Caltech scientists are being forced to rethink their understanding of the San Andreas Fault – previously thought to be incapable of producing a quake larger than magnitude 8.0. An 8.6 magnitude quake in Sumatra in April struck along a similar fault system as the one that threatens SoCal with the “Big One.” Patrick Healy reports from Cal Tech for the…
Californians know it's coming: waves of shaking across the Southland when the San Andreas fault inevitably lets loose the “Big One.”
The scenario developed in recent years for planning purposes assumes a magnitude as large as 7.8. But what if it might be even bigger? Maybe ten times bigger than previously thought possible?
"I think we do need to rethink how large an event it could be," said Victor Tsai, PhD, a geophysicist at the California Institute of Technology.
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Tsai is a member of the research team that came to that eye-opening conclusion after studying a recent large quake that occurred half a world away, off the western coast of Sumatra.
The April 11 quake was larger than previously thought possible – magnitude 8.6 from a type of fault system that's not supposed to produce quakes that large.
Significantly for Southern California, it is the same type of fault mechanism as the San Andreas.
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Both are so-called strike-slip faults, which are not capable of producing the giant quakes of magnitude 9-plus that occur when tectonic plates suddenly move in subduction zones. In fact, conventional wisdom among seismologists has long held that strike-slip faults are not capable of producing quakes much larger than magnitude 8.0.
The Sumatra quake three months ago changes that.
"It's not clear what the ceiling would be," Tsai said.
Quake magnitude is measured on a logarithmic scale. An increase in magnitude of one point represents a 30-fold increase in the power of the quake. Even half a point larger can dramatically increase the destructive power of a quake.
So how large could California's Big One be? Tsai is cautious.
"As our research has shown, we don't really know what's possible," he said.
But it did reveal the apparent mechanism that explains the Sumatra quake's unexpected extra magnitude.
Using sophisticated analysis to determine what happened under the ocean, the CalTech team concluded that the April 11 quake occurred on at least four distinct faults, the quake spreading east to west, and at one point simultaneously north and south, propagating from one fault to the next almost like dominoes toppling.
What's more, the quake movement extended far deeper than is typical, extending below the earth's crust and miles into the mantle, the additional depth releasing even more energy.
"We might have to change our modeling of the San Andreas," Tsai said.
Modeling the impact of a 7.8 magnitude quake on the lower San Andreas yielded projections of thousands of casualties and billions of dollars in damage from desert communities to the Los Angeles basin.
It would take weeks or months to restore utilities and transportation corridors. Most tall buildings would survive, but a handful of downtown's skyscrapers would be expected to collapse.
Tsai emphasized there is no certainty the San Andreas will someday behave in the same fashion as the April 11 quake off Sumatra and produce a quake even larger than 7.8. But it raises the possibility the scenario for Southern California's anticipated "Big One" significantly underestimates the harm it inevitably will do.
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