The last time the southern end of the San Andreas Fault produced a massive magnitude 7 earthquake or greater was 300 years ago — an unusually long time for a seismically active area.
So where is “the big one” southern Californians have been told is coming, and why hasn’t it hit yet? New research points to a possible answer that has to do with the Salton Sea.
Ryley Hill, a Ph.D. candidate in geophysics at Scripps Institution of Oceanography and San Diego State University, did a new study finding that over the past 1000 years, six out of seven major quakes along the southern end of the San Andreas Fault occurred when the ancient Lake Cahuilla was filled or filling. A remnant of that lake is now the Salton Sea.
So why would a filled lake produce seismic activity? Hill chalks it up to the weight of the water on the lakebed and the pressure it exerts onto the fault.
He compares the Earth’s crust to a ruler, and says that when pressure is applied in the middle, it makes it easier for the strike-slip fault below to shift.
“The fault itself is accumulating stress based off of the tectonics. And this stress accumulates whether or not the lake is there. And in the future, regardless of if the lake is there, that stress needs to be released, and when that stress is released, it will be an earthquake.”
Hill hopes that all the pressure release will be broken up into smaller magnitude events, as opposed to the big one.
“It's like a lose-lose. With the lake, we might be triggering large events. But without the lake, we're just accumulating more and more stress that will have to be released in the future no matter what.”
LA is definitely overdue for another big quake, Hill concludes.
“This section is … 10 months pregnant, it's locked and loaded. … This section of the San Andreas Fault poses the largest seismic hazard in all of the state of California, for the simple fact that if there is a northward propagating rupture from this fault, it would severely damage the Los Angeles metropolitan area. You can think of the energy that an earthquake of magnitude 7+ would have — the energy would bounce around the LA basin … like when you swish water in a bathtub.”