It's been about three centuries since the last major earthquake on the southern San Andreas fault, California's most dangerous seismic hazard.
For decades, researchers have wondered why so much time has passed.
The average interval of large earthquakes along that part of the fault has been 180 years over the past 1,000 years.
While seismologists agree that Southern California awaits the Great Earthquake, a group of researchers published a paper Wednesday in the journal Nature that offers a reason for the period of seismic silence along southern San Andreas, the meeting point of North American and Pacific tectonic plates.
The San Andreas seismic fault cuts through all roads and railroads connecting Southern California metropolitan areas, as well as the nation's busiest port complex, to the rest of the United States. Photo David McNew/Getty Images/AFP
The theory is based on the idea that, although the friction of tectonic plates is the main driving force of earthquakes, there are other factors, such as the weight of large bodies of water.
Based on previous research, scientists established a link between the occurrence of large earthquakes and the filling of a lake that has waxed and waned over the centuries.
"We're not trying to predict any earthquakes in the future, but we might be able to say why we haven't had any in the last 300 years," says Ryley G. Hill, a doctoral candidate in the Department of Geological Sciences at San Diego State University and Scripps Oceanographic Institution in La Jolla, California.
Birds at sunset in the Salton Sea in Desert Shores, California. (Mette Lampcov/The New York Times)
Hill and his co-authors found that large earthquakes along the southern San Andreas fault tended to occur when a large body of water, Lake Cahuilla, filled or filled with water from the Colorado River in what are now the Coachella and Imperial Valleys.
The lake has dried up over the past three centuries and all that remains is the vestige of the Salton Sea.
A segment of the San Andreas fault (vertical line c) near the city of Palmdale, California.
The authors of the paper believe that the process of emptying and disappearing the ancient lake stabilized the fault to some extent.
Lucy Jones, a seismologist and chief scientist at the Dr. Lucy Jones Center for Science and Society, called the explanation "plausible reason for the current long interval since the last earthquake."
But the research, he said, "doesn't make me mean we don't worry about the next one."
Jones was not involved in the study.
Tension
Seismologists say one consequence of the three-century interval since the last major earthquake, defined as magnitude 7 or higher, is that more stress has built up asthe two crushing tectonic plates have shifted in opposite directions.
"A lot of energy has accumulated analogous to that of a taut rubber band," says Belle Philibosian, a research geologist with the U.S. Geological Survey in San Francisco Bay.
"We should anticipate that the rubber band could break at any time."
South San Andreas, part of a network of faults that run through California, has the most potential for destruction due to the large number of people living in the area:
10 million people in Los Angeles County alone.
The 1994 Northridge earthquake in Los Angeles, with a magnitude of 6.7, killed more than 70 people and caused $20 billion in damage.
It was caused by one of the many faults that make up the broad San Andreas system, but not by the main San Andreas Fault, which is the longest in California and is capable of a much more powerful event.
The research published in Nature, which is based on a paper Philibosian participated in in 2011, raises questions about plans to rehabilitate parts of the Salton Sea, which was formed when an irrigation canal broke in the early <>th century.
Today it is heavily polluted and shrinking, relying on runoff from irrigation from nearby farms for water.
As the sea dries, toxic dust is left and spreads through the air, posing a danger to nearby residents.
Increasing the volume of Salton Seawater could reduce dust.
Ideas such as importing desalinated seawater — which a group of experts rejected last year — and paying farmers to divert their allocations from the Colorado River have recently been floated.
But a major change in water level could also trigger seismic activity, according to Philibosian.
"This earthquake will eventually happen, probably sooner rather than later, whatever we do," he said.
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