Natural sedimentary basins are home to some of the world’s most populous cities, including Los Angeles,
Mexico City, and Santiago.
When you factor in the fact that these cities are prone to earthquakes,
you have a prescription for disaster:
According to numerical modelling,
ground shaking is amplified within basins.
However, such modelling—
which is frequently used to analyze ground motion in sedimentary basins
—is frequently limited in geographic resolution and further constrained by the equations it receives as input.
Researchers have now completed a series of seismic experiments using 3D printed models of Los Angeles’
underbelly to better understand how seismic waves flow through a sedimentary basin.
They discovered that the highest-frequency seismic waves,
which cause abrupt changes in acceleration and
are thus the most destructive to buildings,
were dampened within
the basins of the models.
The team pointed out that numerical models could not have predicted this.
Consider the following trade-offs
“We don’t want to be running our model for the next 20 years.”
Sedimentary basins are geologically complicated structures.
They begin as depressions that fill in with lower-density material deposited by rivers and landslides throughout time.
“Imagine a bowl full of material,” said Chukwuebuka C. Nweke
, A civil engineer at the University of Southern California in Los Angeles
who specializes in natural hazards and was not involved in the study.
Consider the following trade-offs:
“We don’t want to be running our model for the next 20 years.”
Sedimentary basins are geologically complicated structures. They begin as depressions that fill in with lower-density material deposited by rivers and landslides throughout time.
“Imagine a bowl full of material,” said Chukwuebuka C. Nweke,
a civil engineer at the University of Southern California in Los Angeles
who specializes in natural hazards and was not involved in the study.
However,
due to intrinsic trade-offs between a model’s spatial resolution and the computational time necessary to execute it,
replicating the small-scale characteristics of a sedimentary basin in a numerical model is difficult,
according to Nweke.
“We don’t want to be running our model for the next 20 years.”
An Increase in Resolution
As a result, Sunyoung “Sunny” Park, a University of Chicago seismologist, and her colleagues
recently began 3D printing replicas of the Los Angeles basin.
Park and her colleagues discovered that their 3D printed models could represent even
minor natural fluctuations in density—
roughly 10 meters in size in real life.
According to Park,
this is about a factor of ten better than the spatial resolution of a numerical model frequently
used to examine the Los Angeles basin.
Park and her colleagues chose stainless steel as their favourite printing medium
after testing with materials such as rubber and plastic.
According to Park, the stiffness of steel was a major factor in his decision.
“It has a much wider variety of material qualities if it’s stiff.”
“It contains all of these structures.”
The researchers used a laser to heat and fuse (“sinter”) the layers together,
similar to how
ink is printed on paper.
It’s feasible to manage how much pore space remains by modifying the printing settings,
such as the sintering laser’s speed and power, according to Park.
“That’s how a wide range of densities can be printed.”
The researchers’ models,
which are around 20 centimetres long,
4 centimetres broad, and 1 centimetre thick on the outside, aren’t particularly attractive, according to Park.
However, at a scale of 1:250,000
, each one depicts a variety of geological features within the 50-kilometre-wide Los Angeles basin.
“It has all of these structures,”
Park explained.
Earthquakes Caused by Lasers
By hitting their models with megahertz-frequency laser light,
the team members were able to create incredibly small earthquakes.
The laser pulses’ thermal energy heated the models,
causing differential tensions that resulted in movement, albeit very small:
Park and her colleagues measured ground motion on the order of tenths of nanometers at the tops of the models.
Higher frequencies of ground motion in their models—
which correlate to real-life frequencies over 1 hertz—
were found to be reduced in basins,
according to the researchers.
The scientists discovered that such waves were selectively
reflected at the basin’s margins.
This is surprising,
according to Park
, because sedimentary basins have long been thought to act as ground
motion amplifiers.
“[These findings] are, in some ways,
counterintuitive to our current knowledge.”
These findings were presented
today at the
American Geophysical Union’s Fall Meeting 2021.
The researchers suggested that
these models could be used to investigate a lot more.
The scientists’ investigations revealed that their laser pulses
created not just seismic waves
but also airborne waves that skimmed over the top surfaces of the models.
Because such waves are greatly influenced by
local geography
, Park suggested that adding elements such as hills and mountains to the models’ surfaces and
then analyzing how the airborne waves propagate would be a reasonable
next step.
Source: EOS
