The solid inner core grows through crystallization of the liquid metallic outer core. This process releases latent heat as well as light elements, providing thermal and chemical buoyancy forces to drive the Earth’s geodynamo. Here we investigate temporal changes in the liquid outer core by measuring travel times of core-penetrating SKS waves produced by pairs of large earthquakes at close hypocenters. While the majority of the measurements do not require a change in the outer core, we observe SKS waves that propagate through the upper half of the outer core in the low latitude Pacific travel about one second faster at the time when the second earthquake occurred, about 20 years after the first earthquake. This observation can be explained by 2–3% of density deficit, possibly associated with high-concentration light elements in localized transient flows in the outer core, with a flow speed in the order of 40 km/year.
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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.