Staff and wire reports
Man's interference with Colorado River floods that used to regularly flow to the Salton Sea may have "stopped the clock'' on a regular series of big earthquakes, setting the stage for a megaquake that could wreck Southern California, according to researchers at Scripps Institution of Oceanography.
A massive 7.5 or larger quake may be the result when the southern San Andreas Fault finally jolts back to life, causing waves of enormous destruction in the Inland Empire and Los Angeles basin, according to a study by Danny Brothers, Debi Kilb and Neal Driscoll published in the scientific journal
- Kilb and Driscoll were set to detail the study during a morning press conference.
The team examined "displacement indicators preserved in pristine sedimentary deposits, ... reconstructed their earthquake history and found evidence for coincident timing between flooding of the ancient Salton Sea and fault rupture," according to a press release. "Rupture on these newly discovered 'stepover' faults has the potential to trigger large earthquakes on the southern San Andreas Fault."
Driscoll, a geologist, said in a press release that "earthquake simulations reveal that shaking of large metropolitan areas such as Riverside and Los Angeles will be larger if the earthquake propagates from south to north — our research suggests that the Salton Sea stepover zone may provide a trigger for such a propagation direction."
The seismically active southern end of the San Andreas Fault Zone lies under the Salton Sea, a wide depression whose bottom is about 250 feet below sea level. The area was regularly flooded by the Colorado River over the relatively recent millenia, a practice that ended 100 years ago when levees were built to force the Colorado to flow into the Sea of Cortez just south of
Between that diversion, the construction of upstream dams near Las Vegas and regional droughts, the Colorado has not flooded into the Imperial Valley and Salton Sink, a dry lake bed that was converted to the Salton Sea in gigantic floods in 1906. The Colorado last flooded and reached the Sea of Cortez in 1982, but it now trickles into the sand about where the San Andreas
fault crosses the International Boundary 155 miles east of San Diego.
The new Scripps study shows that several heretofore unknown fingers of the San Andreas system sit beneath the Salton Sea, and the sand and dirt of the Imperial Valley. The faults let loose with magnitude 7.0 quakes or larger every 180 years until the early 20th century — the same time the the Colorado foods that had brought billions of pounds of water to the area were staunched.
The Scripps study suggests that the Colorado River flooding may have affected the timing of the smaller, stress-relieving faults.
"We've been baffled as to why the Southern San Andreas hasn't gone. It's been compared to a woman who is 15 months pregnant," said seismologist Kilb, in a press release. "Now this paper offers one explanation why."
Lead author Brothers, a researcher now at the USGS who conducted most of the research while a graduate student at Scripps, noted in the release that the research does not improve the ability to predict such a quake but suggests that heightened preparedness for a major quake immediately following smaller quakes in the stepover zone is warranted.
The study covers the area just north of the zone struck by the 7.2 magnitude Mexicali Easter Sunday quake of 2010. It killed two people in Mexico, and although it was felt as a nasty sway in the Inland Empire and Los Angeles, Orange and San Diego counties, damage was limited to Imperial County and the Baja California state capital, Mexicali.
Because the magnitude scale is logarithmic, a 7.5 quake is 1,000 times stronger than a 7.2 quake.
The new Scripps study includes maps that show the San Jacinto and southern San Andreas fault lines bracketing the Imperial and Coachella valleys, and a curlicue-shaped Imperial Fault beneath El Centro and Mexicali, home to more than 1 million people.