A surprising re-characterization of earthquake risk along a strand from San Andreas

A recent study reports increased slip rates along the Mission Creek to the San Andreas Fault, indicating a new seismic hazard formation at this latitude.

Written by Ben Wolman, science writer (bythewolman)

Citation: Wolman, B, 2021, Surprising Restatements of Earthquake Hazards Along a Thread of San Andreas, Temblor, http://doi.org/10.32858/temblor.168

The San Andreas Fault is as close to a celebrity as its geological features can get – it even has a movie named in his honor. Since it was officially identified in the late 19th century, the error has been analyzed, dated, drawn, and modeled by thousands of scholars. But its southern portion, which splits into strings like the ragged ends of a rope, still baffles scientists.

The northern and central parts of San Andreas were ruptured relatively recently, geologically (in 1906 and 1857, respectively), giving rise to earthquakes of magnitude 7+ (Fialko, 2006). However, the southeastern portion of Los Angeles was last torn apart in 1726 and a significant ancestry has accumulated since then. This is partly the reason why people often say the Great Earthquake is “late” (although mistakes can’t really be late).

A recent study published in Science Advances indicates that the Mission Creek thread in San Andreas, which runs along the northeastern side of the Coachella Valley, is the predominant error at this latitude, accounting for about 90% of the overall slip rate south. San Andreas system error. This means Mission Creek filaments – not threads previously identified as accumulating the most stress – could host the next major earthquake in southern San Andreas.

The Southern San Andreas Fault has multiple strands. Mission Creek may pose a greater threat to Southern California than previously thought. Credit: Modified from Kimberly Blisniuk

Slipping and effort

Slip rates describe the speed at which the two sides of the fault move relative to each other. Slippage rates are usually verified through geological measurements of landforms corresponding to fault movement, such as outcrops in alluvial fans, broken flow channels (those cut off from their sources), and vegetation patterns (where dense vegetation meets less dense vegetation in a surprising line, Most likely due to a groundwater cut-off error where the line is occurring). Geodetic data obtained by ground motion observed in GPS or radar imaging can also be used to model slip rates.

Previous geological and geodetic data indicated that a single piece of San Andreas in the Coachella Valley called the Banning Strip was likely responsible for the bulk of the northwestern slide across the San Giorgio Pass. Banning and Mission Creek run roughly parallel to each other. The new study examines slip rates from two new sites in the valley.

Terrain compensation

Kimberly Plisniuk, a seismologist and seismologist at San Jose State University, and her team began reconstructing and dating the geomorphic offsets in Indio Hills (Banning) and Pashawala Valley (Mission Canyon). They used lidar imaging and field mapping to determine displacement of ancient flow channels and other forms of terrain. The team then combined two different dating techniques – uranium-thorium dating of soils and beryllium-10 dating of surface exposures – to provide a minimum age estimate and an estimate of the maximum age of the terrestrial formations.

The team noticed that in Pushawalla Canyon, canals exit a steep mountain front and hit the canyon and swell in a unique slope-like pattern, says Richard Hermance, a geologist at California State University, Northridge, who was not involved in new research. By matching the sediments to their possible original locations, and with “a distance and age for each surface when it was just forming,” the team calculated slip rates simply by dividing the distance by the age, Hermans says. He adds that the uniqueness of the Pushawalla Canyon terrain enabled this mapping. “This part of the story is awesome.”

Tape cut headless canals from the San Andreas Fault. Credit: Kimberly Plisniuk

Changes in Earth’s shape and history together indicate that the Mission Creek strand at this latitude, which was previously thought to be inactive, has hosted most of the earthquakes in this region over the past 100,000 years, Plesniuk and her team reported.

Slipping on a thread of a different error

Additionally, Blisniuk and her team found that the Mission Creek thread in Pushawalla Canyon appears to slip about 0.9 inches (21.6 mm) per year – compared to 0.1 inches (2.5 mm) in the Banning strand per year. This means that in the last 295 years, the Mission Creek chain has accumulated 20-30 feet (6-9 meters) of elastic stress, which is a gauge of pressure. Think of elastic stress like a taut rubber band: If you stop pulling the rubber band, the pressure is released and it can return to its normal shape. But if it is pulled too tight for too long, it will burst and release this pressure in the form of energy. Rocks along the fault do the same, releasing pressure in an earthquake. Thus, the Mission Creek chain may keep the lion’s share of the possibility of an earthquake in Pushawalla Canyon.

Dangers to Los Angeles

The results could be significant for the densely populated Los Angeles area. “Previously, we only had this one pathway where the South San Andreas earthquake could explode through the greater Los Angeles area,” Plesniuk says. “Now we’re actually seeing it, kind of like [2019] Ridgecrest earthquakes, which occurred on faults that have not been identified, there are faults that we have identified as likely inactive, and that may still be active. ”

Mapping the risks of southern soldiers in the future

“This study highlights the need for more detailed studies,” says Hermance, particularly through the San Giorgio Corridor northwest of Banning and Mission Creek where much of the stress in this area is currently believed to accumulate. But Hermance says the roughly 0.9 inches (21.6 mm) of annual slip found in the study cannot yet be conclusively attributed to the entire Mission Creek trail northwest of Pushawalla Canyon: he says it’s still an open question, noting that the mapping errors The future should help reduce uncertainty.

Blisniuk is doing field work along the Southern San Andreas Fault. Credit: Thomas Rockwell, San Jose State University

Blisniuk agrees to further terrain compensation and dating analyzes for additional sites along the strands. She says she is excited about the promise of new data that will be discovered in these threads. “This is one of the best studied mistakes in the world. Now with new technology and new dating techniques, we can test it all [earthquake] Models. “This new data indicates that there is a lot to learn, and still a lot to investigate.” The past is the key to the present. ”

References

Blisniuk, K., Scharer, K., Sharp, WD, Burgmann, R., Amos, C., Rymer, M., 2021. A revised site for the baseline of the Pleistocene-Holocene San Andreas fracture in Southern California. Sci Adv 7, eaaz5691. https://doi.org/10.1126/sciadv.aaz5691

Fialko, Y., 2006. Interseismic stress accumulation and earthquake potential on the southern San Andreas fault system. Nature 441, 968-971. https://doi.org/10.1038/nature04797

Further reading

Guns, KA, Bennett, RA, Spinler, JC, McGill, SF, 2020. New geodetic restrictions on fault slip rates at the South San Andreas Corridor, San Giorgio Corridor, California. Geosphere 17, 39-68. https://doi.org/10.1130/GES02239.1

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