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Two Deadly Quakes: Is Earth Unusually Active?


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DazzaMc
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« on: October 09, 2009, 04:48:59 pm »

Two Deadly Quakes: Is Earth Unusually Active?

Between the earthquakes that struck the Samoas and Indonesia yesterday and the temblor that devastated L'Aquila, Italy earlier this year, it might seem like Earth has been particularly shaky this year.

But that's not the case: "This is not out of the ordinary as far as the year goes," said John Bellini, a geophysicist with the U.S. Geological Survey.

While it's not unusual to have several large quakes in any given year, these rumblings of the Earth can have different profiles, occurring on different types of faults and triggering varying series of events, including tsunamis possibly even other earthquakes.

Scientists said it's possible the two quakes this week are related, but they don't know yet. Meanwhile, researchers have some preliminary explanations as to why the Samoan temblor generated a deadly tsunami but the Indonesian quake did not.

A year in earthquakes

More than a million earthquakes are estimated to shake the Earth's surface every year, though most of these are minor tremors, barely perceptible to humans if at all.

As you go up in earthquake magnitude (a measure of a quake's strength), these events become rarer: About 17 magnitude 7.0 to 7.9 quakes occur around the globe each year, while on average only one magnitude 8.0 or higher strikes over the period of a year. (The logarithmic nature of the magnitude scale means that an 8.0-magnitude quake is 10 times stronger than a 7.0-magnitude.)

The underwater quake that struck of the coast of Samoa and American Samoa yesterday measured about 8.0, according to the USGS. The quake off the coast of Sumatra, Indonesia came in at an estimated 7.6 magnitude. (The L'Aquila quake, which struck in April, was only a 6.3-magnitude quake.)

The occurrence of all these earthquakes falls entirely within the average for a given year. "We're just having a busy week," Bellini said.

Different effects

While the Samoan earthquake generated a large tsunami that rushed ashore on the South Pacific islands, leveling towns with waves estimated to be 10 foot (3 meters) or higher and killing scores, the Indonesian quake sprouted only a small local tsunami. The shaking did most of the damage in the Indonesian event.

The damage done by a quake and how it is done isn't just a matter of how strong the quake is, it's "dependent on how the fault breaks," Bellini told LiveScience.

The shift in Earth's crust off the coast of Sumatra occurred at a depth of about 50 miles (80 km). The Samoa tsunami-generating quake occurred much closer to the surface at 11 miles (18 km).

"You need a shallow earthquake to generate a tsunami," Bellini said.

The type of fault that ruptures to produce the earthquake also influences whether or not a tsunami develops and how big it becomes. The Samoan quake occurred along a normal fault, where one chunk of a tectonic plate lifts upward, acting as a paddle and transferring energy to the overlying water.

But sometimes this "paddle effect" is minimal, or the fault moves side-to-side instead of up and down.

The 2004 Indian Ocean tsunami was generated by a colossal 9.3-magnitude quake. An 8.7-magnitude earthquake in 2005 originating at the same location failed to produce a tsunami. It was certainly large enough to generate one, scientists say. The exact reasons it did not remain mysterious.

Related quakes?

One curious aspect of yesterday's events is how close in time and space they occurred to each other. This proximity can fuel speculation that the first earthquake triggered the second.

"I wouldn't say it's impossible," said Leonardo Seeber, a senior researcher at Lamont-Doherty Earth Observatory in New York.

In recent decades, scientists have shown that one earthquake can trigger other seismic events far away, Seeber said.

For example, a 2008 study did find that monstrous earthquakes can trigger much smaller tremors in distant locations around the globe. Another study from last year found that so-called "earthquake doublets" where one large quake triggers another of similar magnitude (versus aftershocks which are weaker than the original quake) can also occur, though they are rare.

And today, a new study announced that large earthquakes could weaken faults on the other side of the world. The study found that the 2004 Indian Ocean quake had weakened California's San Andreas fault. 

But, "it's not easy to prove one way or the other," Seeber told LiveScience. It's also possible that the closeness of the quakes in time was a matter of chance, "because there's enough earthquakes around" to have two occur in the same region around the same time, Seeber said.

However, it's too early for seismologists to know one way or the other, though Bellini agreed it's possible they were related. "They're on the borders of the same plate," he explained, so the stresses could have transferred down the fault.

"That's not something that can be determined overnight," Bellini said.

http://www.livescience.com/environment/090930-earthquake-year.html
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DazzaMc
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« Reply #1 on: October 09, 2009, 04:50:43 pm »

Big Quakes Weaken Faults on Other Side of Earth

Massive earthquakes on one side of the globe can weaken faults half a world away, scientists announced today.

A group of seismologists studying the massive 2004 earthquake that triggered killer tsunamis throughout the Indian Ocean found that the quake had weakened at least a portion of California's famed San Andreas Fault.

The finding, detailed in the Oct. 1 issue of the journal Nature, suggests Earth's largest earthquakes can weaken fault zones worldwide and trigger periods of increased global seismic activity.

The announcement of the new link comes just one day after two different earthquakes rattled the Samoan Islands and Indonesia, the former generating a tsunami that has killed scores of people. Scientists aren't sure if these two temblors were related, but they said it is possible.

Suspected link

Scientists began suspecting a global quake linkage after the devastating 2004 earthquake, an estimated 9.3-magnitude temblor.

"An unusually high number of magnitude 8 earthquakes occurred worldwide in 2005 and 2006," said study co-author Fenglin Niu of Rice University. "There has been speculation that these were somehow triggered by the Sumatran-Andaman earthquake that occurred on Dec. 26, 2004, but this is the first direct evidence that the quake could change fault strength of a fault remotely."

Niu and co-authors Taka'aki Taira and Paul Silver, both of the Carnegie Institution of Science in Washington, D.C., and Robert Nadeau of the University of California, Berkeley, examined more than 20 years of seismic records from Parkfield, Calif., which sits astride the San Andreas Fault in Southern California.

The team zeroed in on a set of repeating microearthquakes that occurred near Parkfield over two decades. Each of these tiny quakes originated in almost exactly the same location.

By closely comparing seismic readings from these quakes, the team was able to determine the "fault strength" — the shear stress level required to cause the fault to slip — at Parkfield between 1987 and 2008.

Three fault changes

The team found fault strength changed markedly at three times during the 20-year period.

The authors surmised that the 1992 Landers earthquake, a magnitude 7 quake north of Palm Springs, Calif. — about 200 miles from Parkfield — caused the first of these changes. The study found the Landers quake destabilized the fault near Parkfield, causing a series of magnitude 4 quakes and a notable "aseismic" event — a movement of the fault that played out over several months — in 1993.

The second change in fault strength occurred in conjunction with a magnitude 6 event at Parkfield in September 2004.

The team found another change at Parkfield later that year that could not be accounted for by the September quake alone. Eventually, they were able to narrow the onset of this third shift to a five-day window in late December during which the Sumatran quake occurred.

"The long-range influence of the 2004 Sumatran-Andaman earthquake on this patch of the San Andreas suggests that the quake may have affected other faults, bringing a significant fraction of them closer to failure," Taira said.

The research was supported by the National Science Foundation, the Carnegie Institution of Washington, the University of California, Berkeley, and the U.S. Geological Survey.

http://www.livescience.com/environment/090930-fault-weakening.html
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