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New Study Reveals the Behavior of the Puente Hills Thrust Fault
In 1999, a team of scientists, led by Harvard University professor John Shaw, announced the discovery of a blind thrust fault running from northern Orange County to downtown Los Angeles. Though its discovery was, in some ways, foretold by the 1987 Whittier Narrows earthquake, which ruptured a small segment of the fault, the revelation of a "new" major fault stunned many residents of the Los Angeles area. Dubbed the Puente Hills thrust fault after the highlands above the eastern end of its 25-mile (40-kilometer) length, such a large fault located directly beneath downtown and other heavily developed areas seemed ominous and threatening. At the time, however, there was no information on how much energy might be released along this newly-mapped fault, and how frequently these releases of energy -- what we all know as earthquakes -- might occur.
Now SCEC scientist and USC earth sciences professor James Dolan, along with USC graduate student Shari Christofferson and the Puente Hills fault's original discoverer, John Shaw of Harvard University, have published the results of a study, supported by the USGS and SCEC, designed to answer those questions. Found in the April 4 edition of the journal Science, these results show that the Puente Hills thrust fault has experienced four major earthquakes in the past 11,000 years, and will almost certainly produce another major earthquake at some point in the future.
The good news, Dolan explained in interviews with local media, is that these findings show that it is several thousand years between devastating earthquakes on the Puente Hills fault. As just one of a few dozen major faults in and around the Los Angeles metropolitan area, it doesn't add much to the overall seismic hazard of Los Angeles. The findings of Dolan's group will, however, help refine the seismic hazard assessment for the area; knowing the threat is the first step in managing it. Another plus: the methods developed during this research may be applied to other, similar areas of seismic hazard that are difficult to study using more conventional methods.
The bad news, Dolan continued, is that when the Puente Hills thrust fault ruptures in an earthquake, it tends to do so in a very big way. Evidence from the new study, the first ever to probe the behavior of the Puente Hills fault, indicates that the four past earthquakes discovered along the fault had magnitudes between MW7.2 and 7.5 -- 10 to 15 times larger than the 1994 Northridge earthquake (MW6.7), the costliest earthquake disaster in U.S. history. In a magnitude 7 earthquake, not only would more energy be released, causing a larger area to experience damaging levels of shaking, but the shaking would last longer and more of the lower frequency seismic waves that affect tall buildings and other large structures would be generated.
"So, obviously, very large earthquakes this close to the high-rise district are of concern," Dolan said.
Worrisome, too, are the changes that would result from ground uplift during such an earthquake. According to Dolan, in a single magnitude 7 earthquake along the Puente Hills thrust fault, "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." Adding to the bad news is the fact that the Puente Hills fault 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.
Some of these sediments, however, were the key to the study released this week. The Puente Hills thrust fault is "blind" because it never breaks through at the surface, instead producing folds in the rocks above it, which may sometimes be expressed as chain of low hills, raised up by successive earthquakes along the fault below. Each year, the Los Angeles basin is squeezed a few millimeters from north to south as tectonic plates of the Earth's crust carrying North America and part of the Pacific Ocean floor converge. This squeezing is accommodated by the many faults in and around the basin, which shift to release the energy of this compression as earthquakes.
Sediments from nearby mountains wash down into the basin and are deposited in the flood plains along rivers and streams. As a rule, these sediments are deposited horizontally. When an earthquake uplifts the ground surface, this disrupts the smooth horizontal profile of the sediments. Gradually, this disturbance is overlain with new, horizontal sediments. Eventually, another earthquake occurs, and a new co-seismic folding event bends the layers of sediment above the blind fault. As this cycle repeats over thousands of years, a record of past earthquakes in stored in the folded sediments.
At the site chosen for this study -- along the San Gabriel River in the city of Bellflower, some two miles (three kilometers) above the Puente Hills fault -- Dolan and his team sank 15 boreholes into the floodplain sediments. In the cores of sediment removed from each hole, they looked for evidence of folding in the sedimentary layers. Because 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. Through the use of radiocarbon age dating, they were able to estimate how long ago these displacements -- the result of major earthquakes -- occurred. The relative offset of the sedimentary layers provided evidence of the size of each ancient earthquake.
"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.
"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."
by John Marquis, SCEC
Portions of this article were provided by Usha Sutliff, USC News.
Los Angeles Times article, "New Quake Threat Found Under L.A.," by Usha Lee McFarling (free registration required).
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