“Mountain formation is still one of the big mysteries left in Earth science.” — George Zandt and Susan Beck, UA geoscientists

Which is why Susan Beck and George Zandt, two geoscientists at the UA, are devoting their careers to figuring out the rest.

Funded by the National Science Foundation, their research aims to understand the basic science behind mountain formation beyond what plate tectonics explains. “Mountain formation is still one of the big mysteries left in Earth science,” says Zandt, a professor of geosciences at the UA. “These plates are not rigid. There are all sorts of processes that are going on that are major.”

“To understand the mountains,” they say, “You have to go to the mountains.” Lucky for them, traveling every three to four months to South America, the locus of their research, doesn’t pull them away from their families. They’re married.

“We were both interested in mountain building and we just asked, ‘Where can we make a difference? Where in the world can we go to study this?’ says Beck, chair of the UA geosciences department. “South America seemed like the obvious natural laboratory.”

The Andes, they say, is an ideal mountain range to study. There, a classic model of the subduction of an oceanic plate, in this case the Nazca plate, under a continental plate, the South American plate, occurs. The subduction pushes against the continental plate forming the high mountains of the Andes.

“What is happening in the Andes now,” Beck says, “is what happened in the Western United States a long time ago. It’s the same tectonic setting.”

“A modern analog,” Zandt says.

George Zandt supervises the installation of a seismic station in South America.

To study the Earth’s crust and mantle within the Andean range, they have placed seismic stations in Bolivia, Chile, and Argentina. The seismometers, buried deep in bedrock, record all ground motion.

As a rule, both Beck and Zandt get excited when the Earth moves. Last June, off the coast of Peru, the biggest earthquake in 30 years occurred. “It measured 8.4 on the Movement Magnitude scale, and 10 to 12 meters of slip were recorded,” Beck says.

In June of 1994 their equipment in Bolivia caught the biggest deep earthquake ever recorded ­ an 8.3 quake at 645 kilometers below the Earth’s surface. “The energy from that earthquake shook high rises in Toronto, because of seismic waves that traveled deep in the Earth from Bolivia to Canada,” Zandt says. “But we had students in Bolivia who didn’t even feel it.”

The earthquakes and other ground motion help them understand what’s going on within the Earth’s crust and mantle.

“The seismographs work like echo sounding,” explains Zandt. “We use earthquakes as a source. Instead of measuring echo waves that go down and up, however, we use waves that are generated on the way up to the seismic station. That way we can measure the depth to interfaces in the Earth beneath the station.”

The current project, CHARGE ­ the Chile and Argentina Geophysical Experiment, is a collaboration with the University of San Jean and INPRES in San Juan, Argentina and with the University of Chile in Santiago. The equipment is supplied by IRIS, a consortium of 100 or more universities whose researchers share equipment.

Partnering with colleagues in Chile and Argentina is critical to their work, they say. The placement of the equipment requires that they travel into remote areas. “At first people in the villages were suspicious,” Zandt says. “Some of them thought we were generating earthquakes. We have a habit of going in and then earthquakes happen.”

“But earthquakes are happening there all the time,” Beck adds.

Though their project currently focuses on Chile and Argentina, they have also conducted studies in Bolivia and Peru. “In Bolivia, one of the first things we discovered was that the Earth’s crust was up to 75 kilometers thick. Most continental crust is about 40 kilometers thick. We thought that would be the case, but no one had measured it accurately before,” Beck says.

“We think we know the tectonic implication of this,” Zandt says. We think thick crust is produced through large amounts of shortening and contraction. That much is pretty well known. But how much shortening is happening?”

Through seismographic data they can “map” where the Earth’s crust ends. In the Andes there is east-west shortening within the Earth’s crust, which results in thicker crust that “floats” higher. That explains, in part, why the mountains there are so high.

One unexpected finding, Beck says, has been related to seismic velocity, or the speed of the seismic waves propagated through the Earth. “Once you know the thickness of the crust you can look at the speed of the waves. This helps us characterize crust.”

This closeup of instruments buried in two barrels in the ground reveals batteries in the one to the left and electronic equipment in the one on the right.

Though their work is considered basic research, it holds potential important future applications, Zandt says. Most important, understanding the formation of the Andes could hold important implications for earthquake hazards. “Actually predicting earthquakes has turned out to be extremely difficult. Some people believe it’s so random that we’ll never be able to predict them. But never is not such a good word in this profession,” Zandt says. “Still, predictability is a long way off. What we can do is deal with probabilities. And understanding probabilities is very valuable to urban planners and structural engineers.”

Also, their findings could provide important information about mineral deposits and where they are located. “South America is one of the most important mining regions of the world. Copper production there rivals, actually surpasses, that in Arizona. A better understanding of structure and tectonics will eventually give us a better understanding of ore deposits.”

Beck and Zandt met in 1988 at the Lawrence Livermore Laboratory in Northern California, where they both were working. Beck came to the UA in 1990 and after years of long-distance commuting Zandt was hired here in 1997. They were married in 1998. “We’ve been very fortunate as far as the dual career opportunities we’ve been given,” Beck says.

Unfortunately, Beck says, the two don’t usually go into field together very often. “Once we managed to rendezvous for a whole day together in La Paz. That was nice.” Still, she says working with her husband, when it happens, is “great fun.” Out of the field, she says, “We spend a lot of time talking about the Andes. Pretty much all the time.”