Southland Has Hundreds of Faults

The discovery of a minor fault trace running beneath the Belmont Learning Center site is no surprise to Southern California geologists: The entire region is crisscrossed by hundreds of large and small faults, many of them unidentified.

“It’s very difficult to go to any part of Southern California and find a piece that doesn’t have faults cutting through it,” said James Dolan, a geologist at USC. “The question is, is the fault active? Most of them probably aren’t.”

What complicates matters in the Belmont case -- and for any school in the Los Angeles Basin -- is that the major faults in this area are blind-thrust faults that are buried and therefore difficult to detect or avoid.

Surface-rupturing faults such the San Andreas are easier to build around because they leave a huge trace on the surface. State law prohibits the construction of schools or occupied buildings within 50 feet of surface ruptures but does not specifically ban construction above blind-thrust faults, which can cause damage over a vast territory, geological survey officials said.

“When the faults are miles underground and angled, like the fault system running beneath Belmont, it is difficult to say exactly which parts of the ground above are vulnerable to earthquake shaking,” said Susan Hough, a seismologist with the U.S. Geological Survey office in Pasadena.

“You just can’t pinpoint a line and say this is where it’s going to shake. Strong shaking would be smeared across a larger area.... You’re hard-pressed to say one site in L.A. is much worse than any other.”

Thomas Heaton, a professor of engineering and seismology at Caltech agreed: “How do you site anything in the L.A. Basin?” he asked. “The issue becomes one not of where do you site them but how do you build them?”

Experts say the legal requirements for placement of schools and other critical buildings are lagging behind earthquake science, which has only in the last decade revealed the hidden dangers of thrust faults. Scientists say they are now on the verge of predicting shaking and other hazards locally -- work that will help guide school construction.

For now, many experts advocate making schools and other critical buildings such as hospital and fire stations strong enough to withstand the largest quake predicted for a particular building site, even if there is only a small chance such a large quake would occur.

District officials said Wednesday that they wanted to err on the side of caution. Although the newly discovered fault is part of a blind-thrust fault system, geologists hired by the district said they fear the fault could cause damage close to the surface or even rupture the ground and harm buildings to the point that children could be injured.

The Belmont complex, like much of downtown, is above the Elysian Park fault system. The fault system, smaller and less active than the San Andreas, is thought to be capable of generating 6.7 magnitude quakes.

Further complicating matters are the toxic gases buried within the site. It is possible for an earthquake to unleash a toxic cloud during a rupture, even with mitigation measures, but geologists are uncertain about the likelihood of such a scenario.

Geologists studying this fault system, like Dolan, say it may not be active at all. At the very least, it appears to be far less active than it was millions of years ago. Trying to determine whether the fault is still active and how often it may have spawned large temblors is difficult because such faults are often miles underground.

The current survey, conducted by Caltech geologist Kerry Sieh, could not determine whether the fault underneath Belmont is active because much of the soil at the surface needed for analysis had been removed for oil recovery or during construction of the school.

Lest there be any doubt of the potential for damage by an active blind-thrust fault, the 6.7 Northridge quake in 1994 was caused by one. Since then, geologists have become increasingly aware that the populous Los Angeles Basin faces more of a hazard from widespread shaking caused by the half-dozen major blind thrusts here than from the distant San Andreas.

“If any of those faults go,” Hough said, “the L.A. Basin is going to shake like a bowl of Jell-O.”