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Riding the 3D seismic wave

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  • First full-physics 3D view of Earth's interior
  • Simulations require large amounts of computational firepower
  • Research was an internatonal collaboration decades in the making

Scientists investigating Earth’s interior face a simple, yet complicated problem. How is it possible to visualize Earth’s insides when the layer of mantle beneath the crust can be as hot as 3,200º C near the core-mantle boundary?

Seismologists usually use a technique known as seismic tomography, similar to a CAT scan of the human body, to map our planet’s inner structure.

Pillars of the earth. Using data gathered from 253 earthquakes around the world, this visualization is the first global tomographic model constructed based on adjoint tomography, an iterative full-waveform inversion technique. Courtesy David Pugmire, ORNL.

However, classical tomography cannot fully capture the complexity of seismic waves, says Ebru Bozdag of the Colorado School of Mines.

This full range of physics data coming from these seismic waves includes anelasticity, the topography, depth, and load of the ocean, as well as the ellipticity, gravity, and rotation of the earth.

“In a realistic medium, wave dissemination can be much more complex,” says Bozdag. “The waves do not necessarily follow straight lines.”

To solve this problem, Bozdag collaborated with a team led by Jeroen Tromp of Princeton University and other colleagues on a project that allows researchers to visualize and improve our understanding of the inner workings of the earth with 3D simulations of seismic waves.

Picturing the unseen world

Capturing the unseen physics of seismic wave distribution has been a key challenge for seismologists. Compressional (P) waves, for example, can travel through any kind of material and move like a slinky being tossed down a flight of stairs whereas shear (S) waves cannot travel through liquids.

We had to wait [decades] for advancements in high-performance computing to perform seismic tomography and to image Earth’s interior globally with full 3D wave simulations. ~Ebru Bozdag

If researchers could access higher-resolution 3D images, they could learn more about how these waves move through the Earth. The opportunities for observing other phenomenon, such as the composition of earthquakes in high-risk areas, would be limitless.

“Having accurate images of Earth’s interior is crucial and is the first step to better understanding the dynamics, composition, and structure of the mantle,” Bozdag says. “This will ultimately help us know more about the mechanism of earthquakes and their locations which is important for assessing seismic hazards, such as in Los Angeles.”

Journey of 1,000 GPUs

Properly visualizing these waves in 3D is a goal that took scholars decades to achieve.  

The idea of using 3D wave simulations to map Earth’s interior dates to the 1980s. Albert Tarantola, a physicist at Institut de Physique du Globe de Paris, published a paper in 1984 that was a major step forward in seismological imaging.

“However, we had to wait about 25 years for advancements in high-performance computing to perform seismic tomography based on 3D wave simulations,” Bozdag says, “and more than 30 years to image Earth’s interior globally.”

The computational demands of waveform data analysis have been a challenge for scientists. Bozdag and her team originally used a CPU version of the SPECFEM3D_GLOBE that was developed in 2002, before accessing the 18,688 GPUs on the Titan supercomputer at Oak Ridge National Laboratory.

<strong> Titan. </strong> 18,688 GPUs enabled the first underworld visualization to incorporate the full complexity offered by seismic wave propagation data. Courtesy ORNL.

“We constructed the first global adjoint tomography model with 253 earthquakes. Our aim now is to use all earthquakes, more than 4,000, to image Earth globally,” Bozdag says. “We need powerful supercomputers like Titan to continue running higher-resolution simulations in shorter periods, ultimately down to just one second.”

3D resolutions of Earth’s interior will enable scientists to better understand how the inner workings of our world affect us. From understanding the physics of earthquakes, to learning more about volcanoes, researchers can gain a vast array of knowledge about Earth’s inner world without risking the dangers of up to 3,200º C (5,792º F) in the mantle.

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