The disappearance of the ancient lake may explain the earthquake gap in Southern California

The new models add to the evidence that stress changes from prehistoric Lake Cahuilla helped trigger major earthquakes in the past.

Written by Laura Vataroso, MA, PhD. Candidate, UMass Amherst (labtalk_laura)

Quote: Vataroso, El, 2021, Disappearance of an Ancient Lake May Explain Southern California Earthquake Gap, Tumbledore, http://doi.org/10.32858/temblor.162

Over the past millennium, the Southern San Andreas Fault has caused a major earthquake approximately once every 180 years – more or less 40 years (Rockwell et al., 2018). But the last big shock was in 1726 – only 300 years ago – indicating that the area was late for a large, devastating earthquake.

“Everyone is interested in the question: What’s the next big question?” Riley Hill, PhD student in the Joint Seismology and Applied Geophysics Program between San Diego State University and UCSD explains. Hill recently presented research on a possible cause of the extended calm of the Southern San Andreas Fault at the 2020 meeting of the American Geological Society. Hill explains that based on an analysis of earthquake frequency periods, there was a 95% chance that a major earthquake would occur by 2020. So why hasn’t that happened yet?

A huge lake dries up

During the same time period, a huge lake appeared and disappeared from the northern Salton Basin – a large inland basin that includes the Coachella and Imperial Valley, and through which the San Andreas Fault passes. The Colorado River drainage has shifted between the Gulf of Mexico and the Salton Basin, fills up to form a large freshwater lake, and then dries up in the desert in a cycle that has repeated many times over a few thousand years. This was the prehistoric Lake Cahuila, and some geologists believe that its frequent filling and draining may have helped trigger large earthquakes in the past.

Map of Southern California showing the extension of ancient Lake Cahuela with respect to the present-day Salton Sea. Credit: modified from Riley Hill

Geological records indicate that the basin has been mostly dry since about 1725. Floodwaters inundated man-made irrigation canals in 1905, partly filling the depression and forming the present-day Salton Sea. Despite being one of the largest lakes in present-day California, the Salton Sea dwindles in front of the prehistoric Lake Cahuela. The Salton Sea is about 43 feet (13 meters) deep, while Lake Cahuela is 330 feet (100 meters). At its peak, Cahuilla Lake covered a map area approximately 6 times larger than the Salton Sea.

No lake, no earthquake?

“It’s an interesting alignment of two sets of data; no lakes and no earthquakes in the last 300 years is something that people have noticed for a long time. The more we get.” [data] “About the timing of lakes and earthquakes, it sounds more interesting,” says Karen Lotrell, a geophysicist at Louisiana State University who was not involved in the new study.

In a previous study, Lotrell and colleagues found that the bending and failure to bend the Earth’s crust from the weight of lake water could have affected the pressure on faults under Lake Cahuila. The resulting change in stress may trigger an earthquake (Luttrell et al., 2007). New research by Hill and colleagues adds hydrological complexity to the story by looking at how the diffusion of water across the crust plays a role in changing stress over time.

In his presentation, Hill explains that water from the lake affects pressure within the pore space of the rocks below in several different ways. First, the added weight of the water increases the pressure in the crust underneath – and this happens almost immediately when the lake fills up. Then, as water seeps through the ground and through the deeper layers of the crust, the pore pressure continues to change and develop over time in a more complex way. Hill explores how these interacting processes affect stress conditions as the San Andreas Fault penetrates the crust. Even portions of the rift north of Lake Cahuilla are still undergoing pressure changes.

Hill says adding propagation effects to simulations of fault failure increases estimates of stress changes that cause earthquakes by 2-3 times. “[Increasing] Pore ​​pressure always leads to failure. ”

Salton Sea from space. Credit: NASA

The danger remains

However, the Salton Dry Basin mostly does not mean that Southern California can forget about the “big” that is coming soon.

“Regardless of whether the lake has modified the process in the past – from tectonic loading – the current probability of an earthquake is still very high,” Hill says.

Basically, earthquakes on the San Andreas Fault are driven by plate tectonics. While other factors such as the emergence and departure of lakes or glaciers can have an effect on the timing of major earthquakes, tectonic stress still accumulates. Since the last major rupture, the southern San Andreas fault has accumulated about 6 meters of slip yet to be achieved – sufficient to induce an earthquake of greater than 7.0 magnitude (Fialco, 2006; Rockwell et al., 2018).

“Understanding the southern San Andreas Fault is a worthwhile endeavor, and that’s part of the puzzle,” says Lautrell.

References

Brothers, D., Kilb, D., Luttrell, K., Driscoll, N., & Kent, G. (2011). The San Andreas Fault Loaded by Flood-Induced Fault Rupture Under the Salton Sea. Nature Geoscience, 4 (7), 486-492.

Fialco, Y (2006). Interseismic Strain Accumulation and Earthquake Potential on the Southern San Andreas Fault System, Nature 441, 968-971.

Luttrell, K., Sandwell, D., Smith-Konter, B., Bills, B., & Bock, Y. (2007). Modification of earthquake cycle in the southern San Andreas fault by lake loading. Journal of Geophysical Research: Solid Earth, 112 (B8).

Rockwell, TK, Meltzner, AJ, Haaker, EC (2018). The dates of the most recent surface rupture on the San Andreas Fault in the far south have been recalculated through the accurate dating of droughts in Lake Cahuila. Bulletin of the American Seismological Society, 108 (5A), 2634-2649.

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