The southern end of the San Andreas Fault that runs directly under the Salton Sea in Southern California currently poses the state’s greatest earthquake hazard, but the area hasn’t experienced a major earthquake in more than 300 years.
Now, a new study in the journal NATURE of scientists at San Diego State University in collaboration with the Scripps Institution of Oceanography at UC San Diego found that as the Salton Sea dries up, it eases the weight of this part of the fault, potentially postponing an earthquake that could destroy Los Angeles.
SDSU NewsCenter’s Susanne Clara Bard asked geophysicist graduate student Ryley Hill and her advisor and co-author Matthew Weingarten about what this means for future earthquakes in the region.
The Salton Sea, which is a remnant of the larger ancient Lake Cahuilla, has undergone several cycles of drying and refilling due to climate change. Your study found that over the past thousand years major earthquakes have occurred every time the lake’s water level reaches a high point, about every 180 years. Why?
Hill: The ancient Lake Cahuilla contained approximately 40 times the volume of water of today’s Salton Sea, with a width six times the size. The lake has filled and drained periodically over the past 1,000 years. When the lake is full, it is large enough to bend the upper crust of the Earth. It’s like taking a ruler and bending it in half, removing both sides of the fault and causing it to slip.
Another effect is the fluid in the lake itself. This causes the pore pressure to increase which also makes the fault slip easily. A puck on an air hockey table is a good analogy. The increased pore pressure from the fluid is like driving wind—it pushes both sides of the fault to slide more easily, thus triggering an earthquake.
The Salton Sea has experienced a relatively dry period since the 1730s—300 years ago. Northwest Los Angeles has experienced several devastating earthquakes emanating from the Southern San Andreas Fault since then, but none have exceeded magnitude 7.0 on the Richter scale. Does this drying time explain why?
Hill: The drying of the last lake contributed, in small part, to the clamping of the fault. However, this does not fully explain why we do not have strong earthquakes during this period. We have direct geodetic measurements of this section of the San Andreas Fault. Regardless of any small clamping effect of drying out the last lake, the accumulated stress from tectonic plate movement has grown exponentially over the last 300 years and must be released. If all the stress were released at once the fault would now have the potential for a large (magnitude 7.0+) earthquake.
So are we still overdue for the ‘Big One’?
Weingarten: Yes. Plate tectonics moves faults around the world every day. We know from earthquake physics that the longer you go without relieving the stress built up on a fault, the more likely you are to have a strong earthquake. That theory holds true for the Southern San Andreas, which poses the highest earthquake risk in the state.
The US Geological Survey estimates that the Southern San Andreas has a nearly three times greater chance than the Northern San Andreas of hosting a magnitude M7.5+ earthquake in the next 30 years (17.3% vs. 5.7%, respectively). The Northern San Andreas hosted the Great 1906 San Francisco Earthquake with an estimated magnitude of M7.9.
Hill: However, we cannot determine if all the accumulated stress will be released in one big event (worst case scenario) or if the stress will be released in a series of small earthquakes (best-case scenario).
How might San Diego be affected by a major South San Andreas earthquake if/when it happens?
Hill: Fortunately, San Diego is in a fortunate position regarding a major earthquake on the Southern San Andreas Fault. Shaking is felt, but not as bad as other places like LA. However, the Rose Canyon fault, which runs directly under San Diego, is capable of a magnitude 6.7–7.0 earthquake. It recurs every 700–800 years, the last time in the mid-1700s. This fault poses the greatest risk for San Diego in large part because of its location.
What is your team investigating next?
Weingarten: We are interested in all the fluid-fault interactions that occur within the Earth’s crust. Ultimately, our goal is to better understand the physics of earthquakes—both natural, like those in the San Andreas, and man-made, like those in Oklahoma and other oil and gas producing states.
Hill: There were three magnitude 5.0+ earthquakes in central Oklahoma in 2016 related to the dumping of fracking wastewater into the Earth. These earthquakes caused significant structural damage. The goal is to better understand how to reduce the seismic hazard caused by humans during the injection of wastewater. As with the San Andreas work, we build physics-based models to understand stress and pore pressure within the Earth.
More information:
L. Astete Vasquez et al, Impact of modified methods of waste introduction on the short and long term use of the onsite sanitation system, Scientific reports (2023). DOI: 10.1038/s41598-023-35110-x
Provided by San Diego State University
Citation: Q&A: Salton Sea drying prevents earthquakes, current (2023, July 25) retrieved 25 July 2023 from https://phys.org/news/2023-07-qa-drying-salton-sea-staved.html
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