Earthquakes ripple through 3D models printed in Los Angeles

Some of the world’s largest cities—including Los Angeles, Mexico City, and Santiago—are located in naturally occurring sedimentary basins. Add to the fact that these cities are prone to earthquakes, and that’s likely a recipe for disaster: Numerical modeling has suggested that Earth’s shaking is amplified within the basins.

But such modeling—a frequently used resource for understanding the motion of the Earth in sedimentary basins—is often limited in spatial resolution as well as constrained by the equations it receives as input. Now, to do a more comprehensive study of how seismic waves travel through a sedimentary basin, researchers have conducted a series of seismic experiments using 3D-printed models of the Los Angeles abdomen. They found that high-frequency seismic waves—those that generate abrupt changes in acceleration and are thus the most destructive to buildings—were actually attenuated within the models’ basin. The team noted that the numerical models are completely unexpected.

Tradeoffs to consider

“We don’t want to run our model for 20 years.”

Sedimentary basins are complex geological structures. They begin as depressions that become filled with low-density material over time, deposited by rivers and landslides. “Imagine a bowl full of stuff,” said Chukwuebuka C. Nweke, a civil engineer working on natural hazards at the University of Southern California in Los Angeles who was not involved in the research.

Reproducing the small details of a sedimentary basin in a digital model is challenging, said Nowicki, due to the inherent trade-offs between the model’s spatial resolution and the computational time required to run it. “We don’t want to run our model for 20 years.”

boost in resolution

For this reason, Sunyoung “Sunny” Park, a seismologist at the University of Chicago, and her colleagues recently started 3D-printing models of the Los Angeles Basin. Park and her team realized that they could reproduce relatively small natural variations in density – corresponding to about 10 meters in size in real life – in their 3D-printed models. This is about 10 percent better than the spatial resolution of the numerical model commonly used to study the Los Angeles Basin, Park said.

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 dictated primarily by the hardness of the steel, Park said. “If it’s a solid, it has a much larger range of material properties.”

“It has all these structures inside of it.”

The researchers printed their models the same way ink is printed on paper: They applied successive layers of stainless steel powder and then used a laser to heat and bind (sinter) the layers together. By changing the printing parameters — including the laser’s sintering speed and power — it’s possible to control how much pore space is left, Park said. “This is how you can print a variable range of densities.”

Park said the models produced by the researchers, which are about 20cm long, 4cm wide and 1cm thick, aren’t much to look at from the outside. But in fact, each one captures a range of geological structures within the 50-kilometre-wide Los Angeles Basin on a 1:250,000 scale. “It has all these structures in it,” Park said.

earthquakes from laser

The team members generated very small earthquakes in their models by bombarding them with laser light at a frequency of megahertz. The thermal energy of the laser pulses heated up the models, resulting in differential pressures that translated into motion, albeit very small: Park and colleagues recorded the motion of the Earth at the top of the models on the order of tenths of a nanometer.

The researchers found that the higher frequencies of Earth’s motion in their models – corresponding to real-life frequencies above 1 Hz – generally decreased within the basins. The team showed that those waves tend to selectively reflect off the edges of the basin.

That’s a surprise, Park said, because sedimentary basins have long been thought to be amplifiers of Earth’s motion. “[These results] It is somewhat antithetical to our traditional understanding.”

These results were presented today at the fall 2021 meeting of the Gulf University.

The researchers suggested that there is much more to research using these models. One of the unexpected discoveries from the scientists’ experiments was that their laser pulses caused not only seismic waves, but also airborne waves that passed over the upper surfaces of the models. Because such waves are strongly influenced by local terrain, Park said, follow-up logic work could include adding features such as hills and mountains to the surfaces of models and then measuring how the airborne waves propagate.

—Katherine Kornei (KatherineKornei), science writer

Citation: Kornei, K. (2021), Earthquakes ripple through 3D print Models of Los Angeles, Eos, 102, https://doi.org/10.1029/2021EO210657. Posted on December 13, 2021. Text © 2021. authors. CC BY-NC-ND 3.0 Unless otherwise noted, images are subject to copyright. Any reuse without the express permission of the copyright owner is prohibited. Related

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