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The westernmost Nankai Trough, southwest Japan, exhibits a rapid along-strike reduction in plate coupling in the proximity to the subducting Kyushu-Palau ridge. Yet how and to what extent the ridge subduction impacts physical properties at the megathrust have not been investigated. Here we present high-resolution seismic P-wave velocity models along the forearc wedge in the western Nankai Trough derived from full-waveform inversion analyses of seismic refraction data. The velocity models show that where the plate coupling is weak and the plate boundary presumably hosts slow earthquakes, the upper plate exhibits lower seismic velocities indicating higher degree of fracturing over a ~ 100 km length along trough. Intriguingly, the extent of the upper-plate low-velocity features is significantly larger than the surficial width of the Kyushu-Palau ridge, and this low-velocity zone is underthrust by the slab with increased crustal thickness by 2–4 km. Seismic reflection images consistently reveal that the thicker slab crust has appreciable basement roughness extending ~ 60 km from the eastern margin of the Kyushu-Palau ridge beneath the western Shikoku basin. We suggest that such a thicker and rugged slab crust, together with the main body of the Kyushu-Palau ridge, can cause significant fracture zones in the overriding plate, decrease the interplate coupling and produce preferable conditions for shallow slow earthquakes to occur when subducted. The results may also provide structural constraints on the western limit of future megathrust earthquakes in the Nankai Trough. [ABSTRACT FROM AUTHOR]

Marine multichannel and wide‐angle seismic data constrain the distribution of seamounts, sediment cover sequence and crustal structure along a 460 km margin‐parallel transect of the Hikurangi Plateau. Seismic reflection data reveals five seamount up‐to 4.5 km high and 35–75 km wide, with heterogeneous internal velocity structure. Sediment cover decreases south‐to‐north from ∼4.5 km to ∼1–2 km. The Hikurangi Plateau crust (VP 5.5–7.5 km/s) is 11 ± 1 km thick in the south, but thins by 3–4 km further north (∼7–8 km). Gravity models constructed along two seismic lines show the reduction in crustal thickness persists further east, coinciding with a bathymetric scarp. Gravity data suggest the transition in crustal thickness may reflect spatial variability in deformation and lithospheric extension associated with plateau breakup. Variability in the thickness of subducting crust may contribute to differences in megathrust geometry, upper‐plate stress state and high‐rates of contraction and uplift along the southern Hikurangi margin. Plain Language Summary: The thickness of crust arriving at subduction zones exerts a major influence on the configuration, structure, and distribution of stresses at plate boundaries. A region of thickened oceanic crust, the Hikurangi Plateau, is subducting beneath the North Island of New Zealand. To understand the structure of this Plateau, we analyze seismic data along a 460 km long transect, which reveals five large seamounts, and a 3–4 km reduction crustal thickness between the southern and northern areas of the Plateau. Gravity data show this difference is persistent across the plateau and reveal structures suggesting the crust may have thinned when the Hikurangi plateau separated from a second plateau (Manihiki), which is thought to have formed at the same time. We propose that the subduction of thicker crustal along the southern Hikurangi margin may contribute to the shallow angle of the subducting plate, and high rates of uplift and deformation in the overthrusting plate. Key Points: Crustal thickness of the Hikurangi Plateau reduces by ∼30% between the southern (10–11 km) and northern (7–8 km) PlateauCoincident contrasts in plateau fabric may attribute the thickness contrast to rifting from the Manihiki PlateauThickness contrasts may impact the geometry of the subducting plate and rates of upper‐plate compression and uplift along the Hikurangi margin [ABSTRACT FROM AUTHOR]

Marine multichannel and wide‐angle seismic data constrain crustal structure along a 530 km margin‐parallel transect of the Hikurangi subduction zone. The subducting Hikurangi Plateau crust (VP 5.0–7.4 km/s) is ∼1 km thicker (11 ± 1 km) and mantle velocities are ∼0.2 km/s higher (VP 8.3–8.5 km/s) beneath south/central Hikurangi relative to north Hikurangi. In the overthrusting plate, an abrupt 0.5 km/s south‐to‐north reduction in forearc wavespeeds occurs in concert with a change in seismic reflection character. We analyze legacy seismic data to show that the forearc transition likely reflects lateral variability in the updip extent of the Torlesse Backstop. Furthermore, we map this unit along‐strike and note a broad correlation between the backstop and down‐dip extent of shallow slow‐slip. We propose that the geological architecture of the overthrusting plate contributes to spatial variability in the location of shallow frictional transitions along the Hikurangi margin, impacting both seismic and tsunami hazard. Plain Language Summary: Some subduction zones produce the largest earthquakes and tsunami on Earth, while others slip freely. To understand what factors impact subduction zone slip behavior, we analyze seismic data along a 530 km long transect spanning a transition from strong (south Hikurangi) to weak (central/north Hikurangi) interseismic locking. From south‐to‐north, we find that seismic wavespeeds in the overthrusting plate undergo an abrupt (∼10%) reduction, which coincides with a reduction in seismic reflectivity. We show that these changes likely reflect differences in the offshore (updip) extent of basement rocks within the forearc crust, which we map using seismic data. These maps also show that the offshore extent of basement rocks is broadly correlated with the maximum depth of shallow slow‐slip events. We propose that geological architecture of the overthrusting plate may contribute to spatial variability in megathrust slip‐behavior, impacting both seismic and tsunami hazard along the Hikurangi margin. Key Points: Coincident reductions in seismic wavespeeds and lower‐crustal reflectivity occur from south‐to‐north along the Hikurangi forearcLegacy seismic reflection data attribute the forearc transition to along‐strike differences in the updip extent of the Torlesse BackstopShallow slow‐slip is focused updip of the backstop, suggesting it may impact the location of shallow frictional transitions at Hikurangi [ABSTRACT FROM AUTHOR]

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 topographyWe identify a deformed inner prism with elevated seismic velocity and an accreted frontal prism divided by a network of thrust faultsLow P‐wave velocities in the overthrusting plate near Gisborne may contribute to tsunamigenesis and enhanced ground motion duration [ABSTRACT FROM AUTHOR]

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 distributionMagnetic anomaly modeling shows seismic tremor focused on the landward flanks and downdip of subducting seamountsStructural heterogeneity of the plate interface may influence the distribution of slow slip and tremor [ABSTRACT FROM AUTHOR]

• Hikurangi Plateau crust at the southern Hikurangi margin is ≈12 km thick. • A fast-upper mantle resides at a depth of ≈50 km beneath Hikurangi trough. • Crustal and upper mantle P-wave-speeds reduced by ≈10% beneath the Hikurangi trough. • Hikurangi Plateau is hydrated to depths of ≈50 km beneath the Hikurangi trough. Controlled-source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave-speeds in the crust and upper mantle near the trench. Similar studies also have found unusually high P-wave-speeds (V p) in the upper mantle under oceanic plateaus. Onshore-offshore seismic profiling at the southern Hikurangi margin, where the ≈120 Ma old oceanic Hikurangi Plateau is subducting, indicates that a fast (V p ≈8.7±0.2 km/s) upper mantle layer lies beneath a ≈25 km thick mantle layer with more regular wave-speeds (V p ≈8.0±0.2 km/s) under the Hikurangi trough. This is consistent with previous findings of upper mantle V p ≈8.7-9.0 km/s in the margin-parallel direction under the North Island (≈100 km northwest of the deformation front) at depths ≈8-10 km below the Moho. Our profiles are margin-perpendicular, thus we show that the upper mantle lid of the subducting Pacific Plate is characterized by unusually high P-wave-speeds along all azimuths. We find an area of lowered V p in the ≈12±1 km thick Hikurangi Plateau crust beneath the trough. This drop in V p is ≈10%, and a similar drop in V p is deduced to depths of 25±2 km into the upper mantle. We interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low-velocity zone in the faster upper mantle layer; this zone formed from serpentinisation by hydration through bending-induced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. Thus the "regular mantle" (V p ≈8 km/s) is not in fact regular, but rather the high-speed mantle has mechanically bent, fractured, and altered. The absolute depth of fast mantle V p under the Hikurangi trough is around 50 km. The onset of the lower band of seismicity of the double seismic zone and high upper mantle V p under the North Island is observed at similar depths. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough. Our study on the Hikurangi margin is different, as the subducting plate here contains a ≈120 Ma old oceanic plateau with a ≈12 km thick crust, but the results are similar to other subduction margins where regular oceanic crust is subducting. [ABSTRACT FROM AUTHOR]