Dolan Unearths Subterranean Terror
By Usha Sutliff, USC News Service
The Big One could be near at hand. A fault system that runs under downtown Los Angeles could generate an earthquake of magnitude 7.0 or greater, according to James Dolan, an earth sciences professor at USC College. Dolan’s research is the first to determine the frequency and magnitude of past earthquakes along the Puente Hills fault. His work also offers insight into when the active fault may rupture next and what havoc it could wreak.
“What we’ve determined is that the Puente Hills fault breaks infrequently, every several thousand years,” Dolan says. “[But] if this fault were to rupture in its entirety, it would certainly be capable of generating an earthquake in excess of a magnitude 7. USC would go up a meter or two relative to the area just south of the Coliseum. Downtown, and everything north of USC, would also be lifted by a meter or two.”
These findings are important because they give geologists a clearer picture of the hazard the fault poses to the city, says Joseph Aoun, dean of USC College. “Eventually, this will help us better forecast where the most damaging earthquakes might occur in Los Angeles and help city leaders better prepare for that potential.”
The fault snakes underground for at least 25 miles, from Puente Hills in northern Orange County through downtown Los Angeles and west toward Beverly Hills. Its depth varies. It runs about two miles beneath USC, for example, and dips to about four miles underneath the downtown high-rise district.
Dolan and his team—which included Harvard University earth sciences professor John Shaw and USC graduate student Shari Christofferson—bored 15 holes into the earth in Bellflower near the San Gabriel River. The spot they chose was ideal because layer upon layer of sediment had been deposited in a flat area during floods.
As the team pulled the cores from the earth, they looked for places where pressure had forced the rock above the fault to fold, or bend, after major earthquakes. Since the same layers existed in each core, they were able to connect them, creating a three-dimensional picture that revealed major displacements of the earth’s crust.
“What we’ve demonstrated is that, during the past 11,000 years, the Puente Hills fault has broken at least four times, generating very large earthquakes well in excess of magnitude 7,” Dolan says, noting that while the last event happened during the past several thousand years, it was impossible to say exactly when.
An important aspect of the Puente Hills fault is that it extends through the Los Angeles basin, a highly populated area made up of soft sediments that amplify shaking. “So, obviously, very large earthquakes this close to the high-rise district are of concern,” Dolan says. “This fault is in one of the worst places you could think of to put a fault of this size and geometry.”
Each year, the Los Angeles basin is squeezed a few millimeters between colliding tectonic plates. The resulting stress is stored along the Puente Hills fault and a web of other faults that crisscross the Los Angeles region—until the day it is released in an earthquake.
But Dolan can’t say when that day will come. “We are currently in a seismic lull that has lasted at least 200 years,” he says. “And that can’t last forever.”
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