Some prior active-source seismic studies beneath oceanic seamount chains indicate a sub-crustal layer of what is considered to be underplated magmatic material. The classic example is the Hawaiian Ridge, where a seismic study in the early 1980s first proposed the existence of such a layer. Since then, Hawaii has been considered one end-member in a range of possible scenarios, extending from underplating to no underplating, but with possible magmatic intrusion into the lower oceanic crust. Magmatic underplating affects mass flux and lithospheric loading calculations. All else being equal, underplating would be expected to lower the amplitude of the gravity anomaly over the crest of the edifice and provide a positive buoyancy force that makes the plate appear more rigid than it actually is. One hypothesis put forward to explain variations in the style of magmatic emplacement at intraplate volcanoes hinges on the age of the lithosphere at the time of volcano formation. In this model, shallow intrusion into the oceanic crust and overlying edifice is favored for seamounts growing on younger lithosphere, while magmatic underplating is favored for seamounts growing on older lithosphere such as Hawaii. However, recent seismic, gravity, and plate flexure studies conducted along the Hawaiian-Emperor Seamount Chain collectively provide clear evidence for shallow magmatic emplacement and contradict the notion of significant magmatic underplating for ages at the time of loading of ~57 Ma (Emperor Seamounts) and ~90 Ma (Hawaiian Ridge). Additionally, reprocessed legacy seismic data has not revealed evidence for underplating beneath the Hawaiian Ridge. In this presentation, results from these recent studies will be shown, compared, and the evidence for a simple mantle structure without underplating will be presented along with seismic synthetic tests that explore the degree of underplating that may be 'hidden' in the data.

Since the introduction of the ocean bottom seismograph (OBS), it has played a pivotal role in the expansion of our comprehension of the Earth’s interior structure and dynamics. Typically deployed through free-fall from a ship, the position of the OBS on the seafloor and its orientation are initially unknown. When using airgun shots to locate and orient an instrument, a severe downside is the often-limited azimuthal distribution of shots around the OBS, possibly leading to large location and, subsequently, orientation errors. We introduce a novel and efficient method aimed at enhancing the accuracy of station locations in active-source seismic experiments. The approach uses airgun shots recorded as part of the experiment, and integrates both acoustic wave travel time information and waveform polarizations. The inverse problem of station location is formulated in terms of Bayesian inference. At each location of a search grid, the misfit of observed and theoretical water wave travel times and the clustering of polarization data are combined into one probabilistic formulation to map the relative likelihood of a station’s position and orientation. We demonstrate the practical utility of the method via application to the location and orientation of OBS deployed during a recent seismic experiment located across the Hawaiian Ridge. Thereafter, we will use the results to compute radial seismograms, extract the travel times of crustal and mantle S waves, and develop S-wave models of the oceanic lithosphere across the Hawaiian Ridge.

Volcanic seamount chains are widespread throughout ocean basins, but their formation and structure are not well understood. Active-source refraction seismology can provide images of the interiors of these volcanoes via P wave travel time tomography, but that requires proper identification of seismic phases that are generated by and propagate across complex volcanic structures. Unfortunately, the current limitation on the number of seismic phases that can be included in tomographic analyses leads to less detailed images and a limited geological understanding of the structures being imaged. The primary objective of this study is to compare recorded seismic wavefields from recent surveys across the Hawaiian-Emperor Seamount Chain with synthetic wavefields generated through waveform modeling. We use simplified yet realistic models of volcanic edifices, the underlying oceanic crust, and the mantle to better understand observed seismic phases and their origins. In previous studies (Dunn et al., 2019; Watts et al., 2021; Xu et al., 2022; MacGregor et al., 2023), several seismic phases were identified in recorded sections along the Hawaiian-Emperor Chain. However, interpreting the origins of some of these phases remains challenging. In this study, we developed an idealized seamount model based on the seismic structure of Jimmu Guyot in the Emperor Ridge (Xu et al., 2022). We calculated the seismic P-SV wavefield for various source and receiver positions using a finite difference wavefield modeling code (Levander, 1988; Lata and Dunn, 2020). Our goal is to model the observed seismograms and identify additional phases, including P-to-S converted waves. Additionally, we aim to verify recent tomographic images of the Hawaiian-Emperor Seamount Chain created by P wave travel time inversion via wavefield comparison. By resolving ambiguities and pinpointing new seismic phases, our aim is to improve seismic images of the Hawaiian-Emperor Chain. This contribution will enhance our understanding of specific structures, such as volcanic cores (a high-density and high-wave-speed interior core of the seamount), the hypothesized magmatic underplating of the oceanic crust by mantle melts rising beneath the volcanic chain, and the nature of the Moho and upper oceanic crust beneath these volcanic edifices.

Accurate seismic phase identification is critical for producing high-quality subsurface images in tomography. Each phase carries unique information about Earth’s structure, so improved understanding of wavefields and phase behavior can lead to better image resolution and reveal new structural features. Recent field studies highlight the complexity of wavefield propagation across submarine volcanoes, where strong lateral heterogeneity and sharp velocity contrasts generate a diverse set of poorly understood seismic phases. Here, we investigate seismic wave propagation through an idealized model of an intraplate volcano, representative velocity structure derived from a recent tomography study of Jimmu Guyot, part of the Hawaiian-Emperor Seamount Chain. We compute wavefields and synthetic record sections using finite difference methods to analyze how key structural features generate different seismic phases. Beyond the standard phases typically observed in oceanic lithosphere, the simulations reveal a broad range of P-wave and P-to-S converted phases generated by complex structures, including thick flexural-moat sediments, a stratified edifice with a high-velocity core, and the underlying oceanic crust-mantle transition. These results provide new insights into wave propagation through submarine volcanic structures, advise caution in phase identification, and highlight opportunities to incorporate additional phases into future tomography studies.

The Hawaii-Emperor seamount chain stretches westward from the “Big Island” of Hawaii for over 6000 km until the oldest part of the chain are subducted at the Kuril and Aleutian trenches. Still regarded as the iconic hotspot-generated seamount chain it has been sampled, mapped, and studied to give insights into numerous oceanic phenomena, including seamount and volcano formation and associated intraplate magma budgets, the past absolute motions of the Pacific plate, the drift of the Hawaiian plume, and the thermal and mechanical properties of oceanic lithosphere. Previous work (Wessel et al., EGU 2023 abstract) used a high-resolution free-air gravity anomaly and high-resolution bathymetry data set, together with fully 3-dimensional flexural models with variable volcano load and infill densities, to estimate the optimal effective elastic thickness, Te, and load and infill densities along the Emperor Seamount chain. Here, we use these parameters to calculate the tectonic tilt of a pre-existing volcano that occurs as each new volcano in a seamount chain is progressively added by flexure to the Pacific oceanic plate. We found tilts in the range 0.1-2.1 degrees which are modest compared to other cases of progressive flexure, for example, at seaward dipping reflector sequences in volcanic rifted margins (~5-15 degrees) but may be significant enough to modify the morphology of volcano summits and the stratigraphy of the sequences that accumulate in their flanking moats. They may also modify the physical properties of the edifice such as their magnetisation vectors. Wessel, P., A. B. Watts, B. Boston, C. Xu, R. Dunn, and D. J. Shillington “Variation in Elastic Thickness along the Emperor Seamount Chain”, EGU 2023 Abstract

Deep-sea hydrothermal systems at slow/ultraslow-spreading mid-ocean ridges are often located within ultramafic rocks that are part of oceanic core complexes. These complexes contain lower-crustal and mantle sections exhumed due to detachment faulting. Hydrothermal circulation in these environments leads to massive sulfide deposits, hydration of oceanic lithosphere and conditions resembling early Earth's life origin. However, the relationship between hydrothermal pathways in these environments and crustal and mantle lithologies, faulting, magmatism, serpentinization and alteration is poorly understood. Here we present seismic models of a Mid-Atlantic Ridge core complex and its ultramafic-hosted hydrothermal system derived from full waveform inversion of controlled-source seismic data and from local earthquake tomography. The models and derived rock properties reveal high-permeability channels within serpentinized peridotite along the flanks of the core complex. These channels converge beneath active and fossil hydrothermal fields and are diverted around mechanically strong, impermeable shallow mafic intrusions (2-3 km wide, similar to 1 km thick), causing hydrothermal outflow and the formation of massive sulfide deposits around the intrusions' edges. These mafic intrusions also act as lids that limit fluid downflow-and thus serpentinization-in the centre of the core complex. Our results demonstrate that hydrothermal flow in ultramafic settings is controlled by lithology contacts, with mafic intrusions modulating hydrothermal pathways and extent of mantle serpentinization at depth.

The architecture of magma plumbing systems plays a fundamental role in volcano eruption and evolution. However, the precise configuration of crustal magma reservoirs and conduits responsible for supplying eruptions are difficult to explore across most active volcanic systems. Consequently, our understanding of their correlation with eruption dynamics is limited. Axial Seamount is an active submarine volcano located along the Juan de Fuca Ridge, with known eruptions in 1998, 2011, and 2015. Here we present high-resolution images of P-wave velocity, attenuation, and estimates of temperature and partial melt beneath the summit of Axial Seamount, derived from multi-parameter full waveform inversion of a 2D multi-channel seismic line. Multiple magma reservoirs, including a newly discovered western magma reservoir, are identified in the upper crust, with the maximum melt fraction of ~15–32% in the upper main magma reservoir (MMR) and lower fractions of 10% to 26% in other satellite reservoirs. In addition, a feeding conduit below the MMR with a melt fraction of ~4–11% and a low-velocity throat beneath the eastern caldera wall connecting the MMR roof with eruptive fissures are imaged. These findings delineate an asymmetric shallow plumbing system beneath Axial Seamount, providing insights into the magma pathways that fed recent eruptions.

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 similar to 7 km of material to pre-existing similar to 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 similar to 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.

The Hawaiian Ridge has long been a focus site for studying lithospheric flexure due to intraplate volcano loading, but crucial load and flexure details remain unclear. We address this problem using wide‐angle seismic refraction and reflection data acquired along a ∼535‐km‐long profile that intersects the ridge between the islands of Maui and Hawai'i and crosses 80–95 Myr‐old lithosphere. A tomographic image constructed using travel time data of several seismic phases reveals broad flexure of Pacific oceanic crust extending up to ∼200–250 km either side of the Hawaiian Ridge, and vertically up to ∼6–7 km. The P ‐wave velocity structure, verified by gravity modeling, reveals that the west flank of Hawaii is comprised of extrusive lavas overlain by volcanoclastic sediments and a carbonate platform. In contrast, the Hāna Ridge, southeast of Maui, contains a high‐velocity core consistent with mafic or ultramafic intrusive rocks. Magmatic underplating along the seismic line is not evident. Reflectors at the top and bottom of the pre‐existing oceanic crust suggest a ∼4.5–6 km crustal thickness. Simple three‐dimensional flexure modeling with an elastic plate thickness, T e , of 26.7 km shows that the depths to the reflectors beneath the western flank of Hawai'i can be explained by volcano loading in which Maui and the older islands in the ridge contribute ∼43% to the flexure and the island of Hawai'i ∼51%. Previous studies, however, revealed a higher T e beneath the eastern flank of Hawai'i suggesting that isostatic compensation may not yet be complete at the youngest end of the ridge.

<p>The Hawaii-Emperor seamount chain stretches westward from the “Big Island” of Hawaii for over 6000 km until the oldest part of the Emperor chain is subducted at the Kuril and Aleutian trenches. Still regarded as <em>the</em> iconic hotspot-generated seamount chain it has been sampled, mapped, and studied to give insights into numerous oceanic phenomena, such as seamount and volcano formation and associated intraplate magma budgets, the past absolute motions of the Pacific plate and the drift of the Hawaiian plume, and the thermal and mechanical properties of oceanic lithosphere. Much early work on determining the flexural rigidity and equivalent elastic plate thickness that supports the large volcano loads that comprise the chain was focussed on the Hawaiian Ridge, with a major multichannel seismic expedition to the Hawaiian Islands in 1982 providing clear and direct evidence of plate flexure, as well as the indirect effect this deformation has on Earth’s gravity field. Numerous studies have since followed. However, the older part of the chain, beyond the ~50 Ma “bend”, has been much less well studied due to its remoteness, but recent expeditions have provided new marine seismic data to allow an estimation of elastic thickness along the Emperor chain and how they compare to the information we have along the Hawaiian Ridge. Here, we present preliminary work on determining the elastic thickness beneath the Emperor Seamounts. Unlike the Hawaiian Ridge, where the age of the lithosphere at the time of loading (i.e., the difference in age between the underlying seafloor and the formation age of a seamount or oceanic island) is remarkably constant, along the Emperor chain there are major variations in the age of loading, compounded by higher uncertainty due to limited seamount age sampling and the chain’s location within the Cretaceous Quiet Zone. Thus, models with variable elastic thickness as a function of location along the Emperor chain are required. In this presentation, we discuss several models that seek to account for the new seismic imaging of the top and base of flexed oceanic crust (i.e. Moho) at Jimmu guyot while at the same time honouring the characteristic gravimetric signature of the Emperor seamount edifices and their flanking moats. The Optimal Regional Separation (ORS) method is used to isolate the flexural loads, while seismic tomography and different velocity/density relations are explored for assigning suitable load and infill densities that vary spatially, and we search for optimal density and elastic parameters which minimize the misfit to both the residual gravity as well as the seismically observed flexure in the vicinity of Jimmu guyot. The first-order result is a clear thinning of the elastic thickness as we move from south to north: the implications of which we examine here for the tectonic evolution of the northwest Pacific Ocean and the long-term (>10<sup>6</sup> a) mechanical properties of oceanic lithosphere.</p>

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 tomographic P wave 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 Vp and density, the velocity model is verified by gravity modeling, and plate flexure modeling indicates an effective elastic thickness (Te) of ∼14 km. Finally, we find no evidence for large‐scale magmatic underplating beneath the pre‐existing crust.

Abstract Data from three co‐located wide‐angle ocean‐bottom seismograph experiments were used to make a new two‐dimensional tomographic image of the East Pacific Rise at 9°N. The upper half of the crustal magmatic system has a different seismic structure than the lower half, presumably corresponding with differences in melt storage and crustal cooling. The upper region is narrow (∼4.5 km wide at 2–3 km depth bsf) and estimated to contain higher melt fractions; the lower region broadens with depth and contains lower melt fractions. The shape of the system suggests steeply‐dipping isotherms and relatively stronger hydrothermal circulation in the upper gabbroic section of the crust, with moderately flattening isotherms and less circulation in the lower gabbroic section.

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 with P wave 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 strong PmP arrival 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/V Marcus G. Langseth cruise 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.

P and S wave data from the L-SCAN active-source wide-angle reflection/refraction experiment are modelled to investigate upper crustal structure in the Lau backarc basin. A combination of ray tracing and finite difference numerical wavefield simulation is used to identify P and P-to-S converted seismic phases. The phases primarily arise from two shallow interfaces, one at ~ 80 m depth or less, and the other at 500–650 m depth. The shallower interface is deeper than the sediment base, is observed across the study area, and is interpreted as a ‘layer 2Aa’ boundary, proposed to result from a rapid change in crack density. The deeper interface is interpreted as the layer 2A–2B boundary, corresponding to a transition from lavas to sheeted dykes. Layer 2A, on average, is 150 m thicker in crust that formed at the spreading center when spreading was located near the arc (< 50 km away), as compared to when spreading was located farther away from the arc (> 70 km away). Layer 2A thickness and Vp/Vs values indicate that a thicker and more porous lava layer, dominated by basalts to basalt-andesites, cap near-arc crust, while a thinner and less-porous, mostly basaltic, volcanic layer caps the far-arc crust. These results are consistent with the waning influence of slab-derived volatiles on crustal formation as seafloor spreading moves away from the active arc.

B. BOSTON1*, D. J. SHILLINGTON2, R. DUNN3, A. B. WATTS4, I. GREVEMEYER5, L. GÓMEZ DE LA PEÑA5, G. ITO3, P. WESSEL3, U. TEN BRINK6, N. MILLER6 1 Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA (*correspondence:brianb@ldeo.columbia.edu) 2Northern Arizona University, School of Earth and Sustainability, 624 S. Knoles Drive, Flagstaff, AZ 86011, USA 3University of Hawaii, Department of Earth Sciences, 1680 East-West Rd., Honolulu, HI 96822, USA 4University of Oxford, Department of Earth Sciences, South Parks Road, Oxford OX13AN, UK 5GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany 6Woods Hole Coastal and Marine Science Center, US Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543, USA

Very few constraints exist on the structure and evolution of old oceanic crust due to a paucity of observations. Existing sparse seismic reflection data from old oceanic crust in the Pacific and Atlantic Oceans reveal diverse attributes of the oceanic crust and upper mantle, including variable crustal thickness, Moho reflectivity, and intracrustal features. To what extent does this heterogeneity reflect variations in processes during accretion at the mid-ocean ridge or modification by post-accretion magmatism? Here we present a series of long seismic reflection profiles around and across the Hawaiian-Emperor Seamount Chain, which can be used to address this question. Multichannel seismic reflection data were acquired around and across the Hawaiian Chain in 2018. In 2019, comparable seismic reflection data were acquired around and across the Emperor. Oceanic lithosphere in both study areas is Cretaceous in age. Seismic data from both cruises were collected with the 15-km-long streamer and 6600 cu in tuned airgun array of the R/V Marcus G. Langseth . We

Spreading centers in the Lau back-arc basin exhibit significant changes in the style of seafloor volcanism, crustal structure, and lava composition based on their proximity to the active arc. The L-SCAN active source seismic tomography experiment along the Eastern Lau Spreading Center identified a change in crustal thickness and structure close to the arc, with corresponding changes in the shallow magmatic system beneath the spreading center [1,2]. To investigate deeper portions of the magmatic system, lower crust and mantle, we have extended the previous tomographic imaging to include all primary crustal P-wave arrivals, mantle refraction arrivals (Pn), and Moho reflection arrivals (PmP), incorporating >500,000 travel time picks into the tomographic analysis. Preliminary results indicate significant variations in Moho depth (< 6.0 – 8.4 km), with the thinnest crust found along the ridge axis at spreading center segments located far from the arc. The thickest crust appears to be generated at spreading centers located close to the arc, where subduction-derived water strongly influences melt production and composition. Low velocity anomalies associated with the magmatic system beneath the spreading center extend into the lower crust and upper mantle, but are generally lower amplitude and less continuous than in the upper crust. In addition, these low velocity anomalies are deeper and broader on average for ridge segments located far from the arc, while the near-arc low velocity zone appears to be narrower and more strongly expressed at shallow depths. These variations in the seismically-imaged magmatic system correlate with changes in lava composition, magma differentiation processes (based on petrologic modeling), and variations in eruptive style and ridge morphology along-axis. Results indicate that slab-derived water entrained in the near-arc ridge system not only enhances mantle melting, as commonly proposed to