Big One Remains Overdue

Vehicles stranded on I-5 after the 1994 event.
Brant Ward/©San Francisco Chronicle.

Now we may know why.

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 Yuma, Arizona.

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 200 miles southeast of Los Angeles.

[...]

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 that the Colorado floods that had brought billions of pounds of water to the area were stanched.

"It's possible that the ending of the diversion re-set the earthquake clock; we're more than 100 years overdue for a quake that could be as big as 7.5,'' said Neal Driscoll, quoted by signonsandiego.com .

"The fault could send tremendous energy towards the Los Angeles area if it broke from south to north, and could cause shaking that would make soil liquefy in bays and estuaries in San Diego County,'' Driscoll told the U-T's web site.

So. Minus the billions of lbs. of water, tectonic tension has been lower, & events less frequent. Eventually, something will give; when it does & if it breaks south to north

[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, a Scripps Institution of Oceanography study published Sunday said.

Even worse, there are other dangers associated w/ Salton Sea earthquakes.