Theory Explains Why San Andreas Fault Is Quiet Near LA

Researchers may have discovered why California is overdue for a major earthquake along the southern section of the San Andreas fault. It's been 300 years since a quake of a magnitude of 7 or greater erupted from this section of fault stretching east of Los Angeles from the Salton Sea to Parkfield. That puzzled scientists who studied rocks collected near the fault and determined it had produced a major earthquake every 180 years, plus or minus 40 years, over a millennium. The seismic silence just didn't make sense—until scientists determined that the last six major earthquakes had likely coincided with high water levels on Lake Cahuilla, a massive water body that dried up centuries ago, per the Washington Post.

Using a computer model, they found that a full lake allowed the North American and Pacific tectonic plates to move against each other with less friction, both through its weight on the Earth's crust and through the seeping of water into the crust, which helped separate the plates. "Our study shows that the lake by itself was sufficient enough to trigger events on the southern San Andreas fault—and large events," Ryley Hill, a PhD candidate in the department of geological sciences at San Diego State University and lead author of a study published Wednesday in Nature, tells the Post. When the lake dried up, the fault was somewhat stabilized, Hill argues. Outside experts agree this could plausibly explain the gap in seismic activity in the region.

The present-day Salton Sea sits where the lake once did, though at around 3% of the lake's ancient size, per the Post. Thankfully, it will likely never reach past levels on its own, Hill notes. But there has been talk of refilling the Salton Sea, "which was formed when an irrigation canal burst in the early 1900s," per the New York Times. Though it continues to be fed by irrigation runoff, the sea is drying up, leaving behind toxic dust that poses a hazard to area residents. But "if you suddenly rapidly increase the filling of the lake, that might actually stimulate seismicity," especially as "we already know that so much stress has accumulated on this fault," Hill tells the Post. Indeed, the US Geological Survey says there's a high chance of a major earthquake in the region within the next 30 years. (More San Andreas Fault stories.)