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Picture a fault line, like the San Andreas fault, and you might imagine a perfect slice through rock, like a cut through a cake with a sharp knife. But these geological fractures between blocks of rock in the Earth’s crust and upper mantle are rarely as straightforward or as simple as that. A fault may zigzag back and forth or form a kind of undulating wave, creating bends and gaps between the rock and leaving jagged edges. Or a number of faults may overlap and intersect, creating clusters of fault lines that branch off in different directions like a spider’s web.
Fault lines are, of course, often the starting points for earthquakes. The San Andreas fault—which runs more than 800 miles through California and Baja, Mexico—regularly produces destructive events, like the one that reduced San Francisco to rubble in 1906 or the one that hit the central California coast in 1989, collapsing a freeway in Oakland. But making precise estimates about where the next quake might hit and how powerful it will be is difficult.
The most complex faults have more dangerous earthquakes.
Now a research paper in Nature suggests that how much a fault line zigs, zags, undulates, and branches—its geometrical complexity—may play a major role in how earthquakes occur, and possibly offer a new way to figure out where to expect future quakes. The authors focused their research effort on California’s faults, including the San Andreas, though they say the findings may apply to earthquakes elsewhere. “We are trying to make predictions about where the big events are going to happen,” says coauthor and Brown University geophysicist Victor Tsai.
For decades, geophysicists have used models of friction along faults, tested in the lab, to assess earthquake risk. The amount of friction along a fault, the thinking went, would determine whether and how powerfully an earthquake would strike. When friction is high, rocks along the fault snag against each other, locking into place, and tension builds over time. With enough tension, these slabs of rock eventually give way all at once and lurch forward violently, causing an earthquake. In contrast, if the rocks can consistently slide past one another gently, in slow motion, about a few millimeters a year, the tension is released. No snagging and breaking, no earthquake. This is an ongoing process known as “creep.”
But it turns out complexity is much easier to assess than a fault’s frictional properties. To determine complexity, Tsai and his colleagues measured fault density as well as the average “alignment” of faults—how close they are to parallel—taken from California fault maps from the United States Geological Survey’s Quaternary Fault and Fold Database. The researchers then compared data on complexity of fault networks against data about earthquake activity and the amount of creep measured along each fault. They found that, on average, the most complex faults tend to have lower rates of creep and more dangerous earthquakes while the least complex, simpler faults tend to have higher rates of creep and fewer earthquakes. (Not all simple faults creep, however, suggesting that factors beyond geometry play a role.)
How much a fault line zigs, zags, undulates, and branches may play a major role in how earthquakes occur.
Faults can be complex at different scales—from tiny complexity at the scale of millimeters to large-scale complexity over dozens of miles, such as wiggles in the boundaries between tectonic plates. “If you zoom in, [a fault] might look very complex. If you zoom out, it might look relatively simple. And then maybe you zoom out even farther, and it might look more complicated at this even larger scale,” says Tsai. The maps Tsai and his colleagues used show fault complexity at a large-scale, over a dozen miles or so.
Their research did not explore how smaller-scale complexity might correlate with creep and earthquake intensity, he adds, but in theory small-scale fault complexity would be linked to small earthquakes, while large-scale fault complexity would produce large earthquakes. Large-scale complexity would cause tension to build up over a larger area, leading bigger slabs of rock to lurch and yielding bigger quakes.
Measuring fault complexity to gauge earthquake risk is possible in areas like California where faults are well mapped, but many regions of the world have not yet been mapped in this way, notes geoscientist Romain Jolivet of the École Normale Supérieure in Paris, who was not involved with the study. “A good fault map is the result of decades of field investigations,” he says. It’s a massive undertaking and treacherous terrain, lack of government funding, or political unrest can make such fieldwork difficult in some geographies. “Some places in the world are simply not accessible for fieldwork for political or security reasons,” he says.
Jolivet appreciates the researchers’ novel approach to using fault, creep, and earthquake data, which helped them identify the correlation between fault complexity and earthquakes. However, he notes that it would be helpful to model how fault complexity relates to creep to make sure the correlation isn’t just a coincidence.
If the research findings are correct, it might help us better understand the mathematics and physics of the Earth’s faults, those zones of rupture beneath our feet—and to plot out where tomorrow’s big quakes may occur in California and beyond. 
Lead image: vernStudio / Shutterstock
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