New evidence from short-period earthquake waves may solve a long-standing mystery of Earth's inner core and offers additional support for a layered inner core model, say seismologists at the University of Illinois.
For about a decade, the cause for anomalous waves passing through the innermost portion of the planet has been a mystery. Seismic waves that traverse the solid inner core along north-south paths have a much smaller amplitude and a more complex waveform than those that travel along east-west paths.
As reported in the Feb. 19 online edition of Geophysical Research Letters (and in the Feb. 15 issue to be distributed in early March), UI professor of geology Xiaodong Song and graduate student Xiaoxia Xu have analyzed new data that may help solve the mystery.
"Seismic waves traveling through the inner core along a north-south direction are faster than those traveling along an east-west direction, a feature known as the anisotropy of the inner core," Song said. "Understanding the source of anisotropy in the inner core could be crucial to explaining other phenomena, such as how Earth's magnetic field arises and how the core formed and evolved. Using seismic waves generated by earthquakes, we found the structure of the inner core to be much more complicated than we originally thought."
Earth's core consists of a solid inner core about 2,400 kilometers in diameter and a liquid outer core about 7,000 kilometers in diameter. In addition, the solid inner core also appears to be layered into a lower inner core and an upper inner core. The upper inner core creates a transition zone about 250 to 400 kilometers thick, Song said.
The layered inner core model was first proposed in 1998 by Song and Donald Helmberger, director of the Seismological Laboratory at the California Institute of Technology.
"At that time, we relied heavily upon long-period, broadband data collected from several earthquakes but recorded at very few seismic stations," Song said. "To enhance the model, we needed more short-period data -- which is where most of the anomalies occur."
Song and Xu filled the void by studying seismic waves from an earthquake that occurred on Oct. 5, 1997, in the South Sandwich Islands off the coast of South America. After traveling through Earth's inner core, the short-period waves were recorded by more than 100 stations of the Alaska Seismic Network.
The new evidence from the short-period waves offers additional support for a layered model of Earth's inner core, Song said. "But, to our surprise, we found that such a model could explain the anomalous short-period waves."
The upper part of the inner core is isotropic, but the lower part of the inner core is anisotropic, Song said. "That means seismic waves traveling through the lower inner core will travel at different velocities in different directions."
Because the anisotropy in the lower inner core is aligned in the north-south direction, seismic waves traveling along north-south paths will speed up and spread out, producing complicated waveforms with varying arrival times. The smaller amplitudes are a result of the energy being split into multiple branches of waves, Song said. Seismic waves traveling along east-west paths are unaffected.
Based on the new earthquake data, the scientists conclude that the anisotropy in the lower inner core is much higher than they previously believed.
"Our waveform modeling indicates that the speed of seismic waves in the north-south direction is about 8 percent faster than in the east-west direction," Song said.
This result raises new questions on the source of the inner core anisotropy, which many scientists believe is caused by a preferential alignment of iron crystals, Song said. "To explain the amplitude of the anisotropy would require nearly perfect alignment of the iron crystals, according to most recent measurements and predictions of the elasticity of inner-core iron." - By James E. Kloeppel
[Contact: James E. Kloeppel]