(NaturalNews) For many years, scientists and geologists have wondered about which forces are responsible for continually elevating California's mighty Sierra Nevada and central coastal ranges, which have caused an increase in the number of earthquakes in the central part of the state.
Recently, the Los Angeles Times reported, a group of scientists offered a new and intriguing theory: the quakes are being caused, at least in part, by pumping groundwater into the Central Valley.
"These results suggest that human activity may give rise to a gradual increase in the rate of earthquake occurrence," said the study published in the journal Nature which was written by scientists at Western Washington University, University of Ottawa, University of Nevada, Reno and UC Berkeley.
Researchers used new GPS data to make an unexpected observation - that the mountains closest to California's drought-stricken Central Valley were growing at a faster-than-expected clip compared to mountainous ranges farther away. Scientists said the rate of growth was 1 to 3 millimeters a year, which amounts to less than half a foot over the past 150 years.
'Over the long term, it could give rise to more seismicity'
One theory floated by the scientists that helps explain the phenomenon is the loss of groundwater in the Central Valley as it produced crops that feed the country and parts of the world. The LA Times reported that groundwater is very heavy and, therefore, depresses the upper crust of the earth like a weight.
If the weight is removed, then the earth moves upward, but it is a change in pressure that can trigger additional small earthquakes.
"It reduces the forces that are keeping the fault clamped together - leading to more small earthquakes during dry periods of time," Colin B. Amos, assistant professor of geology at Western Washington University, the lead author of the study, told the paper.
"During wet periods of time when the fault is loaded down, the forces that are keeping the fault clamped down are greater. It inhibits the sliding of the fault," he added.
"Over the long term, because we're losing more groundwater, it could give rise to more seismicity by reducing these overall forces," Amos said. "Our model of what the groundwater is doing might explain those two things: showing that humans may have a hand in changing the state of stress on the fault, and therefore, rates of small earthquakes over time."
The Central Valley region has slowly depleted its supply of groundwater since the mid-1800s, to feed crops grown by scores of farmers and to quench the thirst of millions in cities. In addition, irrigation has led to the loss of Tulare Lake, which was once the largest fresh-water body west of the Mississippi River. Since those times, reserve groundwater in the Central Valley has lost some 38 cubic miles of water, which is enough to drain Lake Tahoe, the world's 27th largest lake.
The U.S. Geological Survey says that some 20 percent of the nation's groundwater comes from aquifers in and around the Central Valley, making the region the second-most-pumped system in the country. Because of the abundance of water and favorable climate, the Central Valley produces about a quarter of the country's food, including 40 percent of the fruits and nuts consumed.
The Times noted that previous research has indicated there are both seasonal increases and long-term increases in the occurrence of smaller earthquakes, or those under the magnitude of 5, in the Parkfield area of Central California, in southern Monterey County.
Uptick in earthquakes not due to Mankind
Further, the paper reported:
Observations show that in Parkfield, there are more small earthquakes in dry months than in wet months. Also, the number of small earthquakes every year has roughly doubled between 1984 and 2005.
The recent study hypothesizing about the loss of groundwater and its relationship to more earthquakes is not suggesting that future large earthquakes that could strike urban areas like Los Angeles or San Francisco are man-caused, said Amos.
"Large earthquakes are going to occur on the San Andreas fault no matter what we do," Amos said. Nevertheless, he said what is important is that human activity could at some point trigger additional seismic activity.
"It's really opening up a possibility that humans are changing stresses on faults," Amos told the paper. "It's a simple realization that human use of groundwater is having small but perhaps measurable impacts on the San Andreas Fault."