Shuyang Sun

The asthenosphere is a weak layer in the upper mantle where geotherm may exceed mantle solidus and partial melt occurs. Although it has been suggested that an increase in seismic wavespeed at about 220 km depth represents the base of the asthenosphere, seismic studies to‐date have not been able to provide evidence for the existence of such a global interface in the oceanic regions. In this study, we report observations of SS precursors reflected at this boundary throughout the global oceans. The average depth of the discontinuity is approximately 250 km, with a velocity jump of about 7% across the interface. Finite‐frequency tomography of SS precursor traveltimes reveals large depth variations of the discontinuity over short spatial distances, which explains the absence of this discontinuity in previous global stacks. The depth perturbations are characterized by alternating linear bands of shallow and deep anomalies that roughly follow seafloor age contours, indicating a fundamental connection between seafloor spreading and asthenosphere convection. The base of the asthenosphere is smoother under seafloors formed at slow‐spreading centers and becomes much rougher under seafloors formed at fast‐spreading centers with a spreading rate greater than mm/yr. This observation suggests that different geophysical processes at slow and fast spreading centers generate lithospheric plates with different chemical compositions and physical properties, which in turn influences the convection in the oceanic asthenosphere.

The Earth's asthenosphere is a mechanically weak layer characterized by low seismic velocity and high attenuation. The nature of this layer has been strongly debated. In this study, we process 12 years of seismic data recorded at the global seismological network stations to investigate SS waves reflected at the upper and lower boundaries of this layer in global oceanic regions. We observe strong reflections from both the top and the bottom of the asthenosphere, dispersive across all major oceans. The average depths of the two discontinuities are 120 and 255 km, respectively. The SS waves reflected at the lithosphere and asthenosphere boundary are characterized by anomalously large amplitudes, which require a ∼12.5% reduction in seismic velocity across the interface. This large velocity drop can not be explained by a thermal cooling model but indicates 1.5%–2% localized melt in the oceanic asthenosphere. The depths of the two discontinuities show large variations, indicating that the asthenosphere is far from a homogeneous layer but likely associated with strong and heterogeneous small‐scale convection in the oceanic mantle. The average depths of the two boundaries are largely constant across different age bands. In contrast to the half‐space cooling model, this observation supports the existence of a constant‐thickness plate in oceanic regions with a complex and heterogeneous origin.

The Earth’s asthenosphere is a mechanically weak layer characterized by low seismic velocity and high attenuation. The nature of this layer has been strongly debated. In this study, we process twelve years of seismic data recorded at the global seismological network (GSN) stations to investigate SS waves reflected at the upper and lower boundaries of this layer in global oceanic regions. We observe strong reflections from both the top and the bottom of the asthenosphere, dispersive across all major oceans. The average depths of the two discontinuities are 120 km and 255 km, respectively. The SS waves reflected at the lithosphere and asthenosphere boundary are characterized by anomalously large amplitudes, which require ∼12.5% reduction in seismic velocity across the interface. This large velocity drop can not be explained by a thermal cooling model but indicates 1.5%-2% localized melt in the oceanic asthenosphere. The depths of the two discontinuities show large variations, indicating that the asthenosphere is far from a homogeneous layer but likely associated with strong and heterogeneous small-scale convections in the oceanic mantle. The average depths of the two boundaries are largely constant across different age bands. In contrast to the half space cooling model, this observation supports the existence of a constant-thickness plate in oceanic regions with a complex and heterogeneous origin.