A huge earthquake that struck beneath the Indian Ocean in April triggered an extraordinary burst of large quakes around the world that alerted scientists to new seismic hazards everywhere.
Seismologists have known for more than 20 years that major earthquakes can trigger small seismic slips thousands of miles away. But the one on April 11 caused what scientists termed an astonishing increase in dangerous quake activity across five continents for at least six days, with temblors in among other places, Japan, Alaska, Mexico and off the coast of Oregon.
It was a highly unusual event: At least four seismic faults deep underground off the island of Sumatra ruptured within 100 seconds to create a single giant quake with a combined magnitude of 8.7. No major problems occurred anywhere on land, but its effects worldwide call for a new assessment of global dangers, said scientists reporting in the current issue of Nature.
The quake was the largest of its kind ever recorded, they said. And two hours later a fifth fault ruptured nearby, resulting in another major quake with a magnitude of 8.2.
Among the scores of temblors that followed for nearly a week, 16 were registered with magnitudes greater than 5.5, and one of the strongest, in Baja California, had a magnitude of 7.0, larger than the deadly Loma Prieta quake of 1989.
''While the likelihood of events this big is extremely rare, it tells us that when it's over it's not over, and that there's no such thing as an all clear," said Ross S. Stein, a U.S. Geological Survey geophysicist and specialist in seismic hazards.
The unexpectedly dangerous quake in the Indian Ocean was a "strike-slip" temblor, the same type as the infamous San Francisco quake of 1906 on the San Andreas fault, where one side of the Earth's crust moved horizontally past the other side.
But in the far larger seabed quake near Sumatra , the fault rupture sent seismic shock waves speeding through the Earth's surface in every direction, triggering quakes as far as 12,500 miles away.
Scientists at the USGS in Menlo Park, Calif., and those at the University of California campuses in Berkeley and Santa Cruz, the University of Utah and at two French seismic institutions analyzed the events and published three major reports Wednesday in Nature.
The number and size of the triggered quakes was "astonishing," Stein said. At least 16 quakes registered at magnitudes larger than 5.5, representing a sudden five-fold increase in seismic activity around the world, he noted.
Surface waves from the undersea quake -- known as Love waves -- raced across the world in three hours, triggering scores of small temblors, the scientists reported. Some faults as far away as Mexico and Japan ruptured six days later with unusually strong ground shaking.
''If those quakes had been in an urban area, they could potential ly have been disastrous," said Roland Burgmann, a UC Berkeley geophysicist and co-author of the Nature report along with Fred Pollitz, Stein and Volkan Sevilgen, of the Geological Survey.
Some of the faults that ruptured into temblors and were triggered by the Indian Ocean quake were already unstable, noted Pollitz, a research geophysicist at USGS.
''We now know that quakes as big as the April one can be felt all over the world and can trigger other quakes everywhere when they are almost ready to fail," he said.
The April quake had struck in the same ocean region as the powerful and much more destructive quake of 2004, which struck with a magnitude 9.1 and generated the tsunami that killed more than 200,000 people. That one was similar to Japan's offshore Tohuku quake of March, 2011, which also caused a tsunami that destroyed the nation's largest nuclear power plant and drowned 20,000 people.
Those two temblors, however, did not trigger any observable quakes anywhere, the scientists said, and their seismic motion was entirely different. They were called "megathrust" quakes, where one side of the fault in each quake suddenly thrust itself over the other side rather than sliding past each other. That kind of seismic motion does not trigger the shock waves that travel as far as strike-slip quakes, nor do they trigger detectable quakes anywhere else, Pollitz said.
In another Nature report, Thorne Lay and Han Yue of UC Santa Cruz and Keith Koper of the University of Utah analyzed the April quake's unusually complicated fault ruptures. The resulting quake, they said, apparently broke through the upper part of a huge tectonic slab of the Earth's crust called the Indo-Australian Plate that butts against the Indian subcontinent.
As a result, Lay said, the plate is apparently continuing to fragment into two separate tectonic plates and ultimately will create a new plate boundary -- a process that will take millions of years and require hundreds if not thousands of similar earthquakes before it is complete, he said.