Seismic Speed Traps: Iron-rich regions may slow deep-Earth vibes

Large quantities of iron-rich minerals may be responsible for the sluggishness of seismic waves traveling through certain regions deep within Earth, a new analysis suggests.

About 2,900 kilometers below Earth’s surface, molten iron from the planet’s core meets a thick, overlying mantle of silicate minerals. Vibrations spreading from large earthquakes slow significantly as they pass through some patches of rock just above that core-mantle boundary, says Ho-kwang Mao, a geophysicist at the Carnegie Institution of Washington (D.C.). Those regions, dubbed ultralow-velocity zones, range between 5 and 40 km thick and can measure more than 100 km across.

Earthquakes cause a variety of vibrations, including seismic-pressure waves that travel through the ground as sound does through air and other waves that transfer shearing stresses through rock. In ultralow-velocity zones, seismic pressure waves slow 5 to 10 percent, says Mao. Shear waves can be hindered by as much as 30 percent.

Because liquids can’t transmit shear waves, scientists previously speculated that partial melting of minerals in the zones was responsible for the seismic-wave slowdown, he notes. Now, Mao and his colleagues offer another explanation: high iron content.

The researchers performed lab tests on samples of an iron-rich silicate having a particular crystal structure, called a postperovskite phase, that scientists presume occurs near the core-mantle boundary. The silicate contained more than twice as much iron as other silicate-crystal structures can hold.

When the researchers squeezed the mineral to the pressure expected near the core-mantle boundary—almost 1.3 million times that exerted by the atmosphere at sea level—they noted that vibrations traveling through the sample slowed considerably. “Pressure waves traveled 7 percent slower than expected, and shear waves progressed about 30 percent slower,” says Mao. The researchers report their findings in the April 28 Science.

The idea that iron-rich minerals cause seismic slowdown is intriguing, says Michael Thorne, a seismologist at the University of Alaska in Fairbanks. High iron concentrations could also explain the high density of minerals inferred from past observations of ultralow-velocity zones, he notes.

Large concentrations of iron-bearing minerals provide an interesting potential explanation of the seismic slowdown in the ultralow-velocity zones, says John Hernlund of the Paris Geophysical Institute. This “iron-sponge” scenario is more feasible than that of partially melted minerals because it doesn’t require the ultralow-velocity zones to be mixtures of melted and solid minerals, he adds. Such areas, especially in layers 40 km thick, probably wouldn’t have been as stable over geologic time as ultralow-density zones seem to have been.

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