An ancient lake contributed to the rupture of the San Andreas Fault – ScienceDaily

The San Andreas Fault, which stretches along the western coast of North America and crosses dense population centers such as Los Angeles, California, is one of the most studied faults in North America due to its high risk. Based on the nearly 150-year frequency of earthquakes of magnitude 7.5 and the fact that more than 300 years have passed since they occurred, the Southern San Andreas Fault has long been described as “late” for such an earthquake. For decades, geologists have been wondering why so long has passed since the Great Rift occurred. Now, some geophysicists believe that the “earthquake drought” can be partly explained by the lakes – or the lack thereof.

Today, at the 2020 annual meeting of the American Geological Society, Ph.D. Student Riley Hill will present new work using geophysical modeling to estimate how the presence of a large lake above the rift affected the timing of the rupture in southern San Andreas in the past. Hundreds of years ago, a giant lake – Lake Cahuila – in southern California and northern Mexico covered the vast swaths of the Mexicali, Imperial and Coachella valleys, through which southern San Andreas passes. The lake served as a focal point for many Native Americans in the area, as evidenced by archaeological remains of fish traps and camping sites. It has been drying slowly since the most recent high water mark (between 1000 and 1500 AD). If the lake over San Andreas dries up and the weight of the water is removed, could that help explain why the San Andreas Fault dries up from the earthquake?

Some researchers have already found an association between high water levels on Lake Cahuilla and fault faults by studying a 1,000-year-old earthquake record written in fractured layers of soil exposed in deeply dug trenches in the Coachella Valley. Hill’s research draws on a host of existing models but expands to this unique 1,000-year record and focuses on improving one major factor: the complexity of water pressures in rocks below the lake.

Hill explores the effects of the lake on the timing of the fault rift, known as lake loading. Loading a lake on the rift is the cumulative effect of two forces: the weight of the lake’s water and the way this water infiltrates or diffuses into the ground beneath the lake. The weight of lake water pressing on the ground increases the pressure on the rocks underneath, weakening it – including any imperfections present. The deeper the lake, the greater the pressure the rocks are subjected to, and the more likely the fault to slip.

More complicated is how water pressure in empty spaces in soil and rocks (wastewater) changes over time and place. “It’s not that [water] Hill spoils the mistake, “Hill explains. It’s more about balancing another force, making it easier or harder for a mistake to give way.” Imagine your hands are stuck together, pressing. If you try to slide them side to side, you don’t want them to slide so easily. But if you imagine water in between, there is pressure to be pushed [your hands] Outside – This basically reduces stress [on your hands], And they slip in really easily. “Together, these two forces create an overall amount of pressure on the fault. Once that pressure builds up to a critical threshold, the rift breaks out, and Los Angeles experiences the“ big ”.

Where previous modeling work focused on a state of complete depletion, with all lake water diffusing directly (and simultaneously), the Hill model is more complex, incorporating different levels of wastewater pressure in sediments and rocks below the lake and allowing pore pressures to be directly affected. By the stresses of the water mass. This, in turn, influences the general error behavior.

As the work continues, Hill says they have found two main answers. When lake water is at its highest, it increases pressures enough to drive the schedule of error reaching this critical stress point of just over 25% sooner. “The lake can adjust this [fault slip] Just rate it slightly, “Hill says. This is what we think might upset the scales to cause [fault] To fail. “

The overall effect of the drying up of Cahuilla Lake makes it difficult for the bug to blast into its model, citing its potential significance to the recent lulling around the bug. However, Hill stresses, this effect pales in comparison to the tectonic forces on a continent scale. “As the pore pressure decreases, technically, the strength of the bedrock increases,” he says. “But how strong it is is related to the rates of tectonic-driven slip. It’s much stronger.”

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Material provided by the American Geological Society. Note: The content can be modified according to the style and length.

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