Age-Independent Oceanic Plate Thickness and Asthenosphere Melting From SS Precursor Imaging

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.