Seismic waves move through Earth, as shown by 3D models

Park holds a prototype model, which is made in translucent material to show the layers. The metal block to the left is a completed model representing the ground below Los Angeles. Credit: Jason Smith

In September 1985, a big earthquake killed more than 9,000 people in Mexico City, even though the epicentre was more than 200 miles away.

The worst damage happened in the city itself, in part because Mexico City is built on an old basin surrounded by mountains. The shaking is thought to have been made worse by the soft foundation, which caused seismic waves to bounce around in the ground.

Scientists are worried because many populated cities, like Los Angeles, are built on basins like this, and it has been hard for researchers to figure out how the ground moves in these kinds of earthquakes. But a new method that uses 3D printing and lasers could help us learn more about what happens when the ground shakes and how different layers and formations below the ground can make earthquake damage less or more severe.

“We know that the same earthquake would feel different if you were in a basin or on a mountain, but it’s hard to predict or simulate that, in part because it’s hard to get the level of detail you need,” said Sunyoung Park, a geophysicist at the University of Chicago and the lead author of a study about the process that was published in Scientific Reports. “With these 3D models, you can get a level of detail that really helps you see patterns you wouldn’t have been able to see before. It’s a really cool way to do things.”

The bottom line

The ground under our feet is made up of different layers that have built up over time. They could be anything from soft clay to hard shale. During an earthquake, each layer responds in a different way. For example, layers that are more flexible can absorb some movement, while other layers make it stronger. Waves can also be sent back and forth by the depth and strength of an earthquake and the shape of the land around it. All of these things make it very hard to predict how bad earthquake damage will be.

Scientists can try to figure out what will happen by using computers, but it’s not perfect. “Simulating all of this is really hard to do, not only because it takes a lot of computing power, but also because we don’t know enough about the physics at small scales,” Park said. “For example, what happens to waves if there are water-filled aquifers or magma chambers? We don’t know much about it.”

In Japan, scientists have been building real models of the ground since the 1920s to try to figure out what happens during earthquakes. But they require a lot of work and aren’t very useful. “You have to carve out many different layers and try to glue them together perfectly. Since there are so many different types of rock, it’s hard to make exact copies,” Park said.

Park and her colleagues thought it would be a great use for 3D printing because of this.

With a special 3D printer for metal, they could make as many layers as they wanted, and they would all fit together perfectly. It starts with metal powder that is heated by a laser to form one layer at a time. By changing the intensity and scanning speed of the laser, they can make each layer more porous or more dense to look like different types of rock.

Sunyoung Park, a geoscientist at the University of Chicago, shows simulations of seismic waves made with 3D metal printers as a new way to learn more about how earthquakes shake. Thanks to: Jason Smith

Using this method, scientists made an eight-inch-long copy (at a scale of 250,000 to 1) of the rock that is under the city of Los Angeles.

Then, they use lasers and lab equipment to simulate an earthquake and watch the model to see how the waves move through the different layers.

Park said that it looked a lot like data that had been collected from real earthquakes. They could see different kinds of seismic waves moving through the model, just like when they watch real earthquakes.

For this study, they wanted to look at how seismic waves with different frequencies move through the ground. Low-frequency waves are easier to find and study than high-frequency waves, so they have been looked at much more. But scientists think that high-frequency waves are the main cause of damage to buildings and infrastructure. With the printed model, the scientists could simulate the higher frequency waves in a more direct way.

Some of what they found was surprising. “This result shows that the high-frequency waves are less strong inside the basin, which is almost the opposite of what was thought before,” said Park.

Scientists had already seen that low-frequency waves got stronger in the basin, so they thought that high-frequency waves might do the same. Instead, it looks like the edge of the basin is where high-frequency waves bounce back. Because of this, they are weaker inside the basin and stronger near the edges.

She said, “It seems to mean that what we know about low-frequency waves doesn’t work for high-frequency waves, and that we may need a different framework to understand these shakes.”

Park said that the metal models can be made in a few hours and can be used many times. The L.A. model has already been used in more than 2,000 tests.

Park can think of more ways than just earthquakes to use the method. We don’t know much about the Earth’s deeper layers, but 3D models might help us learn more. Also, the new method can be used to find air waves. So Park and her group are looking at how mountains and valleys affect the way air waves travel. This can help us learn more about explosions caused by nature or by humans.

“We could even do other planets. For example, we know from seismic sensors on the moon and Mars that they have Marsquakes and moonquakes, but their recordings look very different from those of earthquakes,” Park said. “You could try to understand by making scale models of the moon or Mars.”

Further information: Sunyoung Park et al, Seismic wave simulation using a 3D printed model of the Los Angeles Basin, Scientific Reports (2022). DOI: 10.1038/s41598-022-08732-w

Journal information: Scientific Reports

Source: University of Chicago