Geophysics

Rough Core

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Science  09 Feb 2007:
Vol. 315, Issue 5813, pp. 741
DOI: 10.1126/science.315.5813.741c

Earth's core is split into two zones. A solid inner core lies within a liquid outer core, which drives the geodynamo via heat convection. The smooth and spherical interface comprising the inner core boundary (ICB) is the site of such geochemical changes as solidification and chemical flows. The ICB can be probed by seismic waves that reflect or refract off of it. In the unusual circumstance of repeating earthquakes, sequential rays can travel identical ray paths through Earth. By analyzing one such pair of repeated earthquakes, Cao et al. found changes in the ICB structure over a decade. Two identical ray paths, sampled by earthquakes in the South Sandwich Islands in 1993 and 2003 and recorded in Alaska and Canada, show variations in the shape of their seismic response over this time. Although simple pressure waves were identical, differences were seen between the reflected and refracted waves that intercepted the ICB. These amplitude differences may indicate lumps on the ICB of 10 km in horizontal extent. Features of this size could be created by low convection in the inner core itself, or by rotation of a fixed bump on the inner core by approximately 0.1° per year. — JB

Proc. Natl. Acad. Sci. U.S.A. 104, 31 (2007).

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