There's a new "ride" at Disneyland. Not too dramatic for visitors, but remarkable from a scientific standpoint. The entire theme park moves up and down every year. Just a couple of centimeters. No extra charge.
Nearby, the annual swelling of the ground is far worse.
A new satellite-based study shows parts of the Santa Ana basin, near Los Angeles, rise and fall more than 4 inches (11 centimeters) every year. That would be fine if the land masses returned to some equilibrium. Instead, over time, the elevation of much of the land in the greater Los Angeles area and beyond is gradually dropping as a result of the land's seasonal movements.
Vast areas of the Golden State are sinking, threatening to disrupt the flow of water and sewage. It's a growing problem not confined to California, and difficult to spot in other parts of the country.
Nature has little to do with it.
For Los Angeles, as ever, it's about water. Local water districts that purchase water from outside the area pump it into huge, natural underground reservoirs to store it in winter months. These aquifers, as they are called, are regions of rock, sand and soil that can extend for many square miles and through which water flows freely.
Every summer, the water is withdrawn from these subterranean banks. But each year, the withdrawals exceed the deposits.
"Every year when they pump water out, they're pumping it to a new, lower level," said Gerald Bawden, a U.S. Geological Survey scientist who led the new study, which appears in the Aug. 23 issue of the journal Nature.
The seasonal draining causes rock and soil to compact to an extent that prevents an aquifer from ever filling to its previous capacity -- something scientists have understood since the late 1920s. Like a hardened sponge, the compacted material simply can't hold water as well as before.
The result is that in addition to the seasonal fluctuation, the ground in some places also sinks steadily over time. It's a process geologists call subsidence.
Faulty fault data
Potentially the greatest impact of the process is that it renders meaningless some of the important data used to study earthquake potential in the region. The Global Positioning System (GPS), which geologists rely on to monitor long-term ground movement along earthquake faults that might presage sudden and deadly quakes, is being fooled by the thirst of millions of people.
After a pair of large Southern California earthquakes, including the Northridge temblor that killed 56 people in 1994, geologists installed 250 GPS stations along faults throughout the region to monitor ground movement.
Plates of the Earth's crust are known to creep over the years, and the movement can be detected along fault lines, where two plates meet and move in different directions. This creep is known to produce stress in the rocks near the fault. When the stress builds to a certain point, it breaks, and an earthquake results.
But the seasonal swelling caused by the pumping of water, as well as withdrawals of oil and natural gas in the area, obscure and in some cases mimic the tectonic signals detected by the stations, the new study found.
As the ground rises and falls over an aquifer, it can actually pull at the edges, creating horizontal movement near the perimeter of the aquifer. This relationship is important because earthquake faults often form the boundaries of an aquifer. Along a fault, rock is ground into a fine powder the consistency of potter's clay, and water cannot penetrate this soil. Fault lines can extend a mile or more below the surface.
So the horizontal ground movement caused by seasonal water pumping sometimes occurs right where GPS data collection is most critical -- along the fault.
Working around the problem
Bawden said the new findings will make it more challenging for geologists to study fault movement in Southern California.
"It's not unrecoverable," he said. "We can still learn what's going on. It just has been made a lot more difficult."
One way to work around the problem is to ignore areas where groundwater pumping alters the surface, and instead take measurements in places that are not affected by the problem. And since fault lines crisscross much of California, pockets of ground can be identified where there is no effect from aquifers. Another solution would be to figure out the pattern of seasonal variation over the next three or four years and account for it.
It's the long-term sinking that will be the most challenging to separate from fault-movement data, and Bawden is working on ways to mathematically account for it.
But it will take several years, he said, to accumulate enough information and develop confidence in whether ground movement is caused by human-induced long-term or seasonal fluctuation or a creeping earthquake fault.
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