Understanding the deep earth to predict earthquakes

They only travel about four inches per year. That makes a sloth, which can achieve a top speed of 6.5 feet per minute, seem like a furry blur by comparison. And yet, when they move, they can change the face of the planet.

The slow shifting of the enormous tectonic plates — like puzzle pieces pressing against and pulling away from each other over the surface of the planet — can not only cause devastation through earthquakes, but can also build mountains and form deep-water ocean trenches.

Ever since the 1960s, when scientists accepted the theory of continental drift, they have been trying to understand and predict where those plates move. There has been a rift in the scientific community itself over what forces (and where those forces are located) that determine plate movement.

“Some people have been saying the deep Earth is everything and is the most important role and force in making the plates move,” said Attreyee Ghosh, a post-doctoral student at Stony Brook University’s Geosciences Department. “Another group has been saying that the deep Earth doesn’t have to play that much of a role and they can explain [the movements] looking at the top 100 kilometers of the Earth.”

As it turns out, both groups are right, depending on where you are on the planet, Ghosh explained.

The viscosity (or thickness) of the middle layer in between the top Earth and the deeper parts of the planet determines which contribution can be more important in driving the movement of the plates.

“A higher viscosity increases the contribution from the deeper mantle,” Ghosh suggested. “When the viscosity is weaker, there is a decoupling from the deeper mantle, so the [deeper Earth] doesn’t play as dominant a role” as the lithosphere, or the upper layer.

Working with professor William Holt, Ghosh created a mathematical model to predict movements of the plates. The scientists input readings from “literally hundreds if not thousands” of other researchers using data points from seismic records and earthquakes as they built and tested their computer model, Holt recalled.

The results were so much more effective than most of the models that one of the most prestigious journals in the country, Science, published their findings earlier this year.
Despite its effectiveness, the model, which was created on a relatively simple computer, can do better, Holt said.

“We are not very far away from having models that can predict surface observations at a level of accuracy that approaches the uncertainty in the measurements themselves,” Holt suggested.

What that means is that the models may become as accurate as the data they receive.

So, what does this increased accuracy mean for anticipating earthquakes?

While the science of predicting earthquakes is still years away, this is an important step toward building a better foundation for long-term earthquake forecast models. Holt explained. “We’re getting more accurate forecasts for the probabilities of earthquakes in particular regions,” he explained. “That enables one to prepare through proper retrofitting of buildings and construction for long-term mitigation of potential hazards from earthquakes.”
Holt, who has been working in this field for 20 years, said scientists have boosted dramatically the amount of information they gather about the Earth.

When he started, “we didn’t have observations to understand how the interior of Tibet was moving or how the interior of Nevada was moving, so there was this big revolution through the 1990s, with the advent of space geodesy,” he said. “I can see incredible progress over the last 20 years.”

When he’s not collecting and interpreting data about the planet, Holt is a busy father of two primary school daughters and the husband of Troy Rasbury, a geochemist at Stony Brook.
Holt enjoys sea kayaking and, more recently, fly fishing, although he said most of the time, he “catches nothing.”

As for Ghosh, she is a resident of Manhattan and commutes to Long Island. She expects to finish her post-doctoral work at Stony Brook in the next few months.

Inspired by her progress thus far, Ghosh expects to continue to look deep into the Earth to understand the movement of some of its larger pieces.

“I’m intrigued by how much of our Earth we still don’t know,” she offered.