for National Geographic News
The intensities of strange, long-lasting tremors in North America's Pacific Northwest ramp up and quiet down with the rise and fall of the ocean's tides, according to a new study.
These so-called nonvolcanic tremors are very faint seismic signals that were not discovered until 2002. Their exact cause remains a mystery.
Scientists are almost certain that the tremors are not earthquakes. Unlike quakes, the tremors start very deep in Earth's crust and can last for weeks at a time.
Although the new study does not reveal how the tremors are triggered, it does suggest that the tides—forces controlled by the gravitational tugs of the sun and the moon—play a key role in how intensely they vibrate.
"When the water level is up from tides, we see that the tremor is about 30 percent stronger than average," said study co-author Justin Rubinstein, a seismologist at the University of Washington in Seattle.
"And when the tides are out, it's about 30 percent weaker than average."
Rubinstein and colleagues present their data this week in the online version of the journal Science.
Almost all nonvolcanic tremors have been detected on so-called subduction faults, where one piece of Earth's crust slips beneath another.
The have been picked up in the Cascadia region of the Pacific Northwest, which stretches from Vancouver Island in Canada to northern California in the U.S. (see a map of the region).
They have also been found occurring in the subduction zone along southwest Japan's Nankai trough.
In 2004 researchers detected nonvolcanic tremors deep in California's San Andreas Fault—the first time they had been found occurring on a strike-slip fault, where Earth's plates slide past each other.
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