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Abstract 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. 

SUMMARY The seismic quality factor (Q) of the Earth’s mantle is of great importance for the understanding of the physical and chemical properties that control mantle anelasticity. The radial structure of the Earth’s Q is less well resolved compared to its wave speed structure, and large discrepancies exist among global 1-D Q models. In this study, we build a global data set of amplitude measurements of S, SS, SSS and SSSS waves using earthquakes that occurred between 2009 and 2017 with moment magnitudes ranging from 6.5 to 8.0. Synthetic seismograms for those events are computed in a 1-D reference model PREM, and amplitude ratios between observed and synthetic seismograms are calculated in the frequency domain by spectra division, with measurement windows determined based on visual inspection of seismograms. We simulate wave propagation in a global velocity model S40RTS based on SPECFEM3D and show that the average amplitude ratio as a function of epicentral distance is not sensitive to 3-D focusing and defocusing for the source–receiver configuration of the data set. This data set includes about 5500 S and SS measurements that are not affected by mantle transition zone triplications (multiple ray paths), and those measurements are applied in linear inversions to obtain a preliminary 1-D Q model QMSI. This model reveals a high Q region in the uppermost lower mantle. While model QMSI improves the overall datafit of the entire data set, it does not fully explain SS amplitudes at short epicentral distances or the amplitudes of the SSS and SSSS waves. Using forward modelling, we modify the 1-D model QMSI iteratively to reduce the overall amplitude misfit of the entire data set. The final Q model QMSF requires a stronger and thicker high Q region at depths between 600 and 900 km. This anelastic structure indicates possible viscosity layering in the mid mantle.