Indian Ocean earthquakes triggered quakes globally

• April 11 quakes triggered tsunami warnings
• April quakes tied to rise in quakes around world
• Quakes' sideways motion may explain their distant effects

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."