Some of the world’s largest cities—including Los Angeles, Mexico City, and Santiago—are located in naturally occurring sedimentary basins. Add in the fact that these cities are prone to earthquakes, and that’s potentially a recipe for disaster: Numerical modeling has suggested that ground shaking is amplified within basins.
But such modeling—an oft-used resource for understanding ground motion in sedimentary basins—is often limited in its spatial resolution and is furthermore constrained by the equations it receives as input. Now, to more thoroughly study how seismic waves travel through a sedimentary basin, researchers have conducted a series of seismic experiments using 3D printed models of the underbelly of Los Angeles. They found that the highest-frequency seismic waves—those that generate sudden changes in acceleration and are therefore the most destructive to buildings—were actually attenuated within the models’ basin. That’s wholly unpredicted by numerical models, the team noted.
Trade-Offs to Consider
Sedimentary basins are complex geological structures. They start out as depressions that over time become filled in with lower-density material deposited by rivers and landslides. “Imagine a bowl being filled up with stuff,” said Chukwuebuka C. Nweke, a civil engineer who works on natural hazards at the University of Southern California in Los Angeles who was not involved in the research.
But reproducing the small-scale details of a sedimentary basin in a numerical model is challenging, said Nweke, given inherent trade-offs between a model’s spatial resolution and the computational time required to run it. “We don’t want to have our model running for 20 years.”
A Boost in Resolution
For that reason, Sunyoung “Sunny” Park, a seismologist at the University of Chicago, and her colleagues recently began 3D printing models of the Los Angeles basin. Park and her team realized that they could reproduce even relatively small natural variations in density—corresponding to about 10 meters in size in real life—in their 3D printed models. That’s roughly a factor of 10 better than the spatial resolution of a numerical model that’s commonly used to study the Los Angeles basin, said Park.
After experimenting with materials such as rubber and plastic, Park and her colleagues settled on stainless steel as their preferred printing medium. That choice was mainly dictated by steel’s rigidity, said Park. “If it’s rigid, it has a much larger range of material properties.”
The researchers printed their models much in the same way that ink is printed on paper: They laid down successive layers of powdered stainless steel and then used a laser to heat and join (“sinter”) the layers together. By changing the printing parameters—including the speed of the sintering laser and its power—it’s possible to control how much pore space remains, said Park. “That’s how you can print a variable range of densities.”
The models the researchers produced, measuring roughly 20 centimeters long by 4 centimeters wide by 1 centimeter thick, aren’t much to look at from the outside, said Park. But in actuality each one captures a range of geological structures within the 50-kilometer-wide Los Angeles basin at a scale of 1:250,000. “It has all these structures within it,” said Park.
Earthquakes from Lasers
The team members generated extremely tiny earthquakes in their models by bombarding them with megahertz-frequency laser light. The thermal energy of the laser pulses heated the models, resulting in differential stresses that translated into movement, albeit very small: Park and her colleagues recorded ground motion at the top of the models on the order of tenths of nanometers.
The researchers found that higher frequencies of ground motion in their models—corresponding to real-life frequencies above 1 hertz—were generally reduced within basins. Those waves tended to be selectively reflected back at the edges of a basin, the team showed.
That’s a surprise, said Park, because sedimentary basins have long been believed to be amplifiers of ground motion. “[These results] are in some sense opposite of our conventional understanding.”
These results were presented today at AGU’s Fall Meeting 2021.
There’s plenty more to investigate using these models, the researchers suggested. One unexpected finding from the scientists’ experiments was that their laser pulses triggered not only seismic waves but also airborne waves that skimmed over the models’ top surfaces. Because such waves are strongly affected by local topography, logical follow-on work could include adding features like hills and mountains to the models’ surfaces and then measuring how the airborne waves propagate, Park said.
—Katherine Kornei (@KatherineKornei), Science Writer