By Sid Perkins
On February 22, 2006, a magnitude-7 quake rocked Mozambique. The temblor was an unusually large one for southern Africa, but it also was quick for its size: Motions at the epicenter lasted only eight seconds or so, says George Helffrich, a geophysicist at the University of Bristol in England. While much of the quake’s energy spread along the planet’s surface, some of it radiated downward, traveled through Earth’s core and then returned to the surface in Japan, where more than 700 seismometers picked up the vibes.
Some types of ground motions triggered by earthquakes — in particular, those that travel like sound waves — can pass either through solid rock or through a liquid such as water or molten iron. But the size, shape and timing of some of the vibrations picked up by the Japanese instruments suggest that the waves traveled through the planet’s inner core as shear waves, which can travel only through a solid material, says Helffrich. He and colleague James Wookey, also a geophysicist at the University of Bristol, report their findings in the Aug. 14 Nature.
Most models of Earth’s internal structure include a core that consists of two layers: an outer core of molten material, primarily iron, that surrounds a solid inner core. But direct evidence of the solidity of that inner core has proven elusive. Teams have previously reported data hinting that seismic shear waves pass through Earth’s inner core, but those reports were inconclusive, Helffrich says. Some of those observations involved low-frequency vibrations and were made by small networks of seismic instruments, making it more difficult to determine the direction from which those ground motions arrived, he suggests. In other cases, the purported shear waves didn’t arrive at instruments when most seismic models suggested they should have.
The ground motions reported in the new study, however, were detected at high frequency and by a large number of instruments. Many of the seismometers had been installed in boreholes more than one kilometer below Earth’s surface — a depth that insulated the sensors from noise generated by human activity and that also made the faint ground motions easier to discern, says Helffrich. “It took heroic measures to detect these signals,” he notes.
One reason these particular ground motions have been so elusive is their size compared to seismic background noise. Indeed, the ground motions reported by Helffrich and Wookey are “so small, they’re easily confused with other vibrations bouncing around inside the Earth after an earthquake,” says Ken Creager, a geophysicist at the University of Washington in Seattle. However, the characteristics of the ground motions the team noted after the African quake were consistent over a broad area, a sign that local geological variations probably didn’t create spurious reflections that could have confused the analysis, he adds.
The ground vibrations triggered by the core-crossing shear waves were larger than those estimated by current seismic models, a sign that the solid material in the inner core attenuated the waves much less than expected, says Helffrich. That, in turn, could mean that the solid material of the inner core doesn’t include many vibration-dampening pockets of melted material, as some scientists have proposed.
Helffrich and Wookey are the first to detect two sets of the ground motions associated with core-crossing shear waves: One set had been triggered by left-to-right vibrations and another had resulted from up-and-down shear waves. The seven-second disparity in travel time between the two sets of ground motions indicates that the crystal structure of the inner core is anisotropic — in other words, sound travels faster through the material in some directions than it does in others, says Helffrich.
Detecting that anisotropy, previously predicted by some scientists but never observed, is “amazing,” says Michael Bergman, a geophysicist at Bard College at Simon’s Rock in Great Barrington, Mass. Similar observations made in the wake of future quakes will provide a new way for researchers to probe the structure of Earth’s interior, he notes.
