extend across Cook Strait.

Silent triggering: Aseismic crustal faulting induced by a subduction slow slip event

Slow Slip Events (SSEs) are a form of fault slip behaviour whose importance has only been recognised in the last decade at subduction zone plate boundaries worldwide. Here, we show one of the first examples of the use of Interferometric Synthetic Aperture Radar (InSAR) data to document deformation during an SSE on the Hikurangi subduction interface in New Zealand. Although much of the deformation that we observe is attributable to slow slip on the subduction interface, the InSAR data also reveals surface offsets of up to 1–2 cm across a major crustal fault during the period of the slow slip event. In order to fit the observations, we find that reverse aseismic slip is required along a portion of the Wellington fault where the Coulomb Failure Stress changes (ΔCFS), due to the SSE, are in excess of 0.01 MPa. This is the first-ever documented example of a subduction interface SSE triggering a transient slip event on a major upper plate fault, and it has wide-ranging implications for the role that SSEs can play in time-dependent seismic hazard.

Effects of rupture complexity on local tsunami inundation: Implications for probabilistic tsunami hazard assessment by example

AbstractWe investigated the influence of earthquake source complexity on the extent of inundation caused by the resulting tsunami. We simulated 100 scenarios with collocated sources of variable slip on the Hikurangi subduction interface in the vicinity of Hawke's Bay and Poverty Bay in New Zealand and investigated the tsunami effects on the cities of Napier and Gisborne. Rupture complexity was found to have a first‐order effect on flow depth and inundation extent for local tsunami sources. The position of individual asperities in the slip distribution on the rupture interface control to some extent how severe inundation will be. However, predicting inundation extent in detail from investigating the distribution of slip on the rupture interface proves difficult. Assuming uniform slip on the rupture interface in tsunami models can underestimate the potential impact and extent of inundation. For example, simulation of an Mw 8.7 to Mw 8.8 earthquake with uniform slip reproduced the area that could potentially be inundated by equivalent nonuniform slip events of Mw 8.4. Deaggregation, to establish the contribution of different sources with different slip distributions to the probabilistic hazard, cannot be performed based on magnitude considerations alone. We propose two predictors for inundation severity based on the offshore tsunami wavefield using the linear wave equations in an attempt to keep costly simulations of full inundation to a minimum.

Slow wavespeeds and fluid overpressure in a region of shallow geodetic locking and slow slip, Hikurangi subduction margin, New Zealand

Travel times recorded onshore from active-source marine seismic surveys were used to determine Hikurangi forearc wavespeeds. Ray-path midpoints sample forearc crust above the shallow (<10–12 km) subduction thrust. The southern region is locked to c. 30 km depth, exhibits slow slip at 30-45 km depth, and is similar to other subduction zones. Our 1D southern model has a rapid increase in seismic velocity (Vp) from the seabed to 4 km depth, consistent with rapidly-deposited clastic sediments, and near-constant Vp=5.0±0.2 km/s at depths of 4–10 km within the forearc. The northern region has slow-slip events at shallow depths of c. 5–15 km, beneath the volume sampled by our analysis. Average travel times at offsets of 20–80 km are >1 s more than at equivalent offsets in the south, and Vp increases with depth within the forearc from 3.5±0.1 km/s at 4 km depth to 4.5±0.2 km/s at 10 km depth. We interpret the low wavespeeds in terms of compaction disequilibrium and compare seismic inferences of anomalously-high porosity (>10%) with hydrostatic reference compaction models to show effective stress is low in both the north (27±10 Mpa) and south (36±14 MPa). In the south, pore space and conduits have a high chance of being localized on faults or sand layers, the wedge is clearly compressionally critical, and we suggest the higher seismic velocities recorded primarily reflect higher effective stress levels transmitted through the rock framework. The observed wedge geometry and our estimate of fluid pressure within the wedge (λ=0.87±0.05) suggest a weak overpressured subduction thrust. In the north, we suggest high fluid pressure is maintained by a large fluid inventory from subducting sediment, a pore-space geometry characterized by pervasively-fractured rock and mudstone, and a lower mean stress due to its stable and non-critical wedge geometry. We consider both end-member assumptions of convergent and tensile failure, the latter of which reveals the possibility of near-lithostatic fluid pressure on the north Hikurangi subduction interface, although this remains unproven. Our observations are consistent with high fluid pressures encountered in petroleum wells at depths <3 km and we observe a clear spatial correlation between the distribution of residual travel-times and the maximum depth of geodetic coupling and slow-slip. Our observations provide evidence for a causative link between high fluid pressure and anomalously-shallow slow slip events and depth of geodetic locking.

SAHKE geophysical transect reveals crustal and subduction zone structure at the southern Hikurangi margin, New Zealand

The Seismic Array Hikurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath the southern North Island along a 350 km transect. Tomographic inversion of first‐arrival travel times was used to derive a well‐resolved 15–20 km deep P wave image of the crust. The refracted phases and migrated reflection events image subducting slab geometry and crustal structure. In the west, Australian Plate Moho depth decreases westward across the Taranaki Fault system from 35 to ∼28–30 km. In the east, subducted Pacific Plate oceanic crust is recognized to have a positive velocity gradient, but becomes less distinct beneath the Tararua Ranges, where the interface increases in dip at about 15 km depth from &lt;5° to &gt;15°. This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching, and low‐velocity high‐attenuation material that we interpret to be an underplated subduction sedimentary channel. We infer that a step down in the decollément transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the Tararua Ranges. Reflections from the Wairarapa Fault show that it is listric and soles into the top of underplated sediments, which in turn abut the Moho of the overriding plate at ∼32 km depth, near the downdip end of the strongly locked zone. The change in dip of the Hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface.