Did Hoover Dam Inadvertently Tame the San Andreas Fault?

Is the Big One a bust?

Until the late 1800s, the San Andreas Fault jolted Southern California with a 7.0 quake about once every 180 years. But it's now been three centuries since the famous fault's last major tantrum.

The long pause has led some experts to compare the San Andreas to "a woman who is 15 months pregnant," said seismologist Debi Kilb. But new research suggests the quake pattern no longer applies.

According to a June 26 report in the journal Nature Geoscience, human interference with the Colorado River might have inadvertently calmed the San Andreas. At least for now.

Here's what happened:

The southern end of the fault runs beneath the Salton Sea, a basin whose bottom is about 250 feet below sea level. Until modern times, the area was subject to flooding by the Colorado River, and the weight of all that water pressured smaller faults, which in turn triggered the San Andreas, scientists theorize.

In the early 1900s, levees were built to divert the Colorado into the Sea of Cortez just south of Yuma, Ariz. Because of that diversion, along with regional droughts and construction of Hoover Dam, the Colorado River hasn't flooded into the Imperial Valley and Salton Sink in more than a century.

The timing coincides roughly with the last due date for a major San Andreas shaker.

Cruising the Salton Sea in a sonar-equipped boat, researchers from the Scripps Institution of Oceanography, UC San Diego, the U.S. Geological Survey and the University of Nevada in Reno recently mapped faults and sediment layers beneath the lake.

Using that data, they found a connection between big floods and big quakes over the last 1,200 years.

Unfortunately, controlling the Colorado doesn't mean the San Andreas is no longer a threat, they said. Instead of letting off steam with an occasional 7.0 quake, the fault might be building even more pressure, researcher Daniel Brothers told OurAmazingPlanet.com.

"We don't know if the next earthquake on the San Andreas Fault will be bigger because of the prolonged [quiet] period," he said. "We have to wait and see if we've actually reset the recurrence interval and if the earthquakes are actually larger."

-- City News Service contributed to this report.