By Sandy Kleffman, Contra Costa Times –
WALNUT CREEK, Calif.—This year’s largest earthquake — a magnitude-8.6 shaker in April in the East Indian Ocean — did little damage but had an unexpected impact: It triggered aftershocks around the world as much as a week later, a new study reveals.
The findings suggest that a major earthquake’s effects can extend farther and last longer than previously thought, shedding light on a threat that one day could set off the San Andreas or Hayward faults in the Bay Area.
Five times more quakes than expected occurred throughout the globe in the six days following the April 11 temblor near Sumatra, seismologists at the University of California, Berkeley and the U.S. Geological Survey found. Major aftershocks jarred people thousands of miles away in Mexico and Japan.
“Seismologists have been telling people for years that you don’t need to worry about large aftershocks being triggered far away,” said Fred Pollitz, a study co-author and seismologist with the Geological Survey in Menlo Park.
Well, it may be time to rethink that theory, he said.
The type of quake that can deliver such a far-reaching punch tends to happen only once every 50 years or so. But it is possible that a large shaker on the other side of the globe could trigger the Bay Area’s two big faults, if it hit when they were ready to rupture, Pollitz said.
A co-author of the study, published online in the journal Nature on Wednesday, agreed that a distant quake could set off shaking here.
“This study now says that, while it is very rare — it may only happen every few decades — it is a real possibility if the right kind of earthquake happens,” said Roland Burgmann, a professor of earth and planetary science at UC Berkeley, in a statement.
Scientists have long known that large earthquakes can trigger small ones, of magnitude 3 or less, around the world. This occurred with the 9.0-magnitude Japan earthquake last year that killed thousands, and the deadly 9.2-magnitude Sumatra quake in 2004.
Seismologists also have known that some moderate to large aftershocks, in the 5 to 6 magnitude range, can occur within 600 miles of a major shaker.
But large aftershocks farther away have been considered rare.
It only takes a few hours for seismic waves to travel through the earth’s crust around the world. So how is it possible that some aftershocks could occur as much as several days later?
Seismologists speculate that the seismic waves passing through don’t rattle the faults enough for them to fail immediately, but that those faults are disturbed enough that a quake occurs a few days later.
The April 11 Indian Ocean temblor had little impact in California, except for minor shaking near Parkfield, where scientists study the San Andreas Fault.
But it is believed to have triggered four West Coast quakes of magnitude 6 or larger: one off Oregon, two in the Gulf of California, and one in an ocean area known as the Mexican trench.
“That was within 24 hours after the main shock,” Pollitz said. “That’s unusual because normally we get one magnitude 6 event every three days, so to get four in one day is a big acceleration.”
All told, 16 quakes of magnitude 5.5 or greater happened throughout the world in the week following the Indian Ocean quake. Normally, one such quake occurs per day.
So what made the Indian Ocean quake so unusual? It was on a “strike-slip” fault, which occurs when one land mass slides past the other. This type of quake generates robust seismic waves that travel just underneath the earth’s surface and can reach distant areas.
The San Andreas and Hayward faults are also the strike-slip” type.
Up to 90 percent of the world’s largest quakes, on the other hand, are on what are known as “thrust” faults, in which one large plate of land descends below another, Pollitz said. These are considered much less likely to trigger far-ranging aftershocks.
The Indian Ocean quake also was unusual because an abnormally low number of earthquakes occurred in the six to 12 days leading up to it. That may have resulted in more faults being close to rupturing and thus more easily triggered by a shock wave, the researchers said.