On the Origin of the Hawaiian Swell: Lithosphere and Asthenosphere Seismic Structure From Rayleigh Wave Dispersion.

In this study, we revisit the shear‐wave velocity structure of the lithosphere and asthenosphere surrounding the Hawaiian hotspot and Hawaiian swell using Rayleigh wave data spanning periods of 20–125 s from the PLUME project. A primary goal of this investigation is to probe the origin of the Hawaiian swell and the mechanism that elevates the topography, providing insights into mantle dynamics beneath hotspot swells. In the shear velocity model, the 30–70 km depth range is largely featureless with weak and local anomalies, indicating that the elevation of the Hawaiian swell cannot be attributed to upper lithospheric reheating or replacement. In contrast, at 80–150 km depth, a pronounced region of anomalously low velocities is well‐resolved, with the lowest velocities found beneath the Hawaii‐Maui‐Molokai part of the island chain. Minimum shear velocities are approximately 4.0 km/s at 100–120 km depth, which is an ∼8%‐10% velocity decrease relative to the surrounding velocities away from the swell. This pattern suggests that hot, buoyant mantle from deeper plume sources laterally spread out near the top of the normal oceanic asthenosphere. We find that the low‐velocity pattern in the asthenosphere exhibits a strong correlation with the overall shape of the Hawaiian swell topography. Assuming that density anomalies are proportional to shear velocity anomalies, we demonstrate that the anomalous elevation of the swell can be explained by the uplift of a 30‐km‐thick elastic plate loaded from below by this buoyant, low‐seismic‐velocity layer in the asthenosphere. Plain Language Summary: We collected and analyzed teleseismic surface wave data recorded by the PLUME ocean‐bottom‐seismometer array and adjacent land stations to obtain a three‐dimensional shear‐wave velocity model of the upper mantle around Hawaii. Our goal is to generate a finer velocity model that can accurately reveal the seismic velocity features within the Hawaiian lithosphere and asthenosphere. The issue addressed in this study is the origin of the Hawaiian swell, the region of elevated seafloor surrounding the island hotspot chain. Our shear‐wave velocity structure, at depths of 30–70 km, displays only local features without strong velocity anomalies. In the 80–150 km depth range, there are widespread, strong low‐velocity anomalies that correlate well with the shape of the swell. We infer that these low‐velocity anomalies are hot, buoyant materials from deeper mantle plumes that laterally spread out at the top of the asthenosphere. We calculated the fit between the overall shape of the observed swell and the prediction of deflection assuming density anomalies are proportional to the shear velocity anomalies. The predicted elevation agrees well, implying that the swell originates in the upper asthenosphere and lower lithosphere deeper than 80 km. Key Points: Beneath the Hawaiian swell, our shear‐wave velocity model from surface wave dispersion displays a largely featureless upper lithosphereLow velocities at 80–150 km depth beneath the swell suggest hot, buoyant mantle spreads near the top of the normal oceanic asthenosphereHawaiian swell topography can be explained by loading an elastic plate from below by a buoyant, low‐velocity layer in the asthenosphere [ABSTRACT FROM AUTHOR]