Were Last Week's Pacific Earthquakes Connected?

Richard A. Lovett
for National Geographic News
Updated October 5, 2009

In four days last week 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.

Samoa and Tonga tsunami pictures
Tsunami video from last 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.

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

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 last 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.

Earthquake Fault Weakening

In a study released last 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 last 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."




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