Seismic waves produced by earthquakes tend to move more quickly when traveling through the center of the earth on a north-south path than on an east-west one. But when Miaki Ishii and Adam M. Dziewonski of Harvard University analyzed data from more than 300,000 seismic events collected between 1964 and 1994, they found that this wave propagation pattern changes slightly at the absolute center of the core. In this region, which they dubbed the "innermost inner core" (IMIC), the scientists determined that the waves traveled the slowest on a trajectory 45 degrees off the east-west line, instead of in the east-west direction. This directional dependence, known as anisotropy, could be the result of a changing environment during core formation. Ishii and Dziewonski estimate the radius of the IMIC (red region in image) to be 300 kilometers, although they caution that the sharpness of the boundary cannot be well constrained with the currently available data. By placing more seismometers around the globe, or at the bottom of the ocean, they conclude, "it might be possible to conduct a more detailed survey of the distinct anisotropy that characterizes the very center of the earth."
The earth's inner core has been of interest to scientists ever since it was discovered in 1936. Imaging it has been difficult, however, because relatively few seismic waves reach it and return to the surface. In addition, the core's extreme temperatures and pressures are hard to reproduce in a laboratory. Now a new examination of three decades of seismic data is providing a more detailed picture of our planet's center. According to a report published online this week by the Proceedings of the National Academy of Sciences, within the 2,440-kilometer-wide solid inner core lies a distinct "innermost core" approximately 600 kilometers across. Its existence, the researchers say, could represent evidence of two separate episodes of development for the inner core.