Indian Ocean earthquakes triggered quakes globally

AP

Residents flee to higher ground after tsunami warnings April 11. A quake off Indonesia's western coast shook this Thai province of Phuket.

AP

Residents flee to higher ground after tsunami warnings April 11. A quake off Indonesia's western coast shook this Thai province of Phuket.

Giant earthquakes that rocked the Indian Ocean in April had global effects, triggering sizable quakes off the Oregon and Mexican coasts and elsewhere, geologists reported Wednesday.

The undersea quakes on April 11, which measured magnitude 8.6 and 8.2, struck about 300 miles southwest of Indonesia's Aceh province. The larger quake was among the 20 most powerful recorded in the past century, and the pair triggered tsunami warnings around the Indian Ocean. Those were soon canceled when only small waves washed onto coastal beaches. But their effects now appear to have been more far-reaching, an idea once seen as unlikely by geologists.

"The energy from the earthquakes radiated sideways around the planet and likely triggered many more events," says Fred Pollitz of the U.S. Geological Survey in Menlo Park, Calif. He led the analysis in the journal Nature, which looked for links between the quakes April 11 and the jump in quakes of magnitude 5.5 or stronger seen worldwide in the days afterward. The number was five times higher than normal.

The Indonesia quakes were so-called slip-strike quakes in which portions of the Earth's crust slide sideways against each other. The unusual sideways motion of the quakes April 11 prevented them from raising large tsunami waves but appears to have more efficiently sent the "ground waves" they created traveling worldwide. Normally, aftershocks or quakes triggered by large quakes are contained within a relatively local zone.

"Essentially, we're seeing here the entire globe become an aftershock zone of these two earthquakes," says seismologist Aaron Velasco of the University of Texas-El Paso, who was not part of the study. " A decade ago we would have laughed at thinking there was a connection, but we see pretty clear links in this case."

One related quake was the magnitude-7.0 quake that struck in the Gulf of California on April 12. The study says the odds are 1 in 300 of a swarm of quakes like that one happening by chance so soon after the Indian Ocean events. These triggered quakes fell most heavily in regions that seismological measures show were the most stressed by ground waves from the quakes April 11.

Earthquake experts hotly debate a possible link between the magnitude-9.1 Indian Ocean earthquake in 2004 that killed 230,000 people and a large increase in quakes since, says Georgia Tech earthquake expert Zhigang Peng. The triggered events seen from the quakes April 11 don't settle that debate, he says, but the analysis "provides a hope for scientists to search for further evidence, or lack of evidence, to link those great earthquakes."

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Indian Ocean earthquakes triggered quakes globally

AP

Residents flee to higher ground after tsunami warnings April 11. A quake off Indonesia's western coast shook this Thai province of Phuket.

AP

Residents flee to higher ground after tsunami warnings April 11. A quake off Indonesia's western coast shook this Thai province of Phuket.

Giant earthquakes that rocked the Indian Ocean in April had global effects, triggering sizable quakes off the Oregon and Mexican coasts and elsewhere, geologists reported Wednesday.

The undersea quakes on April 11, which measured magnitude 8.6 and 8.2, struck about 300 miles southwest of Indonesia's Aceh province. The larger quake was among the 20 most powerful recorded in the past century, and the pair triggered tsunami warnings around the Indian Ocean. Those were soon canceled when only small waves washed onto coastal beaches. But their effects now appear to have been more far-reaching, an idea once seen as unlikely by geologists.

"The energy from the earthquakes radiated sideways around the planet and likely triggered many more events," says Fred Pollitz of the U.S. Geological Survey in Menlo Park, Calif. He led the analysis in the journal Nature, which looked for links between the quakes April 11 and the jump in quakes of magnitude 5.5 or stronger seen worldwide in the days afterward. The number was five times higher than normal.

The Indonesia quakes were so-called slip-strike quakes in which portions of the Earth's crust slide sideways against each other. The unusual sideways motion of the quakes April 11 prevented them from raising large tsunami waves but appears to have more efficiently sent the "ground waves" they created traveling worldwide. Normally, aftershocks or quakes triggered by large quakes are contained within a relatively local zone.

"Essentially, we're seeing here the entire globe become an aftershock zone of these two earthquakes," says seismologist Aaron Velasco of the University of Texas-El Paso, who was not part of the study. " A decade ago we would have laughed at thinking there was a connection, but we see pretty clear links in this case."

One related quake was the magnitude-7.0 quake that struck in the Gulf of California on April 12. The study says the odds are 1 in 300 of a swarm of quakes like that one happening by chance so soon after the Indian Ocean events. These triggered quakes fell most heavily in regions that seismological measures show were the most stressed by ground waves from the quakes April 11.

Earthquake experts hotly debate a possible link between the magnitude-9.1 Indian Ocean earthquake in 2004 that killed 230,000 people and a large increase in quakes since, says Georgia Tech earthquake expert Zhigang Peng. The triggered events seen from the quakes April 11 don't settle that debate, he says, but the analysis "provides a hope for scientists to search for further evidence, or lack of evidence, to link those great earthquakes."

For more information about reprints & permissions, visit our FAQ's. To report corrections and clarifications, contact Standards Editor Brent Jones. For publication consideration in the newspaper, send comments to letters@usatoday.com. Include name, phone number, city and state for verification. To view our corrections, go to corrections.usatoday.com.

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USGS: Small earthquakes rattling California not related

The small earthquakes that rattled Central and Southern California late last week aren't related to each other, nor are they predictors of larger, more dangerous quakes, scientists say.

U.S. Geological Survey

Neither the earthquakes near Fresno (orange dot, upper left) nor the ones near Los Angeles (orange dot in L.A.) were on the San Andreas Fault (thick red line).

U.S. Geological Survey

Neither the earthquakes near Fresno (orange dot, upper left) nor the ones near Los Angeles (orange dot in L.A.) were on the San Andreas Fault (thick red line).

"As far as we know, they aren't related," says Paul Caruso, a geophysicist with the U.S. Geological Survey in Golden, Colo.

Two quakes — magnitude 4.0 and 4.1 — struck Fresno County on Friday morning. And about six hours earlier, a magnitude 3.4 quake was felt by thousands of people in Beverly Hills.

Kate Hutton, a seismologist at the California Institute of Technology, said Sunday that she doesn't see how an earthquake as small as the magnitude-3.4 temblor in Beverly Hills could trigger a quake as far away as Fresno County. "That's too far," she said. However, she points out that Beverly Hills was struck by a magnitude-3.2 quake last Monday, and that was probably a foreshock of the 3.4 quake Friday.

Neither the Fresno quakes nor the Beverly Hills quakes were on the famed San Andreas Fault, which runs through California.

"California has quakes all the time," says Caruso. On average, Southern California has about 10,000 earthquakes each year, many so small they cannot be felt, the U.S. Geological Survey reports.

No damage or injuries were reported in Friday's earthquakes. Caruso says significant damage usually occurs only at magnitude 5.5 and above. There haven't been any significant quakes in Southern California since Friday, Hutton said.

Additionally, while a small earthquake may temporarily ease stress on a fault line, it does not prevent a larger quake, according to the California Geological Survey.

Nor are the small quakes indicators of bigger events, says John Vidale, a seismologist at the University of Washington in Seattle.

Could the quakes have been triggered by Thursday's big, magnitude-7.6 quake in Costa Rica?

"There might be some controversy about whether they are related, but actually we are too far away from Costa Rica for one quake to influence another that much," Hutton said.

Vidale concurs: There wasn't enough energy in the Costa Rica quake to trigger other quakes as far away as California, he says. Costa Rica and Southern California are about 2,700 miles apart.

Also, on Aug. 25, an earthquake "swarm" was reported in the California desert near Brawley, a few miles north of the Mexican border. Only minor damage was reported. Again, that swarm can't be tied into Friday's quakes, Caruso says.

Contributing: The Associated Press

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