for National Geographic News
Slow-moving earthquakes happening deep in the Earth's bowels might be silent but deadly, experts say, and until now the quakes have been very hard to detect.
But weak tectonic signals called nonvolcanic tremors can be picked up by sensitive instruments, and they could be by-products of these "silent" quakes, new research suggests.
Because deep silent quakes might spur destructive surface quakes, the nonvolcanic tremors could help scientists predict when a deadly quake might strike.
Tiny Tremors, Big Quakes
Silent earthquakes are tectonic shifts that can last for days or even weeks. They occur up to 25 miles (40 kilometers) underground and barely make a ripple on Earth's surface.
Humans can't feel these silent quakes, but experts think that they may foreshadow powerful surface quakes that can level buildings and topple bridges.
(See photos of earthquake prediction technology.)
The lumbering temblors have been difficult to pick up with modern seismic instruments and register on global positioning systems only long after they first occur.
"Many places have continuous GPS stations that monitor the surface of the Earth [for quake activity]," said David R. Shelly, a doctoral candidate at Stanford University in California.
"With the silent quakes it would be days to weeks or longer [before the surface moved enough to be detectable by such technology]," Shelly said.
Writing in the July 13 issue of the journal Nature, Shelly and colleagues from Stanford and the University of Tokyo in Japan say they have found evidence that silent quakes may generate nonvolcanic tremors.
The tremors are weak vibrations that originate in the depths of active fault zones and aren't related to volcanic activity.
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