Geometry can help us predict earthquakes

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Picture a fault line, such as the San Andreas Fault, and you might imagine a perfect slice through the rock, like cutting a cake with a sharp knife. But these geological fractures between rock masses in the Earth's crust and upper mantle are rarely so clear or simple. A fault may meander back and forth or form a kind of undulating wave, creating bends and gaps between the rocks and leaving jagged edges. Or a number of faults may overlap and intersect, creating groups of fault lines that branch off in different directions like a spider's web.

Of course, fault lines are often the starting points of earthquakes. The San Andreas Fault – which extends more than 800 miles across California and Baja, Mexico – regularly produces devastating events, such as the earthquake that reduced San Francisco to rubble in 1906, or the earthquake that struck California's central coast in 1989, causing a highway to collapse. In Auckland. But making accurate estimates about where the next earthquake might hit and how strong it will be is difficult.

More complex faults have more serious earthquakes.

Now a paper in the journal Nature suggests that the amount of a fault line zigzags, zigzags, undulates and branches — its geometric complexity — may play a key role in how earthquakes occur, perhaps offering a new way of knowing where to expect future quakes. The authors focused their research efforts on faults in California, including the San Andreas, although they say the findings may apply to earthquakes elsewhere. “We're trying to make predictions about where major events will occur,” says co-author and Brown University geophysicist Victor Tsai.

For decades, geophysicists have used models of friction along faults, tested in the laboratory, to assess earthquake hazards. It was thought that the amount of friction along the fault would determine whether an earthquake would strike and how strong it would be. When friction is high, rocks along the fault get stuck against each other, settling into place, and the tension builds over time. With enough tension, these rock slabs eventually collapse all at once and move forward violently, causing an earthquake. In contrast, if the rocks could continually slide over each other gently, in a slow motion, about a few millimeters per year, the tension would be released. No tearing and breaking, no earthquake. This is an ongoing process known as “creep”.

But complexity turns out to be much easier to assess than the frictional properties of a fault. 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 fault maps of California from the U.S. Geological Survey’s Quaternary Fault and Fold Database. The researchers then compared the data on fault network complexity with data on earthquake activity and the amount of creep measured along each fault. They found that, on average, more complex faults tend to have lower creep rates and more serious earthquakes, while less complex, simpler faults tend to have higher creep rates and fewer earthquakes. (Not all simple faults creep, however, suggesting that factors outside of geometry play a role.)

How much a fault line zigzags, undulates, and branches can play a major role in how earthquakes occur.

Faults can be complex at different levels, from small complexity on the scale of millimeters to large-scale complexity over tens of miles, such as shaking at the boundaries between tectonic plates. “If you zoom in, [a fault] It may seem very complicated. If you zoom out, it might seem relatively simple. “Then you might zoom out even further, and it might look more complicated on that larger scale,” Cai says. The maps used by Cai and his colleagues show the complexity of the faults on a large scale, over a dozen miles or so.

He adds that their research did not explore how smaller-scale complexity might relate to creep and earthquake intensity, but in theory, small-scale fault complexity could be linked to small earthquakes, while large-scale fault complexity could produce large earthquakes. Large-scale complexity would cause stress to build up over a larger area, causing larger slabs of rock to sway, leading to larger earthquakes.

It’s possible to measure the complexity of faults to gauge earthquake risk in areas like California where faults are well mapped, but many parts of the world have yet to be mapped this way, says geologist Romain Jolivet of the École Normale Supérieure in Paris, who was not involved in the study. “A good fault map is the result of decades of field investigations,” he says. It’s a huge 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 inaccessible for fieldwork for political or security reasons,” he says.

Jolivet appreciates the researchers' new approach to using fault, creep and earthquake data, which helped them determine the relationship between fault complexity and earthquakes. However, he suggests that it may be useful to model how error complexity relates to creep to ensure that the association is not just a coincidence.

If the research results are correct, they could help us better understand the mathematics and physics of Earth's faults, the rupture zones beneath our feet, and to pinpoint where tomorrow's major earthquakes might occur in California and beyond.

Main image: vernStudio / Shutterstock

Lisa S. Gardiner

Posted on June 25, 2024

Lisa S. Gardiner is a writer, geologist, and educator. She is the author of the award-winning book Tales from an Uncertain World: What Other Diverse Disasters Can Teach Us About Climate Change (2018) and her writing has appeared in Scientific American, Hakai, The Atlantic, and other publications. She is currently working on a book about coral reefs past, present and possible future.

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