We use a prestack depth migration reflection image and magnetic anomaly data across the northern Hikurangi subduction zone, New Zealand, to constrain plate boundary structure and geometry of a ...subducting seamount in a region of shallow slow slip and recent International Ocean Discovery Program drilling. Our 3‐D model reveals the subducting seamount as a SW‐NE striking, lozenge‐shaped ridge approximately 40 km long and 15 km wide, with relief up to 2.5 km. This seamount broadly correlates with a 20‐km‐wide gap separating two patches of large (>10 cm) slow slip and the locus of tectonic tremor associated with the September–October 2014 Gisborne slow slip event. Largest slow slip magnitudes occurred where the décollement is underlain by a 3.0‐km‐thick zone of highly reflective subducting sediments. Wave speeds within this zone are 7% lower than adjacent and overlying strata, supporting the view that high fluid pressures within subducting sediments may facilitate shallow slow slip along the north Hikurangi margin.
Plain Language Summary
Using a suite of geophysical data from the northern Hikurangi margin, New Zealand, we determine the location and geometry of a subducting seamount on the subducting Pacific Plate and establish its spatial relationship with slow slip and tremor that occurred on the plate boundary in September–October 2014. We infer that slow slip appears to occur preferentially where there are sediments with high fluid pressure in pore fluids subducting adjacent to the seamount but is reduced above the seamount itself. This observation has implications for understanding what physical conditions contribute to spatial variation in frictional properties of the plate interface that may control fault slip behavior on large, plate boundary subduction thrusts.
Key Points
Seismic images reveal Hikurangi margin accretionary wedge architecture and seismic velocity distribution
Magnetic anomaly modeling shows seismic tremor focused on the landward flanks and downdip of subducting seamounts
Structural heterogeneity of the plate interface may influence the distribution of slow slip and tremor
Three depth‐converted and geologically interpreted seismic profiles provide a clear image of the offshore outer accretionary wedge associated with oblique subduction of the Pacific Plate beneath the ...central Hikurangi margin. Plio‐Quaternary turbidites deposited over the pelagic cover sequence of the Hikurangi Plateau have been accreted to the margin by imbrication along E‐verging thrust faults that propagated up‐section from the plate boundary décollement. Growth stratigraphy of piggy‐back basins and thrusting of progressively younger horizons trace the eastward advance of the leading thrust front over ∼60 km in the last 2 Myr. Moderate internal shortening of fault‐bounded blocks typically 4–8 km wide reflects rapid creation of thrust faults, with some early formed faults undergoing out‐of‐sequence reactivation to maintain critical wedge taper. Multistage structural restorations show that forward progression of shortening involves: (1) initial development of a ∼10–25 km wide “proto‐thrust” zone, comprising conjugate sets of moderately to steeply dipping low‐displacement (∼10–100 m) reverse faults; and (2) growth of thrust faults that exploit some of the early proto‐thrust faults and propagate up‐section with progressive break‐through of folds localized above the fault tips. The youngest, still unbreached folds deform the present‐day seafloor. Progressive retro‐deformations show that macroscopic thrust faults and folds account for less than 50% of the margin‐perpendicular shortening imposed by plate convergence. Arguably, significant fractions of the missing components can be attributed to mesoscopic and microscopic scale layer‐parallel shortening within the wedge, in the proto‐thrust zones, and in the outer décollement zone.
Key Points
Structural restorations reveal widening of the Hikurangi accretionary wedge by 60 km since 2 (± 0.8) Ma
The low‐taper wedge geometry is consistent with a weak and forward propagating megathrust décollement
Macroscopic thrusting and folding accommodate less than 50% of the total shortening
We use seismic reflection data to map the geometry and character of the subduction interface in the Gisborne area of the Hikurangi subduction margin, New Zealand, which experiences repeated shallow ...(<15 km) slow slip events. The reflection character and geometry in this area is highly variable, which we interpret to be related to the subduction of seamounts and underthrust sediments. Three zones of high-amplitude interface reflectivity (HRZ-1, 2 and 3) are interpreted to be the result of fluid-rich sediments that have been entrained with subducting seamounts. The interface above the HRZ zones is shallower than the surrounding areas by 2–4 km, due to the warping of the interface to accommodate seamount subduction. These zones of high-amplitude reflectivity and shallower interface geometry correlate broadly with locations of recorded slow slip events from 2002 to 2008. We hypothesize that effective stresses on the interface may be lower along the northeast margin in areas of high-amplitude reflectivity due to; (1) the enhanced underthrusting of fluid-rich sediment, (2) reduced overburden stresses where the interface has been warped to shallower depths to accommodate seamount subduction and (3) potential fluid flow concentration effects leading to overpressure along these shallower interface corrugations. From our observations we propose localized reductions in effective stress caused by interface structural relief may be a potential factor in promoting shallow slow slip events.
We use 2800 line km of seismic reflection data to map the offshore character and geometry of the Hikurangi subduction thrust and outer forearc wedge to depths of ∼15 km. For 200 km along‐strike south ...of Hawke Bay, the subduction thrust is relatively smooth, dips less than 8°, and the wedge is characterized by accretion of young sediment and topographic slopes of less than 3°. In Hawke Bay and north for 200 km, a kink in the subduction thrust is apparent, with a downdip increase in dip to angles greater than 8° at depths of 10–15 km; there is a corresponding steepening of the topographic slope to greater than 3° outboard of the kink and the wedge is characterized by lithified sedimentary rock and slope failure. The kink in the subduction thrust is a locus of inherent weakness in the subducting slab; we suggest its occurrence relates to a northward increase in subduction rate that controls initial slab dehydration and fluid release rates and hence intraslab deformation patterns. The subduction thrust geometry, in combination with a northward increase in subducting plate roughness and decrease in the amount of sediment accreted, causes the observed spatial change in character of the subduction thrust and forearc wedge.
Relationships between extensional tectonics and magmatism are ubiquitous in continental rifts and oceanic spreading centers. Yet few studies document interactions between extensional faults and ...mantle melts in volcanic arcs. We constrain the crustal structure of the extensional offshore Taupo Volcanic Zone (TVZ) from a marine multichannel and wide‐angle seismic experiment. The TVZ crust thins from >26 km to ∼18–19 km across ∼50 km in the Bay of Plenty. Elevated P wave velocities in the lower crust indicate mafic additions. Magmatic sills between 4‐ and 15‐km depth lie beneath listric normal faults in a ∼40‐km‐wide active rift zone. P wave velocities in the middle and upper crust along the arc front are ∼0.3–0.5 km/s slower than in the adjacent crust, indicating a possible thermal anomaly imparted by heat from magmatic intrusions. We propose that rifting in the offshore TVZ is partially compensated by intrusions and assisted by thermal weakening of the lithosphere.
Plain Language Summary
The offshore Taupo Volcanic Zone is a rifting volcanic arc located in the North Island of New Zealand. We use seismic waves from man‐made sources to map seismic wave speeds and image fault and magma‐related structures beneath the offshore Taupo Volcanic Zone. We find that rifting has stretched the crust from >26 km to ∼18–19 km. Fast seismic wave speeds in the lower crust suggest that magma has partially filled some of the stretched crust to reduce the amount of thinning. Using seismic reflections, we find that the zone with the largest faults coincides with an area of intruded magma located between 4‐ and 15‐km depth. We propose that a feedback relationship influences the locations of faults and magma. Extension helps magma to rise from deep in the Earth. In response, the intruded magma heats and weakens the rigid outer layer of the earth to assist more rifting. The hazard potential of the fault and magma systems in this region is not fully known, motivating the need for further investigations to map and monitor magma bodies and faults in the offshore Taupo Volcanic Zone.
Key Points
Magmatic intrusions and thermal weakening may facilitate rifting in the offshore Taupo Volcanic Zone
Bay of Plenty crust rifted from >26 km to ∼18–19 km across a 50‐km‐wide zone; magmatic additions compensate ∼20–30% of crustal extension
Listric faulting overlies an ∼40‐km‐wide zone of sill complexes and heterogeneous P wave velocities in the upper and middle crust
Full‐waveform inversion (FWI) can resolve subsurface physical properties to high resolutions, yet high‐performance computing resources have only recently made it practical to invert for high ...frequencies. A benefit of high‐frequency FWI is that recovered velocity models can be differentiated in space to produce high‐quality depth images (FWI images) of a comparable resolution to conventional reflection images.
Here, we demonstrate the generation of high‐fidelity reflection images directly from the FWI process. We applied FWI up to 38 Hz to seismic data across the Hikurangi subduction margin. The resulting velocity models and FWI images reveal a complex faulting system, sediment deformation, and bottom‐simulating reflectors within the shallow accretionary prism. Our FWI images agree with conventional reflection images and better resolve horizons around the Pāpaku thrust fault. Thus, FWI imaging has the potential to replace conventional reflection imaging whilst also providing physical property models that assist geological interpretations.
Plain Language Summary
Seismic reflection imaging has been used for decades by industry and academia to provide high‐resolution images of geological structures below the surface of the Earth. Over the last 5–10 years, the petroleum industry has increasingly used full‐waveform inversion (FWI) to recover well‐resolved seismic velocity models, which they then use to improve their reflection images.
Here, we show that FWI can also produce images of comparable quality to conventional reflection images if higher frequencies are included in the inversion. Until now, limits on computational power have made inverting for higher frequencies prohibitively expensive. We demonstrate the potential of high‐frequency FWI in an application to seismic data across the shallow Hikurangi subduction margin and produce clear images of sedimentary layers and faults in the accretionary prism.
Key Points
Full‐waveform inversion (FWI) using high‐frequency data produces high‐resolution velocity models that can be directly utilized to produce high‐fidelity depth images
FWI images are comparable to conventional reflection images and provide high‐resolution physical property models to assist interpretation
As FWI is applied to raw data, there are few subjective processing steps, making images easily reproducible for time‐lapse imaging
The northern Hikurangi plate boundary fault hosts a range of seismic behaviors, of which the physical mechanisms controlling seismicity are poorly understood, but often related to high pore fluid ...pressures and conditionally stable frictional conditions. Using 2‐D marine seismic streamer data, we employ full‐waveform inversion (FWI) to obtain a high‐resolution 2‐D P wave velocity model across the Hikurangi margin down to depths of ~2 km. The validity of the FWI velocity model is investigated through comparison with the prestack depth‐migrated seismic reflection image, sonic well data, and the match between observed and synthetic waveforms. Our model reveals the shallow structure of the overriding plate, including the fault plumbing system above the zone of slow‐slip events to theoretical resolution of a half seismic wavelength. We find that the hanging walls of thrust faults often have substantially higher velocities than footwalls, consistent with higher compaction. In some cases, intrawedge faults identified from reflection data are associated with low‐velocity anomalies, which may suggest that they are high‐porosity zones acting as conduits for fluid flow. The continuity of velocity structure away from International Ocean Discovery Program drill site U1520 suggests that lithological variations in the incoming sedimentary stratigraphy observed at this site continue to the deformation front and are likely important in controlling seismic behavior. This investigation provides a high‐resolution insight into the shallow parts of subduction zones, which shows promise for the extension of modeling to 3‐D using a recently acquired, longer‐offset, seismic data set.
Key Points
Full‐waveform inversion was used to produce a high‐resolution P wave velocity model of the upper ~2 km of the north Hikurangi subduction zone
Recovered velocities and gradients correlate well with drilled sonic logs, suggesting that the details we see in our FWI velocity models are real
Velocity changes across faults may relate to compaction, and in some cases low velocities along faults may indicate conduits for fluid flow
The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of ...the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrust's seismogenic zone.
Recurring slow slip along near-trench megathrust faults occurs at many subduction zones, but for unknown reasons, this process is not universal. Fluid overpressures are implicated in encouraging slow ...slip; however, links between slow slip, fluid content, and hydrogeology remain poorly known in natural systems. Three-dimensional seismic imaging and ocean drilling at the Hikurangi margin reveal a widespread and previously unknown fluid reservoir within the extensively hydrated (up to 47 vol % H
2
O) volcanic upper crust of the subducting Hikurangi Plateau large igneous province. This ~1.5 km thick volcaniclastic upper crust readily dewaters with subduction but retains half of its fluid content upon reaching regions with well-characterized slow slip. We suggest that volcaniclastic-rich upper crust at volcanic plateaus and seamounts is a major source of water that contributes to the fluid budget in subduction zones and may drive fluid overpressures along the megathrust that give rise to frequent shallow slow slip.
Clay rich volcanic materials supply voluminous water to a subduction zone that hosts recurring slow slip.
Exploring 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.
Plain Language Summary
The northern Hikurangi margin is a subduction zone that generates hazardous earthquakes and tsunami, as well as months‐long slow slip events. Here, we use a controlled seismic energy source to image and map the seismic wavespeed of the overthrusting plate, downgoing plate—which is called the Hikurangi Plateau—and the megathrust fault that generates earthquakes. Thick layers of volcanic sediments and lavas on the Hikurangi Plateau produce bright reflections beneath the megathrust fault. Volcanic mountains on the subducting plate, called seamounts, are shown to influence the volume of sediment that scrapes off the downgoing plate and complicate the way that thrust faults form in the overthrusting plate. An increase in seismic velocity and change in fault geometry in the overthrusting plate indicates that the upper plate can be divided into a weak outer prism of young off‐scraped sediments and a stronger inner prism of older, deformed sediments. However, the strength of the interior of the overthrusting plate is still relatively weak. A weak overthrusting plate with slow seismic wavespeeds is believed to promote larger than predicted tsunami and excessive shaking during earthquakes.
Key Points
Sharp along‐strike contrasts in frontal accretion indicate variable sediment supply and past subduction of topography
We identify a deformed inner prism with elevated seismic velocity and an accreted frontal prism divided by a network of thrust faults
Low P‐wave velocities in the overthrusting plate near Gisborne may contribute to tsunamigenesis and enhanced ground motion duration
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