Using the Santa Cruz Mountains as a natural laboratory, researchers at Stanford University have built a 3D tectonic model that clarifies for the first time the link between earthquakes and mountain formation along the San Andreas fault, can be used to improve seismic hazard maps of the Bay Area.
The research, published in Science Advances on February 25, reveals that more mountains form in the period between major earthquakes along the San Andreas fault, rather than during the earthquakes themselves.
"This project focused on linking ground motions associated with earthquakes to the uplift of mountain ranges over millions of years to paint a complete picture of what the hazard might actually look like in the Bay Area," said the study's lead author, Curtis Baden, a Ph.D. student in geological sciences at Stanford University's School of Earth, Energy and Environmental Sciences.
Geologists estimate that the Santa Cruz Mountains began to rise from sea level about four million years ago, and formed as a result of compression around a bend in the San Andreas fault.
This fault marks the boundary between the Pacific Plate and the North American Plate, which move horizontally in a sliding motion.
The Santa Cruz Mountains define the geography of the Bay Area south of San Francisco, protecting the peninsula from the cold marine layer of the Pacific Ocean and forming the region's notorious microclimates.
The range also represents the dangers of living in Silicon Valley: earthquakes along the San Andreas fault, so there is a need for improved seismic risk mapping.
In bursts lasting seconds to minutes, earthquakes have moved the region's surface meters at a time, but researchers have never been able to reconcile the rapid release of the Earth's stress and the bending of the Earth's crust over years with the formation of mountain ranges over millions of years.
Now, by combining geological, geophysical, geochemical and satellite data, geologists have created a 3D tectonic model that resolves these time scales and could improve seismic hazard maps.
Deformation measurements (changes in rock shapes) have shown that the Earth's surface deforms and stretches around the San Andreas fault during and between earthquakes, and behaves like a rubber band for seconds, years, and even decades.
But that classical approach cannot align with geological observational data because it does not allow rocks to yield or break under the stress of deformation and stretching, as they would eventually do in nature, an effect that has been observed in the Earth's mountain ranges.
"If you treat the Earth like a rubber band and you push it too far forward, you're going to exceed its strength and ... it will no longer behave like a rubber band, it will start to give way, it will start to break," said another of the study's lead authors, George Hilley , professor of geological sciences at Stanford Earth.
"That rupture effect is common to almost all plate boundaries, but it is rarely addressed in a consistent way that allows you to move from earthquakes to long-term effects," he added.
By simply allowing the rocks to break apart in their model, the study authors have illuminated how earthquake-related ground motions and ground motions between earthquakes build mountains over millions of years.
The results were surprising: while the geoscience community conceives of earthquakes as the main drivers of mountain formation processes, the simulation showed that most of the uplift occurred in the period between earthquakes.
"The conventional wisdom is that permanent rock uplift actually occurs as a result of the immense force of the earthquake," Hilley noted. "This argues that the earthquake itself is relieving the accumulated stress, to some extent."
The study authors compiled the existing set of observations and also collected new geochemical data by measuring helium gas trapped within crystals contained in mountain rocks to estimate how quickly these rocks rise to the surface from thousands of feet below.
They then compared these data sets with model predictions to identify how the earthquakes relate to uplift and erosion of the mountain range.
Scientists are currently working on a companion paper detailing how seismic hazard risk maps could be improved using this new model.
"We now have a way forward in terms of having a workable set of mechanisms to account for differences between estimates on different time scales," Hilley explained. "The more we can make it all fit together, the more defensible our hazard assessments can be."
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