The San Andreas Fault is overdue for "the big one," and the efforts by humans to control flooding in the area could be the reason for the recent lull in temblors, a new study suggests.
Ancient floods once helped unleash earthquakes on the San Andreas, a group of researchers has found. The southern portion of the fault has not experienced a large earthquake for about 300 years, though, which makes one long overdue — the previous five major earthquakes in the region occurred at approximately 180-year intervals.
Over the past century, humans have put in place measures to control floods in the region to protect property and infrastructure, which the researchers say might explain the quake lull.
Flooding possibly fueled faults
To investigate both the cause of these earthquakes and the current lull in them, scientists probed the locale where the southern San Andreas Fault ends, the Salton Sea, currently the largest lake in California.
The area in which the Salton Sea now sits was once home to giant prehistoric freshwater Lake Cahuilla, which once would have dwarfed the Salton Sea by filling the Coachella, Imperial and Mexicali valleys of southeastern California and northeastern Baja California. By analyzing seismic images of sediment layers deposited in this ancient lake, researchers can identify activity such as flooding or disruptions such as earthquakes, which affect how the sediment is laid down.
"We have gone back through time and built one of the longest and most robust earthquake records available," said researcher Daniel Brothers, a U.S. Geological Survey marine geologist in Woods Hole, Mass.
Their findings suggest that episodic flooding of the lake by the Colorado River during the past 1,200 years triggered earthquakes on small faults underlying the lake. These, in turn, could have impacted the nearby San Andreas Fault, apparently causing it to rupture with large earthquakes. [Related: 13 Crazy Earthquake Facts]
Past studies had suggested that flooding could trigger seismic activity, such as via rapid filling of lakes, "but as far as I know, nobody has presented convincing evidence of this relationship using geologic records," Brothers told OurAmazingPlanet.
"We didn't set out to study the relationship between flooding and earthquakes — the idea presented itself while we were interpreting our data and realized we couldn't separate the timing of flooding and fault rupture," he added.
Bigger 'big one'?
In the past century, human diversion and control of the Colorado River for municipal and farming needs may have contributed to the dearth in earthquakes currently seen in the southern San Andreas. One concern regarding this lull is that energy might be building up in the fault without floods to help release that stress.
"We don't know if the next earthquake on the San Andreas Fault will be bigger because of the prolonged quiescent period," Brothers said. "We have to wait and see if we've actually reset the recurrence interval and if the earthquakes are actually larger."
Other areas that scientists might want to analyze for similar effects include prehistoric Lake Bonneville in Utah, Mono Lake in California, the Dead Sea in the Middle East, the Rift Valley lakes of East Africa and Lake Baikal in Russia — "really, any tectonically active basin filled with water," Brothers said.
The scientists detailed their findings in the June 26 issue of the journal Nature Geoscience.
This story was provided by OurAmazingPlanet, a sister site to LiveScience.
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