Geophysics and Tectonics Seminar winter-2019

The Earth’s inner core shows variability in the speed of seismic waves travelling through it depending on the direction that the wave travels, a feature known as anisotropy. Seismic waves passing through the inner core along paths propagating roughly north-south (near-parallel to the Earth’s rotation axis) travel faster than those traversing the inner core roughly east-west (in the plane of the equator). Since the discovery of anisotropy 30 years ago, further patterns of the variation in seismic velocity have been discovered. Inner core anisotropy is proposed to result from preferred alignment of iron crystals. However, the strength and distribution of anisotropy are difficult to reconcile with predictions from mineral physics and dynamical models of inner core growth. We examine the trends of travel times on newly acquired polar paths from recent deployments of seismic arrays in Alaska and Antarctica. We observe large travel time anomalies in Alaska associated with the Alaskan subduction zone, indicating upper mantle contamination of these paths. To match our observations, we propose a model of asymmetric growth at the inner core boundary resulting in net translation and flow from viscous relaxation at the equator. The flow from the equator to the poles results in a strong alignment of iron crystals, which is laterally offset from the rotation axis, explaining the patterns of seismic anisotropy observed in our data. The asymmetric growth of the inner core may have influenced or been influenced by outer core and mantle dynamics.