Northern Hikurangi Margin Research Articles

Gas Hydrates, Subsurface Structures and Tectonic Features of the Tuaheni Landslide Complex in the Northern Hikurangi Margin, New Zealand, Revealed by Seismic Attribute Analysis

The Tuaheni Landslide Complex, located on the upper slope of the northern Hikurangi Margin in New Zealand, is a unique place to research on slow slip creep-like deformation and seabed failure, as well as their possible relationship with the presence of gas hydrates, cold seeps, and fluid migration. Based on the visual interpretation of seismic data, it is sometimes very difficult to identify various subsurface structures and tectonic features. We study certain seismic attributes, namely the reflection strength, instantaneous frequency, instantaneous phase, and the Hilbert transform, in the Tuaheni Landslide Complex and observe that these attributes play a very important role in identifying and interpreting various subsurface geological features and bed boundaries that are not clearly visible in the seismic sections. In general, these seismic attributes are studied to identify hydrocarbons such as oil and gas. However, in this present study these seismic attributes nicely illustrate the fluid migration pathways, the decollement of the sediment slide, the base of the debris flow, the base of the deformed sediment and gas migration, etc., along two perpendicular seismic profiles crossing the Site U1517 of IODP Expedition 372. The instantaneous phase and Hilbert transform attribute depict the bed boundaries and discontinuities, whereas the reflection strength and instantaneous frequency attributes characterize the various strata in terms of whether they are associated with fluid at their bases. The possible role of tectonic activity and seafloor slope failure due to gas hydrate dissociation and vice versa is clearly visible through fluid-filled weak zones in the seismic attribute volumes. Gas hydrates are dissociating and BSRs are abruptly pinching out towards the seafloor due to the movement of hot fluid and free gas, enhancing seafloor sliding and local tectonic activities together.

Temporal Relationship of Slow Slip Events and Microearthquake Seismicity: Insights From Earthquake Automatic Detections in the Northern Hikurangi Margin, Aotearoa New Zealand

AbstractSlow slip events in the northern Hikurangi margin of Aotearoa New Zealand occur every 18–24 months and last for several weeks before returning to average convergence rates of around 38 mm/yr. Along this plate boundary, the Hikurangi plateau subducts beneath the overlying Australian plate and slow slip events occur along their plate interface at depths between 2 and 15 km. To explore whether there is a temporal relationship between slow slip events and earthquake occurrence, the Regressive ESTimator automated phase arrival detection and onset estimation algorithm was applied to a data set of continuous waveform data collected by both land and ocean bottom seismometers. This detector uses an autoregressive algorithm with iterative refinement to first detect seismic events and then create a catalog of hypocenters and P and S wave arrival times. Results are compared with an available catalog of manually detected seismic events. The auto‐detector was able to find more than three times the number of events detected by analysts. With our newly assembled data set of automatically detected earthquakes, we were able to determine that there was an increase in the rate of earthquake occurrence during the 2014 slow slip event.

Ultralow frictional healing explains recurring slow slip events.

Plate motion on shallow subduction megathrusts is accommodated by a spectrum of tectonic slip modes. However, the frictional properties and conditions that sustain these diverse slip behaviors remain enigmatic. Frictional healing is one such property, which describes the degree of fault restrengthening between earthquakes. We show that the frictional healing rate of materials entrained along the megathrust at the northern Hikurangi margin, which hosts well-characterized recurring shallow slow slip events (SSEs), is nearly zero (<0.0001 per decade). These low healing rates provide a mechanism for the low stress drops (<50 kilopascals) and short recurrence times (1 to 2 years) characteristic of shallow SSEs at Hikurangi and other subduction margins. We suggest that near-zero frictional healing rates, associated with weak phyllosilicates that are common in subduction zones, may promote frequent, small-stress-drop, slow ruptures near the trench.

Volcanogenic aluminosilicate alteration drives formation of authigenic phases at the northern Hikurangi margin: Implications for subseafloor geochemical cycles

Alteration of volcanogenic aluminosilicates (VAs) in marine sediments is recognized as critical in regulating geochemical cycles and sustaining the oceanic deep biosphere, but rates of VA alteration and its associated authigenic mineral formation are not commonly reported. Here we present results on analyses of sediments and pore water recovered from the upper 150 mbsf of four sites drilled on the northern Hikurangi margin during IODP Expeditions 372 and 375. Petrographic analyses show that volcanogenic materials (glass shards, feldspar, volcanic lithoclasts) constitute important components (15–45 wt%) of the hemipelagic mud, and reveal ongoing glass alteration with accompanying authigenic phase formation. A reaction-transport model constrained by pore water Sr, 87Sr/86Sr, Ca, Mg, and Si was applied to simulate VA diagenetic reactions. Our model results yield VA alteration rates of 0.047–0.64 mmol Sr m−2 yr−1, with substantially higher values at Sites U1517 and U1520 that experienced rapid sediment emplacement. In addition, our simulations show that >99% of the dissolved Si generated by VA alteration is fixed in silica cement and authigenic clay, and that ∼50% of Ca incorporated in the authigenic carbonate is supplied by VA alteration. First-order estimates suggest that, in addition to authigenic carbonate precipitation, authigenic clay formation may represent an important sink for dissolved Mg. This study quantitatively examines the linkage between VA alteration and formation of authigenic phases, highlights its role in subsurface geochemical cycles, and indicates that slope instability may play an important role in promoting VA diagenesis.

The northern Hikurangi margin three-dimensional plate interface in New Zealand remains rough 100 km from the trench

AbstractAt the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction-interface plate motion from landward of the trench to &amp;lt;20 km depth. SSEs may be spatially related to geometrical interface heterogeneity, though kilometer-scale plate-interface roughness imaged by active-source seismic methods is only constrained offshore at &amp;lt;12 km depth. Onshore constraints are comparatively lacking, but we mapped the Hikurangi margin plate interface using receiver functions from data collected by a dense 22 × 10 km array of 49 broadband seismometers. The plate interface manifests as a positive-amplitude conversion (velocity increase with depth) dipping west from 10 to 17 km depth. This interface corroborates relocated earthquake hypocenters, seismic velocity models, and downdip extrapolation of depth-converted two-dimensional active-source lines. Our mapped plate interface has kilometer-amplitude roughness we interpret as oceanic volcanics or seamounts, and is 1–4 km shallower than the regional-scale plate-interface model used in geodetic inversions. Slip during SSEs may thus have different magnitudes and/or distributions than previously thought. We show interface roughness also leads to shear-strength variability, where slip may nucleate in locally weak areas and propagate across areas of low shear-strength gradient. Heterogeneous shear strength throughout the depth range of the northern Hikurangi margin may govern the nature of plate deformation, including the localization of both slow slip and hazardous earthquakes.

A unified effective medium modeling framework for quantitative characterization of hydrate reservoirs

Quantitative characterization of hydrate reservoirs is essential for evaluating hydrate resources and monitoring environmental hazards. Rock-physics models provide the theoretical basis for linking the hydrate saturation and distribution to the elastic properties. However, many of the existing models ignore the coupled effect of hydrate saturation and complex morphologies on the elastic behaviors and thereby fail to accurately describe hydrate reservoirs. To address these problems, the authors have proposed a unified effective medium modeling framework honoring multiple hydrate morphologies. In this framework, the manners in which the different types of hydrates interact with the sediment grains are characterized by different effective medium models. In addition to the hydrate saturation, the modeling results also reveal the effects of the cement factor, friction coefficient, incidence angle, lithology, and hydrate morphology on the elastic properties of the hydrate reservoirs. Then, five experimental data sets for synthetic methane and tetrahydrofuran hydrates formed under different conditions are used to validate the feasibility of the unified rock-physics modeling scheme. The good agreement between the theoretical predictions and laboratory data indicates that this method can effectively determine the hydrate occurrence mechanisms from the acoustic responses in different hydrate formation environments. On the basis of the constructed elastic rock-physics templates, multiple hydrate morphologies are determined with cluster analysis and then used for constraining the modeling scheme. Finally, the proposed modeling method is tested using sonic logs from Ignik Sikumi, North Slope of Alaska, and Hole U1518B, the northern Hikurangi margin, offshore New Zealand. The hydrate saturations estimated from the velocities are consistent with the interpretations based on resistivity and nuclear magnetic resonance logs, as well as chloride measurements, confirming the applicability and feasibility of our unified theoretical modeling framework.

Seafloor overthrusting causes ductile fault deformation and fault sealing along the Northern Hikurangi Margin

IODP Site U1518, drilled during IODP Expeditions 372 and 375, penetrated a large-offset (∼6 km) thrust, the Pāpaku fault, rising from a megathrust that hosts recurring slow slip events along the Hikurangi margin. Although drilling intersected the fault zone at only ∼300 m below the seafloor within porous silty mudstone, it exhibits intense tectonic ductile deformation, including finely banded mudstones contorted into decimeter-scale folds; elongate mudstone clasts with grain tail complexes; stacked and truncated silt beds in distorted mudstones; and soft sediment injections. Locally, these ductile features are overprinted by brittle deformation, including normal faults, fracture arrays, and breccias. The more consolidated hanging wall is dominated by brittle structures, whereas the footwall exhibits ductile and brittle deformation that decreases in intensity with depth. The intense tectonic ductile deformation and asymmetric distribution of structures across the fault zone at Site U1518 can be explained by seafloor overthrusting. The emplacement of the hanging wall upon the footwall flat overrode high-porosity, undeformed, and previously unburied sediments, localizing shear deformation within these weak sediments. In contrast, the overconsolidated hanging wall preferentially experienced brittle deformation during folding and displacement. Interstitial pore water geochemical profiles at Site U1518 show a repetition of near-seafloor diagenetic sequences below the fault, consistent with overthrusting of previously unburied strata. The preserved diagenetic profiles in the footwall suggest that overthrusting occurred within the last 50-100 kyr, and indicate little along- or across-fault fluid flow at the location of Site U1518. Thus the Pāpaku fault appears to define a low-permeability seal that restricts footwall consolidation, maintaining locally high pore fluid pressures and low fault strength. If similar low permeability structures occur elsewhere along the margin, they could support regionally high pore pressure conditions favorable to the occurrence of SSEs on the Hikurangi megathrust fault.

Temporal velocity variations in the northern Hikurangi margin and the relation to slow slip

Slow slip events (SSE) have been studied in increasing detail over the last 20 years, improving our understanding of subduction zone processes. Although the relationship between SSEs and the physical properties of their surrounding materials is still not well-understood, the northern Hikurangi margin in New Zealand is the site of relatively shallow (<10 km deep), frequent SSEs, providing excellent opportunities for near-field investigations. From September to October 2014, an SSE occurred with more than 250 mm slip, and was recorded successfully by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) deployment. This study applies scattered wave interferometry to ambient noise data acquired by nine HOBITSS ocean bottom seismometers (OBS) to study the seismic velocity variations related to the SSE. Single station cross-component correlations are computed within a period band that focuses on the upper plate in our study region. The average velocity variations display a decrease on the order of 0.05% during the SSE, followed by an increase of similar magnitude afterwards. We suggest two possibilities. The first possibility, which has been suggested by other seismological observations, is that the SSE causes a low-permeability seal on the plate boundary to break. The break allows fluid to migrate into the upper plate, causing a seismic velocity decrease during the SSE because of increased pore fluid volume in the upper plate. Under this model, after the SSE, the fluids in the upper plate diffuse gradually and the velocity increases again. The second possibility is the velocity changes are related to changes in crustal strain during the slow slip cycle, whereby elastic strain accumulates prior to the SSE, causing contraction and reduction of porosity and therefore increase of velocity above the SSE source (the seismic velocity increases between SSEs). During the SSE the upper plate goes into extension as the elastic strain is released, which results in dilation and a porosity increase (seismic velocity reduction). After the SSE, stress and strain accumulate again, causing a porosity decrease and a velocity increase.

Frictional and Lithological Controls on Shallow Slow Slip at the Northern Hikurangi Margin

AbstractSlow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide‐hold‐slide experiments over a range of fault slip rates, from plate rate (5 cm/yr or 1.6 × 10−9 m/s) to ∼1 mm/s (10−3 m/s) and quantified the frictional velocity dependence and healing rates for a range of lithologies atdifferent stresses. The frictional velocity dependence (a‐b) and critical slip distance DC increase with fault slip rate in our experiments. We observe atransition from velocity weakening to strengthening at slip rates of ∼0.3 µm/s. This velocity dependence of DC could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay‐rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1–10 years are consistent with the geodetically inferred low stress drops and healing rates characteristic of the Hikurangi SSEs.

Fluid-rich subducting topography generates anomalous forearc porosity.

The role of subducting topography on the mode of fault slip-particularly whether it hinders or facilitates large megathrust earthquakes-remains a controversial topic in subduction dynamics1-5. Models have illustrated the potential for subducting topography to severely alter the structure, stress state and mechanics of subduction zones4,6; however, direct geophysical imaging of the complex fracture networks proposed and the hydrology of both the subducting topography and the associated upper plate damage zones remains elusive. Here we use passive and controlled-source seafloor electromagnetic data collected at the northern Hikurangi Margin, New Zealand, to constrain electrical resistivity in a region of active seamount subduction. We show that a seamount on the incoming plate contains a thin, low-porosity basaltic cap that traps a conductive matrix of porous volcaniclastics and altered material over a resistive core, which allows 3.2 to 4.7 times more water to subduct, compared with normal, unfaulted oceanic lithosphere. In the forearc, we image a sediment-starved plate interface above a subducting seamount with similar electrical structure to the incoming plate seamount. A sharp resistive peak within the subducting seamount lies directly beneath a prominent upper plate conductive anomaly. The coincidence of this upper plate anomaly with the location of burst-type repeating earthquakes and seismicity associated with a recent slow slip event7 directly links subducting topography to the creation of fluid-rich damage zones in the forearc that alter the effective normal stress at the plate interface by modulating the fluid overpressure. In addition to severely modifying the structure and physical conditions of the upper plate, subducting seamounts represent an underappreciated mechanism for transporting a considerable flux of water to the forearc and deeper mantle.

Crustal Structure of the Northern Hikurangi Margin, New Zealand: Variable Accretion and Overthrusting Plate Strength Influenced by Rough Subduction

AbstractExploring the structure of convergent margins is key to understanding megathrust slip behavior and tsunami generation. We present new wide‐angle and marine multichannel seismic data that constrain the crustal structure and accretion dynamics of the northern Hikurangi margin. The top of the basement of the Hikurangi Plateau is overlain by a rough, 2–3 km thick layer of volcanic cover with P‐wave velocities (VP) between 3 and 5 km/s. This volcanic cover contributes significantly to seismic reflectivity beneath the shallow subduction plate boundary. The frontal prism structure varies along‐strike from ∼25 km wide with imbricate thrust faults where accretion of trench sediments is undisrupted, to narrower (∼14 km) with slumps and branching, irregular thrust fault geometries, which may reflect lower sediment supply or past seamount collisions. A large thrust fault network in the inner prism with a seismically fast hanging wall indicates a mechanical boundary between a seismically faster deforming backstop and the seismically slower frontal prism. Near the coastline, VP increases between 2.8 and 4 km/s at 2–8 km depth and is 0.5–1.7 km/s slower than the southern Hikurangi margin. Low seismic wavespeeds and low vertical velocity gradients in the inner prism support the hypothesis that a weak overthrusting plate contributes to historic tsunami‐earthquakes and long duration seismic ground motion.

Fluid Accumulation, Migration and Anaerobic Oxidation of Methane Along a Major Splay Fault at the Hikurangi Subduction Margin (New Zealand): A Magnetic Approach

AbstractUnderstanding the locus of fluid flow along thrust and splay faults is important to understand the hydraulic properties of accretionary systems and fault mechanics. Here, we use rock magnetic techniques in combination with backscattered electron imaging to depict the locus of enhanced magnetic mineral alteration within the Pāpaku fault, an active splay fault of the subduction interface at the northern Hikurangi Margin. The Pāpaku fault was cored at Site U1518 during Expedition 375 of the International Ocean Discovery Program and we report room temperature magnetic parameters, complemented by first‐order reversal and thermomagnetic curves in the depth interval 250–400 m below seafloor (mbsf). The ∼60‐m wide Pāpaku fault zone comprises two main slip zones, referred to as the upper main brittle (304–321 mbsf) and lower subsidiary (351–361 mbsf) fault zones, and an intervening zoned, termed the lower ductile deformation zone. Two narrow zones, at the top of the main brittle fault zone, and one in a sand‐rich interval above the subsidiary fault zone, experienced enhanced magnetic mineral diagenesis, which resulted in the recrystallization of ferrimagnetic greigite to paramagnetic pyrite. We propose that secondary magnetic mineral diagenesis was driven by anaerobic methane oxidation within these intervals, which occurs in the presence of methane and sulfate. We relate the observed changes to the fault parallel transport of fluids which is restricted to two damage zones. Overlying compacted and clay‐rich sediments likely act as a barrier to upward advective flow through the fault zone and into the hanging wall.

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.

Three‐Dimensional P Wave Velocity Structure of the Northern Hikurangi Margin From the NZ3D Experiment: Evidence for Fault‐Bound Anisotropy

AbstractWe present a high‐resolution three‐dimensional (3‐D) anisotropic P wave velocity (Vp) model in the northern Hikurangi margin offshore Gisborne, New Zealand, constructed by tomographic inversion of over 430,000 first arrivals recorded by a dense grid of ocean bottom seismometers. Since the study area covers a region where shallow slow slip events (SSEs) occur repeatedly and the subduction of a seamount is proposed, it offers an ideal location to link our understanding of structural and hydrogeologic properties at megathrust faults to slip behavior. The Vp model reveals an ~30‐km‐wide, low‐velocity accretionary wedge at the toe of the overriding plate, where previous seismic reflection studies show a series of active thrust faults branching from the plate interface. We find some locations with significant Vp azimuthal anisotropy &gt;5% near the branching faults and the deformation front. This finding suggests that the anisotropy is not ubiquitous and homogeneous within the overriding plate, but more localized in the vicinity of active thrust faults. The fast axes of Vp within the accretionary wedge are mostly oriented to the plate convergence direction, which is interpreted as preferentially oriented cracks in a compressional stress regime associated with plate subduction. We find that the magnitudes of anisotropy are roughly equivalent to values found at oceanic spreading centers, where the extensional stress regime is dominant and the crack density is expected to be higher than subduction zones. This consideration may indicate that additional effects such as fault foliation and clay mineral alignment also contribute to upper plate anisotropy along subduction margins.

Pore Fluid Pressures and Strength Contrasts Maintain Frontal Fault Activity, Northern Hikurangi Margin, New Zealand

AbstractDiverse modes of fault slip occur within shallow subduction zones, but their relationships to upper plate deformation are poorly understood. IODP Expedition 375 collected samples and data across the northern Hikurangi margin, where tsunamigenic earthquakes and slow slip events occur. Site U1518 intersected the Pāpaku fault, a splay fault within the frontal accretionary wedge. To constrain in situ physical conditions and strength of the system, we conducted isotropic loading and triaxial compression experiments on hanging wall and foot wall cores and interpreted the results using critical state soil mechanics theory. The hanging wall is relatively strong, reflecting its tectonic history, whereas the foot wall and fault are anomalously weak owing to elevated pore pressure (λ∗ ≥ 0.3). Critical state in both samples corresponds to a friction coefficient of 0.6, constraining the strength of the Pāpaku fault. Low fault strength, maintained by excess pore pressures, suggests that the Pāpaku fault may still be active.

Porosity, Pore Structure, and Fluid Distribution in the Sediments Entering the Northern Hikurangi Margin, New Zealand

AbstractHosting both tsunami earthquakes and slow slip events at shallow depth, the northern Hikurangi margin has motivated strong research efforts over the last two decades to better understand the relation between fluid pressure and fault mechanical behavior. Recently, IODP Expeditions 372 and 375 drilled, cored, and logged the basin entering this subduction zone providing a unique opportunity to characterize initial hydrogeological and petrophysical properties that drive seismic hazards along the margin. We inventory bound and interstitial fluid contents and assess the compaction state of the heterogeneous incoming sequence by correcting shipboard total connected porosity from the bound water content that typically characterizes clay‐rich sediments. By combining mercury injection capillary pressure, nuclear magnetic resonance, and nitrogen gas adsorption, we further document the evolution of pore structure with increasing depth. We also compare porosity data at the logging and sample scales. Both logging and laboratory data evidence contrasted porosity, pore structure, and fluid distribution across the entering section. Shallow macroporous siliciclastic units show very low bound water content and undergo compaction‐induced dewatering as they are normally consolidated during burial. In contrast, mesoporous volcaniclastic Hikurangi Plateau facies are characterized by high potential for bound water release from smectite, zeolite, and opal dehydration at greater depths. They mostly exhibit very low permeability except potentially in uncemented highly porous intervals where the décollement may initiate. Macroporous to mesoporous pelagic carbonates sandwiched in between exhibit mixed behavior and may also host the future décollement, similarly to the southern part of the margin.

Assessment of gas hydrate reservoir from inverted seismic impedance and porosity in the northern Hikurangi margin, New Zealand

Hikurangi margin, New Zealand, the shallowest subduction zone on Earth is the unique place to study the slow slip creep-like deformation and seafloor failure, and their probable link to the presence of gas hydrate, cold seeps and fluid migration. International Ocean Discovery Program expedition 372 discovered gas hydrate within thin intervals (ranging from ~5 to 25 m) of the deformed sediments in the Tuaheni landslide complex area of the northern Hikurangi margin. Seismic profiles show typical BSRs (bottom simulating reflectors) cutting across the dipping strata throughout the sections. To characterize the reservoir, here, we invert both acoustic impedance (product of velocity and density) and porosity along two perpendicular seismic profiles crossing the well using a model-based post-stack seismic inversion technique. Results show the layer structure of alternate high-impedance with low-porosity and low-impedance with high-porosity sediments. Interestingly, each layer is consisting of thin and relatively low impedance intra-layers, which clearly indicates the possible pathways for fluid and gas migration, and one of the reasons for seafloor collapse in this region. Very low impedance observed below the BSR is due to the gas-charged sediments at the dipping strata, and is the probable source of free-gas migrating upward. Next, we apply rock physics theory to know the distribution of gas hydrate at Site U1517 as well as along two perpendicular seismic profiles. Rock physics modeling using sonic velocity at well location shows that gas hydrate is distributed mainly within the depth intervals of 5–50 m, 90–115 m and 130–155 m with an average saturation of about 10–15% and the maximum concentration of about 35% of the pore space at 100 m depth. Prediction of gas hydrate saturation from the resistivity method using neutron porosity and NMR (nuclear magnetic resonance) logs in Archie's empirical formula shows less saturation compared to that from the sonic log. Whereas, estimated gas hydrate saturation from the chloride concentrations ranges from 0 to 45% of the pore space. The spatial distributions of gas hydrate along seismic profiles are predicted by rock physics theory using the inverted impedance and porosity. Saturation of gas hydrate along the seismic profiles varies from 0 to 40% with an average of ~10% of the pore space. The correlation between inverted impedance and porosity indicates that the high impedance layers are due to low porosity consolidated sediment without significant gas hydrate concentration. Gas hydrate is distributed in relatively high porosity and low impedance layers along the seismic profiles. Our result shows that the amount of gas hydrate concentration is low in this region and sediments are highly deformed/disturbed in the presence of many fluid/gas migration pathways.

Impact of iron release by volcanic ash alteration on carbon cycling in sediments of the northern Hikurangi margin

We present geochemical data collected from volcanic ash-bearing sediments on the upper slope of the northern Hikurangi margin during the RV SONNE SO247 expedition in 2016. Gravity coring and seafloor drilling with the MARUM-MeBo200 allowed for collection of sediments down to 105 meters below seafloor (mbsf). Release of dissolved Sr2+ with isotopic composition enriched in 86Sr (87Sr/86Sr minimum = 0.708461 at 83.5 mbsf) is indicative of ash alteration. This reaction releases other cations in the 30-70 mbsf depth interval as reflected by maxima in pore-water Ca2+ and Ba2+ concentrations. In addition, we posit that Fe(III) in volcanogenic glass serves as an electron acceptor for methane oxidation, a reaction that releases Fe2+ measured in the pore fluids to a maximum concentration of 184 μM. Several lines of evidence support our proposed coupling of ash alteration with Fe-mediated anaerobic oxidation of methane (Fe-AOM) beneath the sulfate-methane transition (SMT), which lies at ∼7 mbsf at this site. In the ∼30-70 mbsf interval, we observe a concurrent increase in Fe2+ and a depletion of CH4 with a well-defined decrease in δ13C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in δ13C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO42− concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4 and DIC. Mass balance considerations reveal that the iron cycled through the coupled ash alteration and AOM reactions is consumed as authigenic Fe-bearing minerals. This iron sink term derived from the mass balance is consistent with the amount of iron present as carbonate minerals, as estimated from sequential extraction analyses. Using a numerical modeling approach we estimate the rate of Fe-AOM to be on the order of 0.4 μmol cm−2 yr−1, which accounts for ∼12% of total CH4 removal in the sediments. Although not without uncertainties, the results presented reveal that Fe-AOM in ash-bearing sediments is significantly lower than the sulfate-driven CH4 consumption, which at this site is 3.0 μmol cm−2 yr−1. We highlight that Fe(III) in ash can potentially serve as an electron acceptor for methane oxidation in sulfate-depleted settings. This is relevant to our understanding of C-Fe cycling in the methanic zone that typically underlies the SMT and could be important in supporting the deep biosphere.

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.

Seismicity at the Northern Hikurangi Margin, New Zealand, and Investigation of the Potential Spatial and Temporal Relationships With a Shallow Slow Slip Event

AbstractIn 2014–2015, the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip experiment deployed seafloor absolute pressure gauges and ocean bottom seismometers directly above a large slow slip event, allowing examination of the relationship between slow slip and earthquakes in detail. Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip data were combined with nearby existing land stations to create a catalog of microseismicity consisting of 2,300 earthquakes ranging in magnitude between 0.5 and 4.7 that is complete to magnitude 1.5, yielding almost twice as many events as detected by the onshore networks alone. This greatly improves the seismicity catalog for this active subduction zone margin, especially in the offshore portion that was difficult to study using only the inland permanent seismic network. The new locations for the events within the footprint of the offshore network show that earthquakes near the trench are systematically shallower than and NW (landward) of their locations using only land‐based stations. Our results indicate that Hikurangi seismicity is concentrated in two NE‐SW bands, one offshore beneath the outer forearc wedge, one onshore beneath the eastern Raukumara Peninsula, and the majority of earthquakes are within the subducting Pacific plate with a smaller percent at the plate interface. We find a 20‐km wide northeast trending gap in microseismicity between the two bands and beneath the inner forearc wedge and this gap in seismicity borders the downdip edge of a slow slip patch.