A fault system that runs under downtown Los Angeles has generated at least four major earthquakes - magnitude 7.0 or greater - in roughly the past 11,000 years, according to research published in the April 4 edition of the journal Science.

The research - led by James Dolan, an earth sciences professor in the USC College of Letters, Arts & Sciences - is the first to determine the frequency and magnitude of past earthquakes along the so-called Puente Hills fault.

In broad terms, it also offers insight into when the active fault may rupture next and the havoc it could wreak.

“What we’ve determined is that the Puente Hills fault breaks infrequently, every several thousand years,” Dolan said.

But, he added, “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 Los Angeles Coliseum. Downtown, and everything north of USC, would also be lifted by a meter or two.”

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 the Coliseum and USC, for example, and dips to about four miles deep underneath the downtown high-rise district.

Because it never hits the surface, it is referred to as a “blind-thrust” fault, much like the one that caused the magnitude 6.7 Northridge earthquake in 1994.

The Puente Hills fault was discovered in 1999 by a team led by Harvard University earth sciences professor John Shaw, a co-author on Dolan’s Science paper.

Shaw had a hunch that the 1987 Whittier Narrows earthquake, which happened about nine miles from downtown Los Angeles, was located on an undiscovered, underground fault.

He found his proof in data collected by oil companies that had searched the area for deposits of petroleum trapped in folds of rock above blind-thrust faults.

Shaw and his team used the petroleum industry data - which was gathered with a technique some compare to taking a sonogram of an unborn child - and pinpointed the locations of the Whittier earthquake’s aftershocks.

The researchers determined that a small piece of the Puente Hills fault had broken, triggering the 1987 earthquake.

If the oil company technique was a sonogram, Dolan’s was amniocentesis. He and his team - which included Shaw and USC graduate student Shari Christofferson - bored 15 holes into the earth in Bellflower where the upward tip of the fault lies nearly two miles beneath the surface.

The spot they chose, near the San Gabriel River, 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, it 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, possibly as large as magnitude 7.5,” Dolan said, noting that while the last event happened during the past several thousand years, it was impossible to say exactly when.

The work is important because it gives geologists a clearer picture of the hazard the fault poses to the city of Los Angeles, said Joseph Aoun, dean of the USC College of Letters, Arts & Sciences.

“We have a much clearer picture of the active fault system underlying Los Angeles thanks to this study,” Aoun said.

"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."

Compared to the magnitude 6.7 Northridge earthquake, a major quake along the Puente Hills fault would release 10 to 15 times more energy, Dolan said. That means the earthquake would be felt over a much wider area, shake longer and generate more of the lower frequency seismic waves that affect tall buildings.

“So, obviously, very large earthquakes this close to the high-rise district are of concern,” Dolan said.

Another 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.

“This fault is in one of the worst places you could think of to put a fault of this size and geometry,” Dolan said.

Each year, the Los Angeles basin is squeezed a few millimeters from north to south as plates of the Earth’s crust carrying North America and part of the Pacific Ocean floor converge.

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 since the first Europeans arrived here more than 200 years ago,” he said, “and that can’t last forever.

“Having said that, we don’t know when this lull will end.”

Contact Usha Sutliff at (213) 740-0252.