Reporter Journal / Chris Davis

Picturing the world below: how to predict earthquakes

(China Daily USA) Updated: 2017-08-09 12:03

The latest earthquake in Sichuan province is a reminder of the never-ending wish for a way to predict such deadly cataclysms.

The latest - a magnitude 7.0 that occurred on Tuesday evening at 9:19 pm 177 miles north of Chengdu and 12 miles deep - killed at least 13 and injured 175.

It came along the same Longmen Shan fault line that produced the 2008 magnitude 8.0 temblor that killed 69,000 and left 5 million homeless. That was 50 miles to the west-northwest of Chengdu.

The fault line is basically where the Australian and Indian plates are slamming into Asia, and the most apparent nearby result is the Longmen Mountains range, where elevations rise from about 2,000 feet to 21,000 feet in a space of less than 30 miles, forming the eastern ridge of the Tibetan Plateau. It's one of the planet Earth's most dramatic escarpments, and it's obviously not done being built.

Plate tectonics - the study of the Earth's crust and what makes it do what it does - has a lot of theories that are impossible to test. As one scientist once told me: "It's not like we can go down there and see what's happening."

Another expert suggested that all of the tectonic activity on Earth - the heaving land masses, the exploding volcanoes, the rupturing fault lines on the oceans' floors - are all byproducts of the planet trying to release its inner heat.

Tectonics really comes down to scientists making educated guesses about what's going on deep inside our planet and then seeing how well their theories explain events like Tuesday's earthquake.

A group at Princeton is bringing supercomputers to the task of shining a light on what's going on miles and miles beneath the ocean's floor by creating a picture of it.

Using advanced modeling and simulation techniques, seismic data from earthquakes scattered around the planet and one of the world's fastest supercomputers, a team led by Princeton's Jeroen Tromp has been constructing a 3-D image of the Earth's interior down to 1,800 miles, where the mantle meets what's believed to be the planet's iron core.

The result is a high-fidelity visualization that ties together tectonic plates, magma plumes and hot spots - like the one under the Pacific Plate that has been (and still is) forming the island chain of Hawaii.

It's all about modeling and reading the waves that travel from a quake's epicenter in all directions, the same ones that wreak havoc on things living on the crust's surface - like us - but also leave a trail for scientists to sleuth out, track and chart.

Not a simple task. "You need really big computers to do this," one team member said.

In this case it was Titan, a 27-petaflop Cray XK7 managed by the US Department of Energy at its Oak Ridge National Laboratory in Tennessee, where the team was able to push the envelope of their science - computational seismology - to its limits.

Seismic waves change speed when they pass through hot magma or cooler zones where plates slip under their neighbor. The variations paint a picture that the computer was able to stitch together to create the 3-D image.

Tromp's team used earthquakes between magnitudes of 5.8 and 7 on the Richter scale. The range extended to include more than 6,000 of the earthquakes in their data base.

The team continues to dig deeper and refine the model. As one of the project's investigators - Ebru Bozdag, assistant professor of geophysics at the University of Nice Sophia Antipolis - told a DOE newsletter: "Most global models in seismology agree at large scales but differ from each other significantly at the smaller scale. That's why it's crucial to have a more accurate image of the Earth's interior.

A living, breathing model of the Earth's core would be a first step toward predicting earthquakes below in the way we can forecast storms above.

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