A seismic study of the Salton Basin published in the online version of the journal Nature Geoscience this week may hold a key to better understanding very large quakes on the southern San Andreas Fault. Or it may not. But it definitely provides a reminder that Southern California's landscape is closely connected to places that might seem distant, and events that took place impossibly long ago.
In the study, conducted by the Scripps Institution of Oceanography in partnership with UC San Diego, the USGS and the University of Nevada, Reno, geologists mapped the seismic structure beneath the Salton Sea, finding not only hitherto undiscovered faults but evidence of ancient tectonic movement on those faults, where sediment layers on one side of a fault were displaced relative to the other side.
These new faults are part of a "stepover zone" between the southern end of the San Andreas Fault, which peters out just east of the Salton Sea, and the Imperial Fault. These stepover faults, which transfer tectonic stress between the two longer faults, seem to have had a relatively active history, rupturing fairly often throughout the period recorded in the sediments.
What caught the researchers' attention was that some of those ruptures seemed to correspond to periods when the Colorado River was flooding the Salton Sink. The Salton Sea, formed by an irrigation acccident a little over a century ago, is but the most recent body of water to reside in the Salton Sink. At intervals over the last few million years the Colorado River has shifted its course to flow northward into the Imperial and Coachella valleys rather than out into the Sea of Cortez, creating a lake larger than Lake Tahoe in extent, and 300 feet deep. The researchers speculate that the weight of all that water may have triggered ruptures on the stepover faults.
If not for the Colorado River, the Sea of Cortez would extend to the base of the mountains at the south edge of Joshua Tree National Park. This is due to the activity of the East Pacific Rise, an oceanic ridge that extends from the south end of the San Andreas Fault near Brawley to the vicinity of Antarctica. The East Pacific Rise is a divergent plate boundary: new crust is created at the rise as magma wells up along the ridge, and tectonic plates to either side move away from the rise like conveyor belts.
Where the Rise intersects the West Coast of North America, it has split the Baja California peninsula away from the mainland the way a chisel would peel a sliver off a block of wood. The spreading of the Rise has created a tectonic wedge whose point is approximately at San Gorgonio Pass, and heading northward. The floor of this wedge sinks as its sides are pulled apart, creating the Salton Sink and the Sea of Cortez.
About five and a half million years ago, the Colorado River began carrying a very large amount of sediment when it started carving the Grand Canyon out of the highlands of Northern Arizona. It dumped that sediment into the Sea of Cortez, building a delta that eventually diked off the north end of the sea. The basin continued to sink and the landlocked area north of the delta dropped below sea level, and periodic floods would send the river surging into the basin to fill what geologists now call Ancient Lake Cahuilla, as shown in this animation:
The weight of the water in Lake Cahuilla pressed on stepover faults created by the Rise's spreading.
This becomes relevant to California outside the Salton Sink due to the fact that ruptures on those stepover faults add stress to the San Andreas, potentially triggering much larger quakes as a result. Over a thousand years in which Ancient Lake Cahuilla weighed down the desert floor, five quakes of magnitudes estimated to be larger than 7.0 hit the Coachella section of the San Andreas, about one every 180 years or so. The last large quake on that part of the San Andreas hit somewhere around 300 years ago, and so some geologists think a Big One is overdue.
Could the weight of that large desert lake -- far larger than the Salton Sea is now -- have triggered regular quakes on the San Andreas? Could our habit of building dams on the Colorado, regulating its flow and controlling floods and sending all its water to kitchen taps, and thus preventing the reestablishment of a larger Lake Cahuilla, have deprived the San Andreas Fault of a kind of release valve, making a quake much larger than 7.0 more likely?
The answer is yes, if you take at face value the breathless reporting on the study over the last few days. The scientists themselves are more circumspect, cautioning that the real reason for earthquakes on the San Andreas is the inexorable motion of tectonic plates. A major earthquake will occur at some point along the southern San Andreas, and it will be followed by another, and another after that -- an endless series of cataclysms that will eventually punch the Sea of Cortez up through the western United States.
The next big quake on the Southern San Andreas may happen today or a century from now, and when it happens the faults under the Salton Sea may or may not play a role. But it will come, inevitably. While you're bolting your bookshelves you may as well take a moment to appreciate the fact that you're doing so as a result of forces that have carved the Grand Canyon, and created an underwater mountain range reaching thousands of miles from Imperial County to the South Pacific.
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