California’s San Andreas fault is one of the most feared fault lines in the world. The fault runs through some major metropolises, such as Los Angeles, and research has shown that it unleashes a major 7.5 magnitude quake on average every 150 years.
However, the fault line has not ruptured for 300 years now, leading some to claim it is “overdue”.
Why it has not unleashed a major earthquake in three centuries is a bit of a mystery, but scientists now believe it is down to an ancient lake which is now largely dried up.
Hundreds of years ago, a lake called Lake Cahuilla covered southern California and northern Mexico, and served as a key point to Native American populations, as is evident by the archaeological discovery of fish traps and campsites.
The lake reached its “high point” between 1000 and 1500 CE, but since then it has been slowly drying up.
Research presented at the Geological Society of America’s 2020 Annual Meeting by Ph.D. student Ryley Hill, found that water pressure from the lake may have helped to cause earthquakes.
Large bodies of water on top of a fault line push down on the ground, making the rocks beneath Earth’s surface weaker.
As pressure builds from these lakes, the fault lines become more vulnerable to rupturing.
What is more complicated is how the water in empty spaces in soils and bedrock (porewater) changes over both time and space.
He added: “The lake could modulate this [fault slip] rate just a little bit
“That’s what we think maybe tipped the scales to cause the [fault] failure.”
The drying up on the lake is likely to explain why there has been such a large gap since the last major quake at San Andreas, but Mr Hill was keen to stress the same influence from water does not compare to continent-scale tectonic forces
He concluded: “As pore pressures decrease, technically, the bedrock gets stronger. But how strong it’s getting is all relevant to tectonically driven slip rates. They’re much, much stronger.”