Resolving the influence of mantle heterogeneity on estimates of inner core anisotropy

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The inner core of the Earth grows from the solidification of the liquid iron outer core, which provides the energy that powers the generation of its magnetic field. Thirty years ago, it was discovered that seismic waves that propagate through the inner core with paths parallel to the earth's rotation axis travel faster than those with paths oriented in the equatorial direction. This dependence on the direction in which the waves propagate is known as anisotropy, and is attributed to the alignment of iron crystals in the inner core in the direction of the rotation axis. The mechanisms responsible for the observed seismic anisotropy have not been completely elucidated, and may include large scale flow in the inner core, or preferential growth directions under the influence of the earth's magnetic field. Since the origin and magnitude of the seismic anisotropy in the inner core is not well constrained and these waves always traverse the heterogeneous mantle, that influences their travel time, separating each effect has been limited to date. Another complication comes from the uneven distribution of earthquakes and seismic stations around the globe, resulting in poor sampling of the inner core seismic paths, especially those parallel to the earth's rotation axis (i.e. polar paths). In this project, the researchers will analyze data from recent deployments in Alaska and Antarctica to find out the mysterious source of the large spread of travel time anomalies for earthquakes in South Sandwich Islands observed in Alaska, which is likely not caused by inner core structure. These new data make it possible to fill gaps in coverage for polar paths, while improved images of the structure in the mantle beneath Alaska are becoming available. We will quantify the effect of mantle structure on the propagation of inner core sensitive seismic waves and explore other possible sources outside of the inner core to explain the travel time patterns observed in these polar paths. In particular, a more definitive answer to the question of the origin of the South Sandwih Island-Alaska anomalies will be sought, allowing the construction of more accurate models of inner core seismic anisotropy. Broader impacts of this project include mentoring of a postdoctoral fellow and development of a website aimed towards public outreach.Resolving the magnitude of inner core (IC) seismic anisotropy accurately, as well as its spatial and depth dependence, is critical for our understanding of inner core formation and evolution. Among data sensitive to inner core structure, PKP data are best suited to construct such detailed models, but they suffer from contamination by mantle heterogeneity, which they need to first be corrected for. In particular, the large magnitude of the anisotropy inferred, up to 6-8% in the western hemisphere, is hard to reconcile with mineral physics modeling. Many of the previous studies of the IC based on PKP waves included few data for polar paths and the magnitude of the IC anisotropy in the western hemisphere hinged on observations at stations in Alaska from events in the South Sandwich Islands. Slab structure in this region is known to be strong and thus the trade off between IC anisotropy and mantle structure has been unclear. Identifying sources of contamination of PKP data and making appropriate corrections is crucial for reconciling different estimates of inner core anisotropy strength and building a more robust image of inner core anisotropic structure. New data from recent deployments in Alaska and Antarctica will be analyzed. These data now make it possible to fill gaps in coverage for polar paths, while improved tomographic images of the Alaska slab are now also becoming available. By 1D and 3D ray tracing and 3D waveform modelling through recent tomographic models and synthetic slab models, the effect that upper mantle structure can have on PKP differential travel times will be better quantified, and other sources of contamination will be explored. A more definitive answer to the question of the origin of the SSI-Alaska anomalies will be sought, allowing the construction of more accurate models of IC anisotropy. Broader impacts include making available the new set of PKP travel time collection to be used by future researchers, as well as the mentoring of a postdoctoral fellow.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.