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We 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 >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. Plain Language Summary: We use man‐made sound waves (controlled‐source seismic data) to reveal in three dimensions the speed of sound within rocks and sediments that make up the northern Hikurangi margin off the east coast of North Island, New Zealand. We find that the sediments lying on top of subducting plate (accretionary wedge) have low velocities and host several branching faults. We also find that the speed of sound is faster when measured in the direction that the plates are colliding and slower when measured parallel to the faults (this phenomenon is called seismic anisotropy). This difference in speed suggests that the faults contain cracks that are aligned parallel to the direction of the plate subduction. Key Points: We present a three‐dimensional anisotropic P wave velocity model in northern Hikurangi from traveltime tomography using NZ3D OBS dataWe find evidence for fault‐bound anisotropy that azimuthal anisotropy is larger in the vicinity of the splay faults and deformation frontAnisotropy magnitude >5% may indicate preferentially oriented cracks and along‐fault clay‐rich layers contribute to upper plate anisotropy [ABSTRACT FROM AUTHOR]

To assess the risk of tsunamis from outer‐rise earthquakes, we carried out tsunami simulations using 33 simple rectangular fault models with 60° dip angles based on marine seismic observations and surveys of the Japan Trench. The largest tsunami resulting from these models, produced by a Mw 8.7 normal‐faulting event on a fault 332 km long, had a maximum height of 27.0 m. We tested variations of the predictions due to the uncertainties in the assumed parameters. Because the actual dip angles of the Japan Trench outer‐rise faults range from 45° to 75°, we calculated tsunamis from earthquakes on fault models with 45°, 60°, and 75° dip angles. We also tested a compound fault model with 75° dip in the upper half and 45° dip in the lower half. Rake angles were varied by ±15°. We also tested models consisting of small subfaults with dimensions of about 60 km, models using other earthquake scaling laws, models with heterogeneous slips, and models incorporating dispersive tsunami effects. Predicted tsunami heights changed by 10–15% for heterogeneous slips, up to 10% for varying dip angles, about 5–10% from considering tsunami dispersion, about 2% from varying rake angles, and about 1% from using the model with small subfaults. The use of different earthquake scaling laws changed predicted tsunami heights by about 50% on average for the 33 fault models. We emphasize that the earthquake scaling law used in tsunami predictions for outer‐rise earthquakes should be chosen with great care. Key Points: Tsunamis were accurately predicted using well‐mapped outer‐rise normal faults along the Japan TrenchIt is necessary to carefully select the earthquake scaling law when predicting tsunamis from outer‐rise earthquakes.Heterogeneous slips have a large impact on tsunami predictions for outer‐rise earthquakes. [ABSTRACT FROM AUTHOR]

During the Late Cretaceous, the fragmentation of eastern Gondwana led to the formation of the narrow eastern Australian margin and the wide Lord Howe Rise during the opening of the oceanic Tasman Basin. To provide crustal‐scale constraints on this margin, a 680‐km‐long, east‐west oriented refraction transect with 100 ocean bottom seismometers was acquired from the Tasman Basin to the Lord Howe Rise. After traveltime tomographic inversion of the first refracted arrivals and reflected arrivals from the Moho, the final P wave velocity model reveals strong variations in crustal thickness. The Tasman Basin is floored by a two‐layered and 7‐km‐thick oceanic crust. To the east, the Middleton Basin separates the Dampier Ridge, with 16‐km‐thick continental crust, from the Lord Howe Rise, where the extended continental crust is 20–23 km thick. Below the Middleton Basin, Moho reflections are recorded at the base of 7‐km‐thick crust. The velocity gradient of this two‐layer crust suggests an oceanic origin for the Middleton Basin. Our results show no clear evidence for mantle exhumation or a sizable igneous intrusion within three separate and relatively narrow (<70 km) necking zones. The northwestern Zealandia margin thus appears to be magma poor and, despite the considerable width of this margin (>1,000 km), the lack of evidence for mantle exhumation, the evidence for oceanic crust under the Middleton Basin, and the narrow necking zones combine to suggest that northern Zealandia is not a hyperextended margin. Key Points: The Lord Howe Rise and Dampier Ridge are thinned Zealandia continental fragments separated by oceanic crust in the Middleton BasinThere is no evidence for mantle exhumation or high‐velocity igneous intrusive bodies during rifting and breakup of eastern GondwanaDespite the width and magma‐poor nature of northwestern Zealandia, it lacks other typical features of a hyperextended continental margin [ABSTRACT FROM AUTHOR]

Continental rifting and breakup of eastern Gondwana during the Cretaceous separated northern Zealandia from eastern Australia, but the processes leading to this highly extended and largely submerged block of continental crust are unknown. We acquired and processed multichannel seismic reflection data across northern Zealandia and examine the stratigraphy of the Middleton Basin. We identified a two‐phase formation process for the basin, as evidenced by an unconformity separating two postrift units. After initial basin formation and slow deposition of the lower postrift unit, deposition rates within the Middleton Basin rapidly increased in response to the latest stage of subsidence and to create the modern basin. We propose a tectonic model wherein the Middleton Basin initiated through oceanic spreading and the subsequent postrift subsidence of the newly created oceanic lithosphere was delayed due to thermal buoyancy associated with nearby oceanic spreading in the Tasman Basin. Our results provide new constraints on rifting and breakup processes of wide, magma‐poor, and asymmetric margins and indicate that multiple regions of weak lithosphere may have influenced the breakup. Plain Language Summary: Tens of millions of years ago, an elongate continental fragment now known as Zealandia broke away from eastern Australia. Because most of Zealandia is underwater in a remote part of the southwest Pacific, little is known about the processes that led to its submergence and isolation. We have used a common subsurface imaging technique based on sound waves bouncing off deep rock layers to map sediments below the seafloor that record the geological events leading to the formation of Zealandia. Our focus is on the Middleton Basin, an elongate trough between two continental blocks that has a uniquely flat seafloor and is filled with up to 3.5 km of sediments. Our results suggest that the geological history of the Middleton Basin initially involved formation of a new, narrow ocean between two continents—like the Red Sea today. Ocean formation then jumped further to the west, ultimately resulting in the wide and deep Tasman Sea. This stop‐start formation of two new oceans during the fragmentation of a continent is uncommon and highlights Zealandia's role in revealing the intriguing geological history of Earth's continents. Key Points: Thick postrift sediments covering the oceanic Middleton Basin are divided by an unconformity indicative of delayed subsidenceThis delayed subsidence resulted from dynamic support linked to hot mantle associated with later spreading in the adjacent Tasman BasinThis dual ocean basin formation suggests that multiple zones of lithospheric weakness affected the breakup of the eastern Gondwana margin [ABSTRACT FROM AUTHOR]