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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Major quakes strike in Pacific off Alaska (Reuters)

ANCHORAGE, Alaska (Reuters) – A major earthquake of 7.4 magnitude struck in the Pacific Ocean more than 1,000 miles west of Anchorage on Thursday, prompting a brief tsunami warning for part of the remote Aleutian Islands chain.

No damage or injuries were reported. The warning, which extended for roughly 800 miles -- from Unimak Pass, northeast of Dutch Harbor, westward to Amchitka Pass, west of Adak Island -- was canceled after a little more than an hour.

A tsunami wave measuring just 6 centimeters tall was recorded at Nikolski, a tiny Aleut village on the island of Umnak, and a 10-centimeter wave was observed at Adak, said Becki Legatt, a spokeswoman for the West Coast and Alaska Tsunami Warning Center in Palmer, Alaska.

The coast of the entire Alaska peninsula and all of the Alaska mainland were never considered to be threatened.

The quake struck shortly after 7 p.m. local time at a depth of about 25 miles. A second tremor of magnitude 7.2 hit in the same vicinity of the Aleutians a half-minute later, the U.S. Geological Survey said.

Quakes of 7 to 8 magnitudes and higher are relatively common in the Aleutians but are generally of little consequence because the island chain is so remote and sparsely populated.

"This is a very seismically active area," said Randy Baldwin, a USGS geophysicist with the National Earthquake Information Center in Golden, Colorado.

A tsunami warning means all coastal residents in the warning area who are near the beach or in low-lying regions should move immediately to higher ground and away from harbors and inlets, including those sheltered directly from the sea.

(Reporting by Yereth Rosen in Anchorage; Writing by Steve Gorman; Editing by Philip Barbara)

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