San Andreas Fault is one topic that has not only baffled scientists but has also caught the attention of movie and documentary makers. Scientists and directors alike have time and again predicted a mega-earthquake of over 10-magnitude at the San Andreas fault that could potentially be a cataclysmic event. While we have seen in the movies and documentaries how devastating it would be, it hasn't happened in over 300 years. Now, scientists believe they have found the reason why it is "overdue".

The fault runs along the coast of North America passes through densely populated cities like Los Angeles (California). Based on the studies, there has been an earthquake of over 7.5 magnitudes every 150 years. However, things get more interesting.

While there have been earthquakes of over 7.5-magnitude on the fault's central (1857) and northern (1906) segments, the southern part hasn't seen one in over 300 years. Both earthquakes were of 7.9-magnitude and the one in 1906 is the deadliest so far with nearly 3,000 people killed.

San Andreas Fault
A major earthquake at San Andreas Fault is long overdue Geological Society of America/ Rebecca Dzombak

Changes in Lake Cahuilla

The reason for the earthquake draught is a lake. Ryley Hill, a Ph.D. student at the University of California San Diego, using geophysical simulations presented at the Geological Society of America's 2020 Annual Meeting that Lake Cahuilla that covered parts of southern California and northern Mexico could be responsible for the earthquake drought. The lake has been slowly drying up since 1,000-1,500 CE. As the lake slowly dried up, it lifted the weight off the San Andreas fault, affecting the rupture timing.

Some geophysicists have studied 1,000-year old records of earthquakes from soil layers and found a correlation between the high water levels on the lake and the fault ruptures. Hill's research is based on existing models but he has expanded the scope by incorporating the unique 1,000-year records to focus on water pressure on rocks under the lake.

Known as lake loading, the weight of water greatly impacts the fault's rupture timing. Hill is exploring lake loading by studying the effects of the weight of the lake's water and the way the water has been creeping through the cracks into the ground under it. As the lake's water presses down, it increases the stress on the rocks underneath, impacting the fault. If the lake is deeper, the stress will be more and the fault is likely to slip, creating a major earthquake.

Lake Cahuilla
Ancient shorelines of Lake Cahuilla. The lake which is situated in California is drying up for thousands of years, impacting the rupture of San Andreas fault Wikimedia Commons

"It's not that water lubricates the fault. It's more about one force balancing another, making it easier or harder for the fault to give way," explained Hills. "Imagine your hands stuck together, pressing in. If you try to slip them side by side, they don't want to slip very easily. But if you imagine water between them, there's a pressure that pushes your hands out — that's basically reducing the stress and they slip really easily." His study was published by the Geological Society of America.

Impact of Water Pressure

In the previous models, scientists focused on a fully drained state with the water of the lake sipped in through the crust straight down at once. But Hill's model incorporated different levels of water pressure on the sediments and the rocks underneath the lake. Although his study is ongoing, Hill has found two key answers. When the water level at the lake is at its highest, the stress is high enough to reach the fault's threshold a little over 25 percent sooner. "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," Hill said.

Now as Lake Cahuilla is drying up, scientists are finding it difficult to model and predict the next "big one". While that is one aspect of the San Andreas fault, the tectonic shift can also rupture the fault. "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," Hill added.