Temblor in Chile gives California insight - SignOnSanDiego.com
Temblor in Chile gives California insight
By Scott LaFee, UNION-TRIBUNE STAFF WRITER
Thursday, March 4, 2010 at 12:59 a.m / AP
Soldiers patrol in Concepcion , Chile, Wednesday, March 3, 2010. An 8.8-magnitude earthquake struck central Chile early Saturday. The government sent soldiers and ordered a 6 p.m. to -noon curfew to quell looting. (AP Photo/ Natacha Pisarenko)
When an earthquake is powerful enough to shift Earth’s axis, even the toughest building codes in the world won’t stand tall.
The Saturday temblor in Chile registered a magnitude of 8.8. It was the world’s fifth-largest quake since 1900, strong enough to tweak the planet’s axis more than 3 inches and speed its spin ever so slightly.
Chile’s stringent building codes appear to have limited the quake’s worst effects to mostly older, seismically susceptible structures. They also may have provided a peek into how California might fare in a massive temblor.
In dealing with their long, mutual history of unsettled ground, Chile and California have created some of the toughest, most rigorously enforced seismic building codes and design standards in the world. But those rules and laws are no guarantee of safety, scientists and engineers said.
“People think the codes are intended to eliminate damage. They aren’t,” said Tom Rockwell, a professor of geological sciences at San Diego State University. “They’re meant to mitigate collapse and death. If we have a large earthquake, there will be damage. The hope is that it will not be deadly or catastrophic.”
Earthquake standards and building codes in California reflect an always-evolving assessment of risk and the function and value of different kinds of infrastructure, said Jorge Meneses, chairman of the seismic and geohazards practice group in the San Diego offices of Kleinfelder Inc., a national construction and engineering company.
The degree of acceptable damage depends on the structure’s importance, he said.
While modern homes may not crumble in a temblor, they could still suffer irreparable damage and have to be torn down, Meneses said. “Things like bridges, highways and hospitals are designed to withstand some damage and remain operating because they are vital to any recovery. And something like a nuclear power plant is extremely engineered because even the smallest amount of damage might have catastrophic consequences.”
Then there are the older structures.
“Some people think we don’t have ‘bad buildings’ in California,” said Richard McCarthy, director of the state’s Seismic Safety Commission. “We have thousands of them — structures made of unreinforced masonry or concrete or built to now-inadequate standards and not retrofitted. They’re all vulnerable to coming down.”
A major temblor in California is inevitable.
The U.S. Geological Survey said there’s a more than 99 percent chance that one or more quakes with a magnitude of at least 6.7 will occur somewhere in the state during the next 30 years. The chance of a quake registering 7.5 or greater in that time is 46 percent, and the epicenter would most likely be in Southern California.
But a “Big One” in California would not be like a big quake in Chile. The southern San Andreas fault zone, which extends from the Salton Sea to the town of Parkfield in Monterey County, poses the greatest seismic threat. But it isn’t capable of producing a magnitude-8.8 temblor, seismologists said.
California’s plate tectonics differ fundamentally from Chile’s. The San Andreas fault grinds horizontally rather than subducting — one plate diving below another. Also, the planet’s crust below California is thinner.
“To get a really big quake, you need a very large area and a lot of displacement,” Rockwell said. “The crust (beneath California) isn’t thick enough. Even in a worst-case scenario, I don’t think we would see anything larger than an 8.0 or 8.1.”
California has recorded just two earthquakes approaching magnitude 8: the 7.9 Tejon quake in 1857 and the 7.8 San Francisco quake in 1906.
Locally generated quakes tend to be much smaller, in the range of magnitude 3 to 5, though some active offshore fault zones may be capable of generating quakes of up to 7.7.
The deeper, stronger temblors in Chile often last for up to two minutes, while California quakes typically persist for 10 to 15 seconds, said Frieder Seible, dean of the Jacobs School of Engineering at the University of California San Diego and an international expert on bridge and highway seismic safety.
Still, researchers such as Seible said every strong quake is potentially devastating and that most offer new insights. Both the magnitude-6.9 Loma Prieta temblor in 1989, which killed 63 people and caused $6 billion in damage, and the 6.7 Northridge quake in 1994, which killed 72 people and caused $20 billion in damage, resulted in significant revisions to seismic standards and building codes.
Earthquake damage is essentially the result of seismic motions beneath a structure, the nature of the soil it was built on and its construction.
In the Loma Prieta quake, 42 people died when a half-mile portion of the Cypress Freeway collapsed. The section had been constructed on man-made fill layered over soft mud. Adjacent sections of the freeway, built upon older, firmer sand and gravel deposits, didn’t collapse.
“We learn new things after every quake, and much of it has been implemented, such as the huge, $16 billion retrofitting effort of bridges and highways since 1989,” said Jose Restrepo, a UCSD professor of structural engineering. “I think new homes, too, are now well-constructed and would perform OK in a quake.”
Scott LaFee: (619) 293-1259; scott.lafee@uniontrib.com
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