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The Hawaiian Ridge, a classic intraplate volcanic chain in the Central Pacific Ocean, has long attracted researchers due to its origin, eruption patterns, and impact on lithospheric deformation. Thought to arise from pressure‐release melting within a mantle plume, its mass‐induced deformation of Earth's surface depends on load distribution and lithospheric properties, including elastic thickness (T_e). To investigate these features, a marine geophysical campaign was carried out across the Hawaiian Ridge in 2018. Westward of the island of O'ahu, a seismic tomographic image, validated by gravity data, reveals a large mass of volcanic material emplaced on the oceanic crust, flanked by an apron of volcaniclastic material filling the moat created by plate flexure. The ridge adds ∼7 km of material to pre‐existing ∼6‐km‐thick oceanic crust. A high‐velocity and high‐density core resides within the volcanic edifice, draped by alternating lava flows and mass wasting material. Beneath the edifice, upper mantle velocities are slightly higher than that of the surrounding mantle, and there is no evidence of extensive magmatic underplating of the crust. There is ∼3.5 km of downward deflection of the sediment‐crust and crust‐mantle boundaries due to flexure in response to the volcanic load. At Ka'ena Ridge, the volcanic edifice's height and cross‐sectional area are no more than half as large as those determined at Hawai'i Island. Together, these studies confirm that volcanic loads to the west of Hawai'i are largely compensated by flexure. Comparisons to the Emperor Seamount Chain confirm the Hawaiian Ridge's relatively stronger lithospheric rigidity.

During 1975–1988, an academic research ship, R/VRobert D.Conrad, acquired more than 150,000‐line‐km of multichannel seismic reflection profile data from each of the world's main ocean basins and their margins. This extensive legacy seismic data set, which involved both single ship and two‐ship data acquisition, has been widely used by the marine geoscience community. We report on our experience in reprocessing seismic reflection profile data acquired duringConradcruise RC2308 to the Hawaiian Islands region in August/September 1982. We show that the application of modern, industry standard processing techniques, including filtering, de‐bubble, deconvolution, and migration, can significantly enhance 40+ year old legacy seismic reflection profile data. The reprocessed data reveals more precisely, and with much less scatter, the flexure of Cretaceous Pacific oceanic crust caused by the Pliocene‐Recent volcanic loads that comprise the Hawaiian Islands. A comparison of observed picks of top oceanic crust which has been corrected for the Hawaiian swell and the Molokai Fracture Zone with the calculations of a simple 3‐dimensional elastic plate (flexure) model reveals a best fit elastic plate thickness of the lithosphere,T_e, of 26.7 km, an average infill density of 2,701 kg m⁻³, and a Root Mean Square difference between observations and calculations of 305 m. Tests show these results depend weakly on the load density assumed and that the average infill density is close to what would be predicted from an arithmetic average of the flanking moat infill density and the infill density that immediately underlies the volcanic edifice.

The intraplate Hawaiian‐Emperor Seamount Chain has long been considered a hotspot track generated by the motion of the Pacific plate over a deep mantle plume, and an ideal feature therefore for studies of volcanic structure, magma supply, plume‐crust interaction, flexural loading, and upper mantle rheology. Despite their importance as a major component of the chain, the Emperor Seamounts have been relatively little studied. In this paper, we present the results of an active‐source wide‐angle reflection and refraction experiment conducted along an ocean‐bottom‐seismograph (OBS) line oriented perpendicular to the seamount chain, crossing Jimmu guyot. The tomographicPwave velocity model, using ∼20,000 travel times from 26 OBSs, suggests that there is a high‐velocity (>6.0 km/s) intrusive core within the edifice, and the extrusive‐to‐intrusive ratio is estimated to be ∼2.5, indicating that Jimmu was built mainly by extrusive processes. The total volume for magmatic material above the top of the oceanic crust is ∼5.3 × 10⁴ km³, and the related volume flux is ∼0.96 m³/s during the formation of Jimmu. Under volcanic loading, the ∼5.3‐km‐thick oceanic crust is depressed by ∼3.8 km over a broad region. Using the standard relationships between Vpand density, the velocity model is verified by gravity modeling, and plate flexure modeling indicates an effective elastic thickness (T_e) of ∼14 km. Finally, we find no evidence for large‐scale magmatic underplating beneath the pre‐existing crust.

Accreting neutron stars (NS) can exhibit high frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. These frequencies can be offset from the NS spin frequency by several Hz (where known independently) and can drift by 1–3 Hz. One plausible explanation is that a wave is present in the bursting ocean, the rotating frame frequency of which is the offset. The frequency of the wave should decrease (in the rotating frame) as the burst cools hence explaining the drift. A strong candidate is a buoyant r-mode. To date, models that calculated the frequency of this mode taking into account the radial structure neglected relativistic effects and predicted rotating frame frequencies of ∼4 Hz and frequency drifts of >5 Hz; too large to be consistent with observations. We present a calculation that includes frame-dragging and gravitational redshift that reduces the rotating frame frequency by up to $30 \, {\rm per\, cent}$ and frequency drift by up to $20 \, {\rm per\, cent}$. Updating previous models for the ocean cooling in the aftermath of the burst to a model more representative of detailed calculations of thermonuclear X-ray bursts reduces the frequency of the mode still further. This model, combined with relativistic effects, can reduce the rotating frequency of the mode to ∼2 Hz and frequency drift to ∼2 Hz, which is closer to the observed values.

The Hawaiian‐Emperor seamount chain in the Pacific Ocean has provided fundamental insights into hotspot generated intraplate volcanism and the long‐term strength of oceanic lithosphere. However, only a few seismic experiments to determine crustal and upper mantle structure have been carried out on the Hawaiian Ridge, and no deep imaging has ever been carried out along the Emperor seamounts. Here, we present the results of an active source seismic experiment using 29 Ocean‐Bottom Seismometers (OBS) carried out along a strike profile of the seamounts in the region of Jimmu and Suiko guyots. Joint reflection and refraction tomographic inversion of the OBS data show the upper crust is highly heterogeneous withPwave velocities <4–5 km s⁻¹, which are attributed to extrusive lavas and clastics. In contrast, the lower crust is remarkably homogeneous with velocities of 6.5–7.2 km s⁻¹, which we attribute to oceanic crust and mafic intrusions. Moho is identified by a strongPmParrival at offsets of 20–80 km, yielding depths of 13–16 km. The underlying mantle is generally homogeneous with velocities in the range 7.9–8.0 km s⁻¹. The crust and mantle velocity structure has been verified by gravity modeling. While top of oceanic crust prior to volcano loading is not recognized as a seismic or gravity discontinuity, flexural modeling reveals a ∼5.0–5.5 km thick preexisting oceanic crust that is overlain by a ∼8 km thick volcanic edifice. Unlike at the Hawaiian Ridge, we find no evidence of magmatic underplating.

Earth's surface topography/bathymetry and gravity fields provide important constraints on crustal structure and the tectonic processes that act on it due, for example, to plate flexure and mantle convection. Such studies require, however, high accuracy measurements at a wide range of spatial scales. During the past few decades much progress has been made in the acquisition of bathymetry and gravity data using both shipboard and satellite altimeter methods. Surprisingly, there have been few comparisons of these data. During April–June, 2019 we had the opportunity onboard a R/VMarcus G. Langsethcruise in the northwest Pacific Ocean to compare data acquired with an EM122 Kongsberg swath bathymetry system and a refurbished Bell Aerospace BGM‐3 gravimeter with the most recent global bathymetry and gravity fields. We find that while the recovery of bathymetry and gravity from satellite radar altimeter data in areas of sparse shipboard data has been impressive, root mean square discrepancies in the range 175.5–303.4 m and 2.6–6.3 mGal exist between shipboard and satellite‐derived data. While these discrepancies are small, they are highly correlated and therefore have implications for the density structure, rock type and geological processes occurring on the deep seafloor. Shipboard data should continue to be acquired, especially over features such as seamounts, banks, and ridges that are associated with short wavelength (<25 km wavelength) bathymetric and gravimetric features beyond that is recoverable in satellite‐derived data.

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