The Santa Cruz Mountains in Northern California were built slowly over time, new research shows.
By Nam Cheah, science writer, Temblor Earthquake News extern
Citation: Cheah, N., 2022, Between earthquakes, mountains rise, Temblor, http://doi.org/10.32858/temblor.263
Earthquakes are seen as one of the keys to building mountains. When quakes strike, they release energy that can cause rocks to move upward, inching peaks toward the sky. However, current models of earthquake-induced mountain building do not take into account the permanent breaking and bending of rocks — particularly between earthquakes — at timescales of millions of years.
A new paper published in Science Advances, led by Stanford University doctoral student Curtis Baden, reveals a more complicated relationship between mountain building and earthquakes — using the Santa Cruz Mountains, which border the western edge of Silicon Valley, as a natural laboratory. Because the active San Andreas Fault slices through this mountain range, the scientists were able to create a model that includes both short-term earthquake-induced movements along the fault and long-term processes like uplift and erosion. This new research shows that by incorporating that permanent deformation into the mountain-building model, strike-slip faults help the earth rise skyward during the time between earthquakes.
Earthquakes, honey and play dough
Mountains and earthquakes are common features present at plate boundaries. Both are a consequence of the constant movement between tectonic plates. Faults created by tectonics, then, are the best place to study the link between earthquakes and mountain building because they tend to be the focus of deformation. However, mountains take millions of years to form. Earthquakes, in which energy accumulated by rocks releases near-instantaneously, occur relatively frequently.
The current earthquake-induced mountain building model is geared toward the shorter timescale in which rocks behave elastically. This means that when a fault plane is stressed by Earth’s movements, it stores that stress until suddenly releasing it in an earthquake, accounting for all that pent up energy. Another way of thinking about it is if rocks behave elastically, a movement of 16 feet (five meters) results in a 16-foot (five-meter) displacement. This may seem obvious, but it’s not always the case.
Rocks behaving elastically is an approximation, says Luca Dal Zilio, at the Institute of Geophysics at ETH Zürich, who was not involved in the new study. “Part of that deformation is not returned by the earthquakes,” he says, explaining that some of the energy dissipates into the surrounding rocks, forcing them either flow viscously (like honey) or plastically deform (like play dough). Both of these ductile behaviors that rocks can experience are not recoverable, he says. Over time, that cumulative deformation builds mountains.
Bridging the gap
Scientists at Stanford University led by Baden’s advisor, George Hilley, have been collecting data on the Santa Cruz Mountains’ topographic relief, rock records and erosion rate over many years, says Baden. In addition to these data, the researchers collected observations from individual crystals of apatite sampled from the region that signal when and how fast the mountains rose at million-year timescales. For shorter timescales — on the order of years — they looked at satellite data collected and compiled from 1993-2003 about the direction and magnitude in which the ground moves and bends between earthquakes.
Together, these metrics allowed Baden and his colleagues to construct a model of the mountains over time that reconciles the elastic earthquakes with the long-term plastic deformation of rocks near faults.
The model stretched back 4 million years, when scientists think the Santa Cruz Mountains began to rise, and showed that between earthquakes, rocks bend and stretch. Cumulatively, these small movements turn out to be the primary mountain-building processes that constructed the Santa Cruz Mountains. This finding came as a surprise to Baden. “Our model suggests that there’s a decent amount of permanent deformation that occurs on the San Andreas in between large earthquakes…[that] goes against what we thought was the case.”
Baden, C. W., Shuster, D. L., Aron, F., Fosdick, J. C., Bürgmann, R., & Hilley, G. E. (2022). Bridging earthquakes and mountain building in the Santa Cruz Mountains, CA. Science Advances, 8(8), eabi6031.
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