A recent study indicates increased slip rates along the Mission Creek strand of the San Andreas Fault, suggesting a novel configuration of earthquake risk at this latitude.
By Ben Wolman, science writer (@bythewolman)
Citation: Wolman, B, 2021, Surprising recharacterization of earthquake risk along a strand of the San Andreas, Temblor, http://doi.org/10.32858/temblor.168
The San Andreas Fault is as close to a celebrity as geological features can get — it even has a movie named in its honor. Since its formal identification in the late 19th century, the fault has been analyzed, dated, mapped and modeled by thousands of scientists. But its southernmost section, which is divided into strands like the frayed ends of a rope, still puzzles scientists.
The northern and central sections of the San Andreas have ruptured relatively recently, geologically speaking (in 1906 and 1857, respectively), producing magnitude-7+ earthquakes (Fialko, 2006). The southernmost section, southeast of Los Angeles, however, last ruptured in 1726 and has accumulated significant strain since. This is partly why people often say it’s “overdue” for a big quake (though faults can’t really be overdue).
A recent study published in Science Advances suggests that the Mission Creek strand of the San Andreas, which runs along the northeastern side of the Coachella Valley, is the dominant fault at this latitude, accounting for about 90% of the overall slip rate of the southern San Andreas Fault system. That means the Mission Creek strand — not the strands previously identified as accumulating the most strain — could host the next major earthquake on the southern San Andreas.
Slipping and straining
Slip rates describe the speed at which the two sides of a fault move relative to each other. Slip rates are typically ascertained through geologic measurements of landforms offset by fault movement, such as jags in alluvial fans, beheaded stream channels (those cut off from their headwaters), and vegetation lineaments (where dense vegetation meets less-dense vegetation in an abrupt line, likely due to a fault cutting off groundwater where the line occurs). Geodetic data obtained by ground motion observed in GPS or radar imaging can also be used to model slip rates.
Previous geologic and geodetic data suggested that one piece of the San Andreas in the Coachella Valley called the Banning strand was likely responsible for the bulk of the slipping northwest through the San Gorgonio Pass. The Banning and Mission Creek strands run roughly parallel to one another. The new study investigates the slip rates from two new locations in the valley.
Kimberly Blisniuk, an earthquake geologist and geochronologist at San Jose State University, and her team started by reconstructing and dating landform offsets in the Indio Hills (Banning) and Pushawalla Canyon (Mission Creek). They used lidar imaging and field mapping to determine the offset of ancient stream channels and other landforms. Then the team combined two different dating techniques — uranium-thorium dating of soil and beryllium-10 dating of surface exposures — to provide a minimum age estimate and a maximum age estimate for the landforms.
The team noted that in Pushawalla Canyon, channels come out of a steep mountain front and hit the valley and aggrade in a unique stairstep-like terrace pattern, says Richard Heermance, a geologist at California State University, Northridge, who was not involved in the new research. By matching the deposits with their likely places of origin, and with “a distance and an age for each surface when they were just forming,” the team computed slip rates by simply dividing distance by age, Heermance says. The uniqueness of the Pushawalla Canyon landforms enabled this mapping, he adds. “That part of the story is great.”
The landform changes and dating together indicate that the Mission Creek strand at this latitude, previously thought to be inactive, has hosted the most earthquakes in this region over the last 100,000 years, Blisniuk and her team reported.
Slip on a different fault strand
In addition, Blisniuk and her team found that the Mission Creek strand at Pushawalla Canyon appears to slip approximately 0.9 inches (21.6 millimeters) per year — compared to the Banning strand’s 0.1 inches (2.5 millimeters) per year. That means in the last 295 years, the Mission Creek strand has accumulated 20-30 feet (6-9 meters) of elastic strain, a measure of stress. Think of elastic strain like a rubber band pulled taut: If you stop pulling on the rubber band, strain is released and it can go back to its normal shape. But if it’s pulled too tight for too long, it will snap and release that strain in the form of energy. Rocks along a fault do the same, releasing the strain in an earthquake. Thus, the Mission Creek strand may instead hold the lion’s share of earthquake potential at Pushawalla Canyon.
Risks to Los Angeles
The findings could be important for the densely populated Los Angeles area. “Before, we only had this one path where a southern San Andreas Fault earthquake could rupture through the greater Los Angeles area,” Blisniuk says. “Now we’re seeing that actually, kind of like the  Ridgecrest earthquakes, which occurred on faults that weren’t identified, there are faults that we’ve identified as likely inactive, that may still be active.”
Future southern strand risk mapping
“This study has highlighted the need for more detailed studies,” Heermance says, especially through San Gorgonio Pass northwest of the Banning and Mission Creek strands where much of the strain in this region is currently thought to be accumulating. But Heermance says the approximately 0.9 inches (21.6 millimeters) of annual slip found in the study cannot yet be definitively attributed to the entire Mission Creek strand northwest of Pushawalla Canyon: It’s still an open question, he says, noting that future fault mapping should help reduce the uncertainty.
Blisniuk agrees further landform offset analyses and dating of additional sites along the strands are needed. She says she’s excited by the promise of new data to be unearthed at these strands. “This is one of the best-studied faults in the world. And now with new technology and new dating techniques, we can test all of these [earthquake] models.” And those new data are suggesting that there’s so much to learn, and there’s so much to still investigate. “The past is the key to the present.”
Blisniuk, K., Scharer, K., Sharp, W.D., Burgmann, R., Amos, C., Rymer, M., 2021. A revised position for the primary strand of the Pleistocene-Holocene San Andreas fault in southern California. Sci Adv 7, eaaz5691. https://doi.org/10.1126/sciadv.aaz5691
Fialko, Y., 2006. Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system. Nature 441, 968–971. https://doi.org/10.1038/nature04797
Guns, K.A., Bennett, R.A., Spinler, J.C., McGill, S.F., 2020. New geodetic constraints on southern San Andreas fault-slip rates, San Gorgonio Pass, California. Geosphere 17, 39–68. https://doi.org/10.1130/GES02239.1
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