Northern Hikurangi Subduction Margin Research Articles

Compactive deformation of incoming calcareous pelagic sediments, northern Hikurangi subduction margin, New Zealand: Implications for subduction processes

Calcareous rocks are commonly found in subduction zones, but few studies have investigated the consolidation and compactive deformation of these rocks prior to subduction, and their potential effects on subduction and accretionary processes are thus poorly understood. Using drilling data obtained during International Ocean Discovery Program (IODP) Expeditions 372 and 375 combined with 2D and 3D seismic reflection data, the structure, growth history, and slip rates of normal faults identified in the incoming pelagic sedimentary sequences of the Hikurangi Margin were investigated. A seismic coherence depth slice and vertical profiles show that these faults exhibit polygonal structure that has rarely been documented at subduction margins. The polygonal faults are closely spaced and layer-bound within sequences dominated by pelagic carbonate and calcareous mudstone of Paleocene-Pliocene age. Kinematic modeling and 2D displacement analysis reveal that fault throws decrease toward the upper and lower tipline. In detail, two groups of throw profiles are defined by locations of displacement maxima, possibly reflecting lateral variations in physical properties. The polygonal fault system (PFS) likely formed by syneresis processes that involve diagenetically induced shear failure and volumetric contraction of the pelagic unit associated with fluid escape. Fault growth sequences reveal multiple, weakly correlated intervals of contemporaneous seafloor deformation and sedimentation and allow estimates of fault slip rates. We find evidence for a significant increase in typical slip rates from 0.5-3 m/Ma during pelagic sedimentation to >20 m/Ma following the onset of terrigenous sedimentation. These observations suggest that rapid loading of the pelagic sediments by the trench-wedge facies was associated with renewed and faster growth of the PFS. The PFS will eventually be transported into the base of the accretionary wedge, enhancing geometric roughness and heterogeneity of materials along the megathrust, and providing inherited zones of weakness. Selective fault reactivation may facilitate deformation and episodic vertical fluid migration in the lower wedge associated with microearthquakes, tremor, and slow slip events.

Trench floor depositional response to glacio-eustatic changes over the last 45 ka, northern Hikurangi subduction margin, New Zealand

ABSTRACT Glacio-eustatic cycles lead to changes in sedimentation on all types of continental margins. There is, however, a paucity of sedimentation rate data over eustatic sea-level cycles in active subduction zones. During International Ocean Discovery Program Expedition 375, coring of the upper ∼110 m of the northern Hikurangi Trough Site U1520 recovered a turbidite-dominated succession deposited during the last ∼45 kyrs (Marine Isotope Stages (MIS) 1–3). We present an age model integrating radiocarbon dates, tephrochronology, and δ18O stratigraphy, to evaluate the bed recurrence interval (RI) and sediment accumulation rate (SAR). Our analyses indicate mean bed RI varies from ∼322 yrs in MIS1, ∼49 yrs in MIS2, and ∼231 yrs in MIS3. Large (6-fold) and abrupt variations in SAR are recorded across MIS transitions, with rates of up to ∼10 m/kyr occurring during the Last Glacial Maximum (LGM), and <1 m/kyr during MIS1 and 3. The pronounced variability in SAR, with extremely high rates during the LGM, even for a subduction zone, are the result of changes in regional sediment supply associated with climate-driven changes in terrestrial catchment erosion, and critical thresholds of eustatic sea-level change altering the degree of sediment bypassing the continental shelf and slope via submarine canyon systems.

Plate‐Rate Frictional Behavior of Sediment Inputs to the Hikurangi Subduction Margin: How Does Lithology Control Slow Slip Events?

AbstractThe northern Hikurangi subduction margin hosts slow slip events (SSEs), which are exceptionally shallow (&lt;15 km). The sedimentary sequence on the incoming plate is therefore representative of the shallow fault material where the SSEs will take place once they enter the subduction zone. Knowledge about the frictional behavior of these sediments is required to know which lithologies are capable of hosting SSEs, and what mechanisms are causing them. Frictional behavior is material specific and depends on sliding velocity, but it is unknown how these natural sediments behave at plate‐rate velocities. We performed laboratory shearing experiments testing the major lithologies sampled during International Ocean Discovery Program (IODP) Expedition 375, at velocities ranging from the plate convergence rate at the Hikurangi margin (5 cm/year), up to those characteristics of the shallow SSEs (160 and 530 cm/year), under simulated in‐situ as well as standardized conditions. We find that the calcite‐rich pelagic sediments are relatively strong and display the velocity‐weakening frictional behavior required for slip events, whereas other lithologies are weaker and show velocity‐neutral to velocity‐strengthening friction. We observe spontaneous laboratory SSEs in the calcareous pelagic sediments, which show partial locking in between sliding events, consistent with the interpretation of SSEs within the spectrum of slow to fast earthquakes. For the Hikurangi margin, our results suggest that SSE occurrence requires the stronger carbonate‐rich unit to be incorporated into the plate‐boundary fault zone, which we suggest occurs because the rough incoming plate introduces geometrical complexity into the fault zone.

A Repeating Earthquake Catalog From 2003 to 2020 for the Raukumara Peninsula, Northern Hikurangi Subduction Margin, New Zealand

AbstractRepeating earthquakes provide a novel way of monitoring how stresses load faults between large earthquakes. To date, however, and despite the availability of long‐duration, high‐quality seismological datasets, little attention has been paid to tectonic repeating earthquakes in New Zealand. We develop a workflow and composite criterion for identifying repeating earthquakes in New Zealand, using data from the GeoNet permanent seismic network, and present New Zealand’s first decadal‐scale repeating earthquake catalog. For events to be identified as repeating in this study, two or more events must have a normalized cross‐correlation of at least 0.95 at two or more seismic stations, when calculated for 75% of the earthquake coda. By applying our composite criterion to seismicity around the Raukumara Peninsula, northern Hikurangi subduction margin, we have identified 61 repeating earthquake families occurring between 2003 and 2020, consisting of 347 individual earthquakes. These families have a magnitude range of ML 1.7–5.2 and recurrence intervals of &lt; 1 to ∼7 yrs. Repeating earthquakes in 9 of the 11 regional groups identified in this study coincide spatiotemporally with previously identified slow‐slip events and tremor. However, the responses shown to slow‐slip are not consistent within families or within regional groups.

Near Trench 3D Seismic Attenuation Offshore Northern Hikurangi Subduction Margin, North Island, New Zealand

AbstractWe image seismic attenuation near the Hikurangi trench offshore New Zealand, using ocean bottom and land‐based seismometers, revealing high attenuation above a recurring shallow slow‐slip event and within the subducting Hikurangi Plateau. The Hikurangi subduction margin east of the North Island, New Zealand is the site of frequent shallow slow slip events. Overpressured fluids are hypothesized to lead to slow slip at shallow depths close to the oceanic trench. Seismic attenuation, energy loss of seismic waves, can be used to detect high temperatures, melt, the presence of fluids, and fractures. We use local earthquake P‐ and S‐waves from 180 earthquakes to invert for t*, and subsequently invert for Qp and Qs, offshore the North Island directly above the area of slow slip. We image Qp and Qs to ∼25 km depth, increasing resolution of previously identified coastal low Q (100–300), and finding a new region of even higher attenuation (Qp and Qs &lt; 50–100) directly above the shallow slow slip event of 2014–2015, beneath the offshore seismic array. This highest attenuation is downdip of a subducting seamount, and is spatially correlated with a high seismic reflectivity zone and Vp/Vs &gt; 1.85, all of which provide evidence for the presence of fluids. The Qp and Qs is low at the trench (&lt;50–100) and in the subducting plate (100–200), suggesting that seismic wave scattering due to faults, fractures, and the inherent heterogeneous composition of the Hikurangi Plateau, a large igneous province, plays a role in seismic attenuation.

The many double BSRs across the northern Hikurangi margin and their implications for subduction processes

The bottom simulating reflection (BSR) is widely observed along continental margins and is believed to mark the base of gas hydrate stability zone (BGHSZ). In some regions, double or multiple overlapping BSRs are observed, yet their formation mechanisms and geologic implications are not well understood. Here we present 3D seismic images from the 2018 NZ3D experiment that covers a 14×60km2 survey area on New Zealand's northern Hikurangi subduction margin. We observe double BSRs in five locations. Beneath the Tuaheni Basin in the mid-slope, a secondary BSR (BSR2) lies ∼100-360 m deeper than the primary BSR (BSR1) and its 3D geometry mimics the unconformity at the base of the basin. At three thrust ridges located 18-38 km from the deformation front, BSR2 lies ∼55-130 m below and is sub-parallel to BSR1. At another thrust ridge ∼14 km from the deformation front, BSR2 forms above the BSR1, and the two BSRs converge towards the peak of the ridge. Through 3D modeling of BGHSZ and analysis of the geometry and reflection characteristics of the double BSRs, we identify three potential mechanisms for their formation (1) rapid sedimentation, (2) tectonic uplift, (3) overpressure/heat advection caused by fluid migration. Our study demonstrates that formation of double BSRs is closely linked to subduction processes along the northern Hikurangi margin, and double BSRs may be used as indicators for areas with recent sedimentation, tectonic and/or fluid activities.

Marine Terraces Reveal Complex Near-Shore Upper-Plate Faulting in the Northern Hikurangi Margin, New Zealand

ABSTRACT Recent earthquakes involving multiple fault ruptures highlight the need to evaluate complex coastal deformation mechanisms, which are important for understanding plate boundary kinematics and seismic and tsunami hazards. We compare ages and uplift of the youngest Holocene marine terraces at Puatai Beach and Pakarae River mouth (∼10 km apart) in the northern Hikurangi subduction margin to examine whether uplift is the result of subduction earthquakes or upper-plate fault earthquakes. From stepped platform-cliff morphology, we infer uplift during 2–3 earthquakes and calculate an average uplift-per-event of 2.9±0.5 m at Puatai Beach and 2.0±0.5 m at Pakarae River mouth. Radiocarbon ages from the youngest beach deposit shells on each terrace and a tephra coverbed on one terrace constrain the timing of earthquakes to 1770–1710, 1100–910, and 420–250 cal. B.P. at Puatai Beach, and 1490–1290 and 660–530 cal. B.P. at Pakarae River mouth. The ages differ at each site indicating uplift is neither the result of subduction earthquakes nor single upper-plate fault earthquakes. A reinterpretation of new and existing bathymetry and seismic reflection data, combined with dislocation modeling, indicates that near-shore fault segmentation is more complex than previously thought and ruptures likely involve multiple upper-plate faults. Future updates of the New Zealand National Seismic Hazard Model should revise the northern Hikurangi subduction seismic sources so that rupture does not uplift Puatai Beach and Pakarae River mouth and include new near-shore upper-plate faults as multifault sources.

A MATLAB GUI for Examining Triggered Tremor: A Case Study in New Zealand

We introduce a user‐friendly MATLAB GUI (graphical user interface) software for searching for triggered tectonic tremor. This software includes the following features: (1) multiple panels, whose primary function screen shows seismograms filtered at various frequency bands, surface‐wave seismograms or spectrograms for comparison, and a station map; (2) a tag function with which users can classify each station by quality and/or group and whose results will show on the map; (3) the locations of tremor sources from which users can manually pick the arrival times of individual tremor bursts and quickly obtain preliminary locations; and (4) output visualization from which users can align triggered tremor as a move‐out plot or save snapshots to a PDF file. For demonstration, we apply this package to examine triggered tremor in New Zealand between 2004 and 2016. We analyze 5300 seismograms from 18 M_w ≥ 7.5 target earthquakes and find that 9 generate triggered tremors in six regions in the central and the northern Hikurangi subduction margin, the central strike‐slip Alpine fault, Kaikōura and Southland in the South Island, and Northland in the North Island. The minimum apparent tremor‐triggering stress observed at the surface stations in these regions varies from 5.0 to 16.6 kPa. This GUI‐based package can facilitate the process of analysis for identifying triggered seismic events from a sizeable seismic dataset.

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).

Fluid budgets along the northern Hikurangi subduction margin, New Zealand: the effect of a subducting seamount on fluid pressure

We estimate fluid sources around a subducted seamount along the northern Hikurangi subduction margin of New Zealand, using thermomechanical numerical modelling informed by wedge structure and porosities from multichannel seismic data. Calculated fluid sources are input into an independent fluid-flow model to explore the key controls on overpressure generation to depths of 12 km. In the thermomechanical models, sediment transport through and beneath the wedge is calculated assuming a pressure-sensitive frictional rheology. The change in porosity, pressure and temperature with calculated rock advection is used to compute fluid release from compaction and dehydration. Our calculations yield more precise information about source locations in time and space than previous averaged estimates for the Hikurangi margin. The volume of fluid release in the wedge is smaller than previously estimated from margin-averaged calculations (∼14 m3 yr−1 m−1), and is exceeded by fluid release from underlying (subducting) sediment (∼16 m3 yr−1 m−1). Clay dehydration contributes only a small quantity of fluid by volume (∼2 m3 yr−1 m−1 from subducted sediment), but the integrated effect is still significant landward of the seamount. Fluid source terms are used to estimate fluid pressures around a subducting seamount in the fluid-flow models, using subducted sediment permeability derived from porosity, and testing two end-members for décollement permeability. Models in which the décollement acts as a fluid conduit predict only moderate fluid overpressure in the wedge and subducting sediment. However, if the subduction interface becomes impermeable with depth, significant fluid overpressure develops in subducting sediment landward of the seamount. The location of predicted fluid overpressure and associated dehydration reactions is consistent with the idea that short duration, shallow, slow slip events (SSEs) landward of the seamount are caused by anomalous fluid pressures; alternatively, it may result from frictional effects of changing clay content along the subduction interface.

Slow Slip drilling Project at the northern Hikurangi subduction Margin, New Zealand

Slow Slip drilling Project at the northern Hikurangi subduction Margin, New Zealand

Submarine paleoseismology of the northern Hikurangi subduction margin of New Zealand as deduced from Turbidite record since 16 ka

Paleoseismic studies seek to characterise the signature of pre-historical earthquakes by deriving quantitative information from the geological record such as the source, magnitude and recurrence of moderate to large earthquakes. In this study, we provide a ∼16,000 yr-long paleo-earthquake record of the 200 km-long northern Hikurangi Margin, New Zealand, using cm-thick deep-sea turbidites identified in sediment cores. Cores were collected in strategic locations across the margin within three distinct morphological re-entrants – the Poverty, Ruatoria and Matakaoa re-entrants. The turbidite facies vary from muddy to sandy with evidence for rare hyperpycnites interbedded with hemipelagites and tephra. We use the Oxal probabilistic software to model the age of each turbidite, using the sedimentation rate of hemipelagite deduced from well-dated tephra layers and radiocarbon ages measurements on planktonic foraminifera.Turbidites are correlated from one core to the other using similarity in sedimentary facies, petrophysical properties and ages. Results show that 46 turbidites are synchronous along the entire margin. Amongst them 41 are interpreted as originating from the upper continental slope in response to earthquake-triggered slope failures between 390 ± 170 to 16,450 ± 310 yr BP. Using well-established empirical relationships that combine peak ground acceleration, magnitude and location of earthquakes, we calculate that synchronous slope failures were triggered by the rupture of 3 of the 26 known active faults in the region, each capable of generating Mw 7.3 to 8.4 earthquakes – two are crustal reverse faults and one is the subduction interface. The 41 Mw ≥ 7.3 earthquakes occurred at an average recurrence interval of ∼400 yr over the last ∼16,000 yr. Among them, twenty are interpreted as subduction interface earthquakes that occurred at an average recurrence interval of ∼800 yr, with alternating periods of high activity and low return times (305–610 yr) and quiescence periods with high return times (1480–2650 yr). Based on turbidite paleoseismology, we propose that subduction interface earthquakes were of lower magnitude during active periods (Mw > 7.5) than during quiescence periods (Mw ≥ 8.2).

Evaluating the coastal deformation mechanisms of the Raukumara Peninsula, northern Hikurangi subduction margin, New Zealand and insights into forearc uplift processes

Coastlines that trend both parallel and normal to the Hikurangi subduction margin along the Raukumara Peninsula, New Zealand, provide a globally unique situation that enables study of the coastal deformation and its spatial variance across and along the forearc. The data suggest there is margin-parallel zoning of the forearc deformation processes across the Raukumara segment of the Hikurangi margin. On forearc promontories closest to the Hikurangi trench (within 80 km), Holocene marine and fluvial terraces record localised zones of intermittent uplift driven by large coseismic events on landward-dipping nearshore reverse faults. At greater distances from the trench (>100 km), Holocene and Pleistocene coastal stratigraphy and geomorphology suggest that surface uplift occurs by a gradual mechanism: either by frequent small earthquakes or by an aseismic process. In concurrence with geophysical models, we suggest the gradual uplift of this area is most likely driven by the buoyancy of underplated sediment with a contribution from subduction of the over-thickened crust of the Hikurangi Plateau. There is an intermediate zone at c. 80–100 km from the trench that is uplifted at low to moderate rates, probably by the effects of Hikurangi Plateau subduction with some contributions from outboard upper plate faulting and inboard sediment underplating.

Three‐dimensional velocity structure of the northern Hikurangi margin, Raukumara, New Zealand: Implications for the growth of continental crust by subduction erosion and tectonic underplating

Traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3‐D P wave velocity model of the northern Hikurangi subduction margin, New Zealand. From north to south between Raukumara Basin and Raukumara Peninsula, the Moho of the overriding plate increases in depth from 17 to ∼35 km. Low seismic P wave velocities of 3.5–5.0 km/s are localized within a ∼10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and Moho and beneath the topographic crest of East Cape Ridge and the Raukumara Range. Southward, this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases. We interpret this low‐velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope. The buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift. In the middle crust, our observations of very low velocity suggest high fluid‐filled porosities of 12%–18%, and the implied buoyancy anomaly may enhance underplating. At greater depths the process is driven by the contrast between upper crustal quartz‐feldspar mineralogy and the denser diabase or olivine‐rich lithologies of the lower crust and mantle. We estimate a rate of lower crustal underplating at the northern Hikurangi margin of 20 ± 7 km3 Ma−1 km−1 since 22 Ma. We suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of fore‐arc crustal rocks into the mantle at subduction zones may be systematically overestimated.