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Shear attenuation provides insights into the physical and chemical state of the upper mantle. Yet, observations of attenuation are infrequent in the oceans, despite recent proliferation of arrays of ocean‐bottom seismometers (OBSs). Studies of attenuation in marine environments must overcome unique challenges associated with strong oceanographic noise at the seafloor and data loss during OBS recovery in addition to untangling the competing influences of elastic focusing, local site amplification, and anelastic attenuation on surface‐wave amplitudes. We apply Helmholtz tomography to OBS data to simultaneously resolve array‐averaged Rayleigh wave attenuation and maps of site amplification at periods of 20–150 s. The approach explicitly accounts for elastic focusing and defocusing due to lateral velocity heterogeneity using wavefield curvature. We validate the approach using realistic wavefield simulations at the NoMelt Experiment and Juan de Fuca (JdF) plate, which represent endmember open‐ocean and coastline‐adjacent environments, respectively. Focusing corrections are successfully recovered at both OBS arrays, including at periods <35 s at JdF where coastline effects result in strong multipathing. When applied to real data, our observations of Rayleigh wave attenuation at NoMelt and JdF revise previous estimates. At NoMelt, we observe a low attenuation lithospheric layer (> 1,500) overlying a highly attenuating asthenospheric layer (∼ 50 to 70). At JdF, we find a broad peak in attenuation (∼ 50 to 60) centered at a depth of 100–130 km. We also report strong local site amplification at the JdF Ridge (>10% at 31 s period), which can be used to refine models of crust and shallow mantle structure.

 

Small‐scale convection beneath the oceanic plates has been invoked to explain off‐axis nonplume volcanism, departure from simple seafloor depth‐age relationships, and intraplate gravity lineations. We deployed 30 broadband ocean bottom seismometer stations on ∼40 Ma Pacific seafloor in a region notable for gravity anomalies, measured by satellite altimetry, elongated parallel to plate motion.P‐wave teleseismic tomography reveals alternating upper mantle velocity anomalies on the order of ±2%, aligned with the gravity lineations. These features, which correspond to ∼300°–500°K lateral temperature contrast, and possible hydrous or carbonatitic partial melt, are—surprisingly—strongest between 150 and 260 km depth, indicating rapid vertical motions through a low‐viscosity asthenospheric channel. Coherence and admittance analysis of gravity and topography using new multibeam bathymetry soundings substantiates the presence of mantle density variations, and forward modeling predicts gravity anomalies that qualitatively match observed lineations. This study provides observational support for small‐scale convective rolls beneath the oceanic plates.

 

Abstract The Pacific ocean-bottom seismometer (OBS) Research into Convecting Asthenosphere (ORCA) experiment deployed two 30-station seismic arrays between 2018 and 2020—a US contribution to the international PacificArray project. The “Young ORCA” array deployed on ∼40 Ma central Pacific seafloor had a ∼68% data recovery rate, whereas the “Old ORCA” array deployed on ∼120 Ma southwest Pacific seafloor had a ∼80% recovery rate. We detail here the seismic data quality, spectral characteristics, and engineering challenges of this experiment. We provide information to assist users of this dataset, including OBS orientations and tables of daily data quality for all channels. Preliminary analysis illustrates the utility of these data for surface- and body-wave seismic imaging. 

Lithospheric seismic anisotropy illuminates mid‐ocean ridge dynamics and the thermal evolution of oceanic plates. We utilize short‐period (5–7.5 s) ambient‐noise surface waves and 15‐ to 150‐s Rayleigh waves measured across the NoMelt ocean‐bottom array to invert for the complete radial and azimuthal anisotropy in the upper ∼35 km of ∼70‐Ma Pacific lithospheric mantle, and azimuthal anisotropy through the underlying asthenosphere. Strong azimuthal variations in Rayleigh‐ and Love‐wave velocity are observed, including the first clearly measured Love‐wave 2θand 4θvariations. Inversion of averaged dispersion requires radial anisotropy in the shallow mantle (2‐3%) and the lower crust (4‐5%), with horizontal velocities (V_SH) faster than vertical velocities (V_SV). Azimuthal anisotropy is strong in the mantle, with 4.5–6% 2θvariation inV_SVwith fast propagation parallel to the fossil‐spreading direction (FSD), and 2–2.5% 4θvariation inV_SHwith a fast direction 45° from FSD. The relative behavior of 2θ, 4θ, and radial anisotropy in the mantle are consistent with ophiolite petrofabrics, linking outcrop and surface‐wave length scales.V_SVremains fast parallel to FSD to ∼80 km depth where the direction changes, suggesting spreading‐dominated deformation at the ridge. The transition at ∼80 km perhaps marks the dehydration boundary and base of the lithosphere. Azimuthal anisotropy strength increases from the Moho to ∼30 km depth, consistent with flow models of passive upwelling at the ridge. Strong azimuthal anisotropy suggests extremely coherent olivine fabric. Weaker radial anisotropy implies slightly nonhorizontal fabric or the presence of alternative (so‐called E‐type) peridotite fabric. Presence of radial anisotropy in the crust suggests subhorizontal layering and/or shearing during crustal accretion.