Major quakes can weaken seismic faults far away, scientists say

Yahoo News 30 Sep 09;

PARIS (AFP) – Huge earthquakes can weaken seismic faults on the other side of the world, scientists in California said on Wednesday.

Their study coincided with a major 8.0-magnitude quake in the Pacific, unleashing a tsunami that killed scores of people in the Samoan islands and Tonga.

Seismologists led by Taka'aki Taira of the University of California at Berkeley found that the 9.1 monster that struck west of Sumatra in December 2004 weakened a closely-monitored segment on California's San Andreas fault, 8,000 kilometers (5,000 miles) distant.

Their investigation is based on a scan of 22 years of data from the Parkfield area, a district so studded with borehole seismometers and other gauges that it has been dubbed "the earthquake capital of the world."

The monitors found that areas of fluid-filled fractures lie within this section of the fault.

Driven by seismic pressure, the fluid migrates along the fault like spidery veins in marble, acting as a lubricant that enables shocks to pry open the rock, they believe.

Proof of this suspicion came with the finding that repetitive background quakes became smaller and smaller during periods of fluid shift -- in other words, as the fault slowly weakened, less energy was needed to shake it.

But the most remarkable finding was unexpected impacts from two big, distant quakes -- a 7.3-magnitude shake near the Californian town of Landers in 1992 and the 2004 Sumatra behemoth that unleashed the Indian Ocean tsunami.

Almost five days after Sumatra event -- one of the biggest quakes in recorded history -- sensors noted dynamic stress on the Parkfield fault at a depth of five kilometres (three miles).

The study, published by the British weekly science journal Nature, provides compelling support for a novel theory that very big quakes can have a cascade effect elsewhere, sometimes months afterwards, say the researchers.

"The long-range influence of the 2004 Sumatra-Andaman earthquake on this patch of the San Andreas fault suggests that many of the world's active faults were affected in the same way, thus bringing a significant number of them to failure," the study says.

"This hypothesis appears to be supported by the unusually high number of quakes of magnitude eight or above occurring in the three years" after the 2004 event, it said.

"No other large earthquake, of magnitude eight or more, since 1900 was followed by as many for a comparable period," it observed.

The team hopes their work will yield a technique for assessing the strength of a seismic fault -- testing whether it has the strength to resist a shock or rip apart and threaten human life.

Discreet changes in the seismic wave, corresponding to periods when the numbers of small earthquakes intensifies, can be quantified into a means of pinpointing faults that are likely to fail, Taira believes.

Predicting when earthquakes will strike remains an over-the-horizon prospect, although strides have been made into assessing how stress builds up in a fault deep underground.

"Earthquakes are caused when a fault fails, either because of the buildup of stress or because of a weakening of the fault," said Taira in a press release.

"Changes in fault strength are much harder to measure than changes in stress, especially for faults deep in the crust. Our results open up exciting possibilities for monitoring seismic risk and understanding the causes of earthquakes."

Earth Shakes Twice: Is the Planet On a Roll?
Andrea Thompson, livescience.com Yahoo News 30 Sep 09;

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.

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.

Asian quake could trigger California's big one
New Scientist 1 Oct 09;

IT'S a kind of geological butterfly effect. Fenglin Niu of Rice University in Houston, Texas, and colleagues believe they have found two clear cases where remote events weakened the San Andreas fault near Parkfield, California. The finding suggests powerful earthquakes - like the one that has just hit Sumatra - may trigger further quakes worldwide.

The first changes to the San Andreas occurred in 1992 after a 7.3-magnitude earthquake several hundred kilometres to the south. The second took place in 2004 after a quake of magnitude 9.1, also in Sumatra, 8000 kilometres away. In both cases, there were distinct changes in the movement of fluids and an increase in the frequency of micro-earthquakes deep within the fault below Parkfield (Nature, DOI: 10.1038/nature08395).

Niu and colleagues believe these changes are linked to a weakening of the fault, and that monitoring them could lead to more accurate earthquake forecasts. They suggest that very large quakes might push faults all round the world closer to the point of failure, and so lead to a temporary increase in global seismicity.
Rumble relation?

So does Niu's finding suggest the two quakes that struck the Pacific on 30 September were linked?

The magnitude-8.0 rumble that sent a tsunami towards the Samoan islands on Wednesday was followed within a matter of hours by the magnitude-7.6 quake on the island of Sumatra in Indonesia. However, other scientists have been quick to rule out a link between the two, pointing out they occurred on separate fault systems.

Niu says this may have been too hasty. "It's possible that the Samoan earthquake did trigger Indonesia's earthquake," says Niu, not least because the epicentres are geographically close.

However, he concedes that the Samoan tremor may not have been big enough to produce the same kind of fault weakening his team observed.

Were This Week's Pacific Earthquakes Connected?
Richard A. Lovett, National Geographic News 2 Oct 09;

In the past four days there've been at least five substantial earthquakes—and a tsunami—in the same general region, the tectonically rambunctious Ring of Fire in the Pacific Ocean.

Coincidence? Maybe yes, but probably not, scientists say.

The Ring of Fire jaggedly circles the Pacific and cuts roughly beneath Samoa, American Samoa, and Tonga—the region where Tuesday's magnitude 8.0 undersea earthquake triggered the tsunami that killed at least 180 people and where a magnitude 6.3 quake struck today.

Also along the ring are Indonesia—where a magnitude 7.6 quake Wednesday and a 6.3 temblor on Thursday may result in a death toll in the thousands—and Peru, where a 5.9 quake struck a remote region on Wednesday.

While new evidence says earthquakes can aggravate far-away fault lines, scientists are reluctant to say that the Samoa, Indonesia, and Peru quakes are linked.

To begin with, earthquakes aren't that infrequent.

Earthquakes the size of the event in Peru, for example, occur about a hundred times a year, said Emile Okal, a geophysics professor at Northwestern University in Illinois.

This means that, on average, there's a quake that powerful somewhere about every three days, Okal said. "It's the normal way of things."

Even the one-two punch of the Tuesday's Samoan and Wednesday's Indonesian earthquakes isn't unprecedented. On average, temblors that size occur once a month.

"The odds of [two quakes on] back-to-back days are not terribly bad," Okal said.