Transient variation in seismic wave speed points to fast fluid movement in the Earth's outer core

Differences in travel time between seismic waves generated by two large earthquakes about 20 years apart suggest a localized density change of 2-3% in the Earth's outer core during this time which could be caused by fast-moving flows rich in light elements 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.

Automated summary

The differences in travel time between seismic waves generated by two large earthquakes about 20 years apart indicate a localized density change of 2-3% in the Earth's outer core, possibly caused by fast-moving flows rich in light elements.