Letter abstract
Nature Geoscience 1, 692 - 696 (2008)
Published online: 28 September 2008 | doi:10.1038/ngeo314
Subject Categories: Geomagnetism, palaeomagnetism and core processes | Seismology | Structural geology, tectonics and geodynamics
Seismic evidence for distinct anisotropy in the innermost inner core
Knowledge of the seismic structure of the innermost inner core is important for understanding the formation of the inner core1, 2, 3. It has been suggested recently that this region may exhibit distinct seismic anisotropy4, 5. Here, we use the difference in travel times between seismic waves reflected at the underside of the inner core boundary and those traversing the inner core to constrain the seismic anisotropy. We calculated travel-time residuals for waves generated by two deep earthquakes that occurred in Indonesia and Argentina respectively, recorded by seismic arrays in Venezuela and China. The travel-time residuals are systematically larger, by about 1.8 s, for waves that travel roughly along the equatorial plane of the inner core (Indonesia–Venezuela) than for those travelling in a direction at an angle of 
28° to the equatorial plane (from Argentina to China). The difference in travel times is arguably most sensitive to the structure near the centre of the Earth, and thus provides evidence for deep layering within the inner core. Our results are consistent with models invoking seismic anisotropy of the innermost inner core with the slowest direction tilted at an angle of 45° to the equatorial plane4.
- Department of Earth Science, Rice University, 6100 Main Street, Houston, Texas 77005, USA
- Institute of Earthquake Science, China Earthquake Administration, No. 63 Fuxing Avenue, Beijing, 100036, China
Correspondence to: Fenglin Niu1 e-mail: niu@rice.edu
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