Monday, July 24, 2006

Silent earthquakes may be a sign of volcano eruptions coming up!

Scientists are puzzled by a series of "silent" earthquakes deep underground that trigger no seismic signals, and by swarms of tremors they have detected in Northern California and other regions of the world where major quakes constantly threaten.

Those mysterious events, the scientists say, might help warn of truly dangerous quakes before they occur -- if only their meaning can be deciphered.

The so-called silent quakes occur as seismic faults slip slowly deep underground. Although instruments on the Earth's surface cannot pick up the slippages, they do cause the surface to move ever so slightly, and are often followed by swarms of underground tremors that are easily detectable.

The deep, slow-moving quakes and the tremors they cause have been called "the chatter of silent slip," and scientists from Stanford University are urging their colleagues around the world to monitor them in order to understand their links to damaging temblors that often follow.

The intriguing events have been detected beneath the Northern California coast and as far inland as Yreka (Siskiyou County); in the Puget Sound area around Seattle and western Canada; in Hawaii on the flanks of the Kilauea volcano; beneath the east coast of Japan; and along Mexico's west coast in the state of Guerrero.

If the tremors and silent quakes can be pinned down -- and it's a big if -- they might provide insights into how the Earth behaves just before certain types of huge, destructive quakes strike -- like the catastrophic undersea Sumatra event of 2004 that triggered the tsunami that killed hundreds of thousands of people around the Indian Ocean, said geophysicists Paul Segall and Gregory C. Beroza of Stanford.

Known as "megathrust" quakes, those types of temblors shove the seafloor abruptly upward, and their powerful force turns immense volumes of ocean water into a tsunami that can sweep across the sea for thousands of miles.

Segall and his colleagues reported recently in the journal Nature on four silent slip earthquakes they detected in Hawaii. Meanwhile, Beroza and David Shelly, together with their Japanese colleagues, reported in the same journal on a series of silent quakes and tremors that have been detected on the island of Shikoku in southeastern Japan, an area where devastating quakes have struck in the past.

All these seismic events are very much a mystery, Segall and Beroza agree. Geophysicists Garry Rogers and Herb Dragert of Central Washington University called them "the chatter of silent slip" after both the tremors and the silently slipping slow quakes were detected three years ago beneath British Columbia's Vancouver Island.

Underground tremors commonly occur inside volcanoes when magma churns upward just before eruptions, but the four sets of tremors Segall's team recorded on Kilauea were not connected with any volcanic activity, he said.

The silent-slip events are extremely slow-moving earthquakes that do not generate seismic waves and so cannot be detected by seismometers. A slipping fault -- sometimes as deep as 25 miles underground -- may take weeks or even months to distort the Earth's surface enough for Global Positioning System satellites to measure the motion, which is often less than an inch -- compared with the 1906 San Francisco quake, which displaced the surface as much as 21 feet at Tomales Bay.

The swarms of tiny quakes with magnitudes of less than 2 that sometimes accompany these events also might provide valuable clues to what's happening underground, the scientists say -- particularly in regions where one huge slab of the Earth's crust is diving beneath another in a process called subduction.

Quakes of moderate magnitude strike frequently in California around Petrolia (Humboldt County), where evidence of at least one silent quake was detected recently. All the land north of that town, including Eureka, lies within the Cascadia Subduction Zone.

Canadian quake experts maintain that a large-scale quake with a magnitude higher than 8 "is very possible" anywhere along the Cascadia Subduction Zone within the next 200 years.

At least one huge earthquake -- or maybe a series of quakes -- beneath Cascadia in 1700 sent a tsunami coursing across the Pacific to Japan, where it ravaged many villages. Japan has fallen victim to many such disasters; so have Alaska and the west coast of South America, and, of course, Indonesia.

"All these tremors and swarms of tiny quakes, as well as the silent earthquakes, appear to be adding stress to the entire fault zones where they occur," Segall said in an interview, "and when we learn to understand them better, they may well serve as signals warning that much bigger quakes are increasingly probable."

"It's almost unthinkable that these slow quakes and the tremors they produce wouldn't trigger much bigger quakes," Beroza said. "But we still have a lot to learn."

Similar nonvolcanic tremors have been detected some 50 miles beneath the San Andreas Fault south of Parkfield (Monterey County), but no silent quakes have been detected there -- only an increased frequency of small, shallow temblors easily detectable about 7 miles deep, according to Robert Nadeau of the UC Berkeley Seismological Laboratory.

Nadeau has been monitoring the deep tremors, and in September 2004, right after a long-awaited magnitude-6 quake struck on the San Andreas Fault, "those swarms just went crazy," he said in an interview Wednesday.

Right now, he said, he and his colleagues are awaiting installation of highly sensitive instruments called strain gauges that can detect the tiniest movement of the Earth's surface caused by the same mysterious silent quakes that are now known to occur so deeply in many of the world's most dangerous subduction zones.

Cascadia Subduction Zone “Silent quakes” and “nonvolcanic” tremors deep underground have been detected in the Pacific Northwest, Japan, Hawaii and other parts of the world. Many occur beneath regions like the Cascadia Subduction Zone which extends into Northern California. Last Wednesday, an offshore quake — not a “silent” one — struck 8 miles west of Petrolia in Humboldt County with a magnitude of 5. An offshore quake of magnitude 5 occurred Wednesday near Petrolia. In the Cascadia Subduction Zone, huge slabs of the Earth’s crust like the Juan De Fuca Plate dive beneath the North American Continental Plate.

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