Aquifer Levels May Lift, Lower L.A. Land
So much water is pumped in and out of underground aquifers in the Los Angeles area that much of the landscape rises and falls more than 4 inches each year--a finding that is unsettling the calculations of the region’s earthquake hazards.
The surprising discovery is the product of a new $20-million seismic monitoring network of 250 satellite surveying stations and an orbiting imaging radar satellite.
The research offers an unexpectedly human explanation for some of the seismic strain building up across the Los Angeles Basin. As a result, the findings suggest that the risk posed by hidden thrust faults like the one responsible for the 1994 Northridge earthquake may be less than previously believed, several experts said. Even so, they added, the region’s earthquake hazard is still substantial.
The finding is important, said UC San Diego geophysicist Bernard Minster, scientific director of the Southern California Earthquake Center. “It makes a difference in terms of estimating the overall seismic risk in blind faults in the Los Angeles Basin.”
The immense annual groundswell caused by pumping practices is 100 times larger than normal seismic fluctuations. It is particularly notable in northern parts of Orange County, where 75% of all the water used is pumped from the ground. The ground movement overshadows the more subtle tectonic forces at work along Southern California’s countless thrust faults, the researchers said.
“It is actually quite astonishing,” said geophysicist Gerald Bawden at the U.S. Geological Survey in Menlo Park, who led the study team. “The magnitude and extent of these motions are a product of Los Angeles’ great thirst for water; they are unprecedented, and have not been observed elsewhere in the world.” The new data--representing the first time the seasonal cycle has been measured--are reported today in the journal Nature.
From fall to early spring, officials pump water into underground aquifers for storage, causing the land to rise. In summer months, these unseen reservoirs slowly collapse, systematically drained to water lawns, wash cars, top off swimming pools and slake the thirst of the area’s 14 million residents. Overall, the level of the water table sinks lower each year, leaving a permanent imprint on the land.
Some Areas Rise as Others Sink
In all, an area 25 miles long and 12 miles wide is affected in Los Angeles and Orange counties, encompassing Anaheim, Garden Grove, Norwalk, Downey, Lakewood and Fountain Valley. As a lasting effect of the pumping, the area around Santa Ana has been sinking at the rate of half an inch a year for the last six years, the researchers from UCLA and the Geological Survey calculated.
A few areas, such as Baldwin Hills and Santa Fe Springs, are rising slowly at a rate of about 6 millimeters--barely a quarter-inch--every year.
The almost imperceptible but steady sinking of a broad swath of the metropolitan area could eventually cause problems for pipelines, sewers and buildings. More careful management of the ground water pumping could eliminate some of the subsidence, experts said.
Aquifers are increasingly popular for water storage. In addition, oil field operators in the region have been more aggressive at pumping to recover petroleum from the area’s many working fields.
The findings took water district managers by surprise, and several who were briefed on the research were initially skeptical.
“I think they’re way off in left field,” said Mel Blevins, the court-appointed water master for the Upper Los Angeles River Area. Blevins oversees exploitation of water and water rights, the technical activities involved in the pumping of water.
“When they say large portions of the L.A. area are rising and falling 4.3 inches a year, I don’t agree with that. I don’t think they’re even close to having any realistic measurements,” Blevins insisted.
Bill Mills, general manager of the Orange County Water District, said his agency has been making greater use of the aquifers for temporary water storage over the last decade as the population has grown. The county does its own surveys periodically to monitor possible subsidence, but Mills said he is not aware of a problem.
“I’d like to see [the new research] confirmed with some actual ground measurements by survey crews,” Mills said. “We’d like a survey to ascertain whether there was any subsidence.”
The new data, generated by measurements using global positioning system satellites, are considered far more precise than those gathered by traditional surveying techniques.
Bawden and his colleagues at UCLA and the Geological Survey did not set out to measure the effects of ground water management practices.
They were trying instead to better calculate the stresses building along thrust faults in the region. As the area between the Palos Verdes Peninsula and Pasadena is squeezed by the action of vast tectonic plates sliding past each other, stress builds in thrust faults. Combined with the titanic stresses along larger strike-slip faults such as the San Andreas, that pent-up energy fuels Southern California’s frequent earthquakes.
In their new calculations of the tectonic forces, the researchers have convincingly demonstrated that there is less seismic energy building along some fault lines in the Los Angeles Basin than many scientists had thought, experts said.
By not taking the pumping into account, seismic experts may have overstated the earthquake risk in some areas, said earthquake geologist Thomas Rockwell of San Diego State University.
“It would make the seismic hazard seem greater than it really is,” he said. “I think it is important that we accurately quantify the tectonic rate.”
Data Don’t Change Basic Quake Risks
Geologists do know that the downward revision of seismic risk “doesn’t change our basic understanding of the fact that we are storing a large amount of strain on faults under metropolitan L.A. at a relatively rapid rate,” said USC earthquake geologist James Dolan.
“At some point in the future, that strain will be released in earthquakes,” he said.
Caltech engineering seismologist Thomas Heaton said common sense suggests there should be some seasonal ground motion from so much water being pumped in and out of the aquifers. But like many other seismologists, he was surprised that the movement was so large.
“I did not expect it to be as big an effect as it is,” said geophysicist Paul Segall of Stanford University. “It is more significant than some of us thought.”
Over time, Segall said, “The subsidence will cause damage to roads or buckling of pipes. It is an engineering management issue that needs to be dealt with.”
A more immediate problem is that the $20-million effort to measure precisely the tectonic strains in the Los Angeles region has been unintentionally distorted by water management practices.
The new seismic network, which uses global positioning system satellites to monitor tectonic activity in the region to within a few millimeters a year, was installed over the last decade and formally dedicated in July.
About half of all the monitoring sites are affected by the seasonal rise and fall of the aquifers, with the most severe distortions occurring over fault zones in the Puente Hills, Elysian Park and Compton, which scientists are most eager to study.
“The data is being contaminated by the effects of the ground water pumping,” said Geological Survey geophysicist Ken W. Hudnut in Pasadena, who collaborated on the survey.
“But we don’t have the option of walking away from this problem,” he said. “We can’t say that because we have the ground water pumping, we have to give up on observing the tectonic motion.”
The researchers acknowledged that eventually they should be able to work around the problem. As one geophysicist noted, astronomers have to contend with clouds and bright city lights in their attempts to study the stars. Earthquake experts will have to take account of the up and down motion of aquifers in their effort to study the Earth below.
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Ground Movement
New measurements show that a 25-by-12-mile area is sinking because of ground water being pumped out.
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Source: O.C. Water District
