Were This Week's Pacific Earthquakes Connected?

[James]

Were This Week's Pacific Earthquakes Connected?
Richard A. Lovett
for National Geographic News
October 2, 2009

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.

Related
• Samoa and Tonga tsunami pictures
• Tsunami video from this week

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.

[James]

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.

Video: Earthquakes Rock Indonesia, Tonga, Samoa

[James]

Hastened by 2004 Indian Ocean Mega-Quake?

Nevertheless, it's possible for earthquakes to trigger each other, scientists say. Tectonic stresses from the quake that sparked the catastrophic 2004 Indian Ocean tsunami, for example, may have triggered this week's Indonesian earthquakes on the island of Sumatra.

"The [recent] earthquake is about 470 kilometers [290 miles] away from the 2004 earthquake [epicenter]," said Fengling Niu, a seismologist at Rice University, in an email.

But, Northwestern's Okal said, that type of stress transfer occurs only at relatively short range—"1,000 kilometers [600 miles] or something like that," he said. "To extend this to Samoa seems far-fetched," given that Samoa is roughly 4,000 miles [6,400 kilometers] from Indonesia.

That's especially true, he added, because there are a number of faults between the two regions that would interfere with stress transfer, even if the distance weren't so great.

[James]

Earthquake Fault Weakening

In a study released this week, though, a team that included Rice's Niu found that earthquake vibrations may affect faults at surprisingly great distances.

In particular, the researchers found that vibrations from the 2004 Indonesian earthquake may have increased the frequency of small earthquakes in California's San Andreas Fault by causing fluids to move into the fault lines. Such an inflow would have lubricated the fault, making it more likely that the two sides would slip and slide.

Still, the study team is cautious about suggesting the same thing happened with this week's two large quakes.

"The question 'Are these associated?' is very interesting, but we do not know yet," said Taka'aki Taira, a seismologist at the University of California, Berkeley, and lead author of the new study, published Thursday in the journal Nature.

"Given our finding, it might be possible that they are linked to each other," he said in an email. "But it is too early to make a conclusion, and we do not yet have any evidence that they are associated."

http://news.nationalgeographic.com/news/2009/10/091002-earthquakes-tsunami-connected-indonesia.html

[James]

Earthquakes big and small, depicted as squares on a USGS map, struck south of the Samoan islands between September 25 and October 2, 2009.

In a four-day period during that time span, big quakes also hit two other regions along the Pacific's seismically active Ring of Fire. The back-to-back earthquakes may—or may not—have set each other off, experts say.


Maps courtesy USGS

[James]

Some 80 percent of all the planet's earthquakes occur along the rim of the Pacific Ocean, called the "Ring of Fire" because of the preponderance of volcanic activity there as well. Most earthquakes occur at fault zones, where tectonic plates—giant rock slabs that make up the Earth's upper layer—collide or slide against each other. These impacts are usually gradual and unnoticeable on the surface; however, immense stress can build up between plates. When this stress is released quickly, it sends massive vibrations, called seismic waves, often hundreds of miles through the rock and up to the surface. Other quakes can occur far from faults zones when plates are stretched or squeezed.

Scientists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves. A quake measuring 3 to 5 is considered minor or light; 5 to 7 is moderate to strong; 7 to 8 is major; and 8 or more is great.

On average, a magnitude 8 quake strikes somewhere every year and some 10,000 people die in earthquakes annually. Collapsing buildings claim by far the majority of lives, but the destruction is often compounded by mud slides, fires, floods, or tsunamis. Smaller temblors that usually occur in the days following a large earthquake can complicate rescue efforts and cause further death and destruction.

[James]

A 1976 earthquake near Guatemala City shattered this bridge in Agua Caliente, cutting off the city’s main supply route to the Atlantic. The 7.5-magnitude quake killed more than 23,000 people and left thousands more injured and homeless.
Photograph by Robert W. Madden

[James]

A crane and several construction vehicles lay toppled on a fractured road in Kobe, Japan, after a 7.2-magnitude temblor shook the quake-prone country. The Great Hanshin Earthquake Disaster of 1995 was one of the worst in Japan’s history, killing 6,433 people and causing more than $100 billion in damages.
Photograph by Karen Kasmauski

[James]

A steel-fortified railroad lies twisted like a toy after a 7.2-magnitude earthquake rocked Kobe, Japan, in 1995. The earthquake was the biggest to hit Japan in 47 years and shook the city for 20 seconds.
Photograph from Pacific Press Service/Alamy

[James]

The San Andreas Fault scars Southern California’s Carrizo Plain like a battle wound. The 800-mile (1,300-kilometer) fault runs through western and southern California, dividing the Pacific and North American plates.
Photograph by Phil Degginger/Alamy

[James]

Workers position support beams to steady tilting homes in San Francisco's Marina District after a disastrous earthquake hit the city in 1989. The 7.1-magnitude earthquake buckled highways and bridges, crushed cars, and toppled homes and buildings throughout the city.
Photograph by Michael K. Nichols