Profile of Don Helmberger

Greetings from Lake Wobegon

Anyone familiar with author and humorist Garrison Keillor will have an instant appreciation for Helmberger’s youth. He was the youngest child of 13 in a rural Minnesota farming community halfway between Brainerd and Fargo, North Dakota. “He’s basically describing my hometown,” says Helmberger of Keillor’s radio segments about the comings and goings in the fictitious town of Lake Wobegon. “I like it when he describes how every farm kid would be associated with some townie who they would stay with when the blizzard was so bad that he couldn’t go home. The humor of it all is very clear to me,” he says.

The low temperatures in Minnesota were also clear to him growing up. “Northern Minnesota is as cold as it looks. It really is a very cold place. It’s healthy, probably, but it’s not too much fun,” says Helmberger. He recalls taking many long walks in the snow to get to and from his country school, and he was more interested in staying warm than in figuring out his academic interests, though it did give him one definite aspiration. “I was motivated to move to a warmer climate,” he says.

Although his parents did not have advanced educations, Helmberger says he was lucky to have a smart older brother, John. The oldest Helmberger boys were drafted into the U.S. Army during World War II, but John’s intelligence kept him out of combat; instead, he became a letter writer for one of the generals. After the war, John went to the University of Minnesota (Minneapolis), where Don also enrolled in 1956. “He kind of set a path for his brothers to follow,” says Helmberger of John, “and three of them, including me, did, and stayed at the same rooming house. There was a Helmberger there for, I don’t know, 10, 15 years.”

Between his semesters at Minnesota, Helmberger first found a sense of direction. “Every summer I had to get a job, since that was important to help me finance my education,” he says, “and, in the summer of 1960, I had the good fortune to go on a cruise organized by The Scripps [Institution] of Oceanography.” The research cruise, called Leapfrog, traveled around the coast of Alaska and the Bering Sea collecting seismic data. “This was the first time I got out of Minnesota for any length of time. It was really quite an eye opener for me,” Helmberger says. During the trip, he made the acquaintance of several Scripps researchers, among them Russell Raitt. Says Helmberger, “He was an interesting fellow. During the war, he worked on submarine communications for the Navy, so he got into seismology as a physicist, which was totally different than the other people who were doing seismology. Seismology was treated like a tool for geologists to learn about the Earth, but they didn’t treat it like a science. He put it on a scientific footing, which turned out to be important for me.”

Warm Climes and Large Explosions

Whether it was the fascinating people he met on the cruise, the research opportunities Scripps presented, or just the chance to move to San Diego, CA, Helmberger was excited upon his return. “When I got back to University of Minnesota, where I was a graduate student in physics at the time, I immediately applied to Scripps and moved out there as soon as I could,” he says.

At Scripps and the University of California at San Diego (La Jolla, CA) Helmberger joined up with Raitt, and the two began applying the wave propagation data obtained on the Leapfrog to create model seismograms. “Besides Russell, who knew a heck of a lot about what he was doing, I also had the good fortune of knowing Freeman Gilbert, a first-rate theoretical seismologist,” says Helmberger. “The two of them did all the work, and the only thing I did was put it all together,” he says. What Helmberger managed to “put together” was ahead of its time, however: his thesis work was the first to show that model seismograms could be designed to provide detailed images of what was in the Earth (2).

Shortly before receiving his Ph.D. in 1967, Helmberger attended his first scientific conference, the annual meeting of the Seismological Society of America, in Reno, NV. He went to the meeting to discuss his modeling studies, and he impressed at least a few people with his talk. “There was a fellow there I didn’t know who offered me a postdoc at MIT. When I went over to Freeman and asked him who that guy was who just offered me a job, he said, ‘Well, that’s Frank Press,’ who happened to be the biggest gun in geophysics. So that was pretty nice,” says Helmberger.

He joined the Massachusetts Institute of Technology (MIT; Cambridge, MA) in 1967, carrying out postdoctoral research under Press and M. Nafi Toksöz. Toksöz conducted research on bomb detection, and Helmberger turned his skills toward modeling the seismograms produced by large explosions. “We were trying to figure out how big the explosions were that were being set off by the Soviets,” he says. This work led Helmberger to a longstanding collaboration with the Defense Advanced Research Projects Agency (DARPA) at the U.S. Department of Defense. He served on early advisory panels monitoring large explosions. “We were checking what countries were testing underground, how big the devices were, and what exactly they were testing,” he says.

After two years at MIT and one year as an assistant professor at Princeton University (Princeton, NJ), Helmberger returned to sunnier climes when Don Anderson offered him a position as Assistant Professor of Geophysics at CalTech. “I really liked the West Coast a lot,” says Helmberger, “so I took this opportunity to come back.” Helmberger continued to model underground explosions when he returned to southern California in 1970, but, shortly after joining CalTech, Helmberger experienced firsthand a powerful explosion of a different sort.

Structure and Source

Just before sunrise on February 9, 1971, the San Fernando earthquake rocked southern California, rousing Helmberger from his sleep. “It was quite impressive. It broke all kinds of glasses in our house, and I think it really ‘woke me up,”’ he says, referring not only to the disruption of his slumber but also to a new field where he could apply his seismographic wave models. “I turned my attention to earthquakes, and we set up a group in our lab to study and model earthquakes to see what kind of motions they make,” he says. Together with graduate student Chuck Langston, Helmberger devised a procedure to model earthquakes by inverting their waveform data (3). “We found we could take and model global seismic records and, from that, figure out how deep the earthquake was and what orientation it was. That paper started a new field about what you could learn about earthquakes,” says Helmberger.

To truly learn about earthquakes, though, Helmberger knew he also needed to learn more about the earth and rock that they shook. “You kind of go back and forth. You can study wave propagation through the complicated earth, or you try to understand the source that produces these waves. Both of these issues have to be modeled to make a complete numerical record of what’s going on,” he says. So Helmberger also began examining the composition of the Earth, especially the boundary zones between the various layers of the planet. Together with another student, Steve Grand, Helmberger modeled the triplications in the upper mantle (4). “There are two big boundaries in the upper mantle,” he says, “and we showed that one could model these structures, and that they varied very strongly depending on where you were on Earth, whether under a continent or a tectonic region. This was a pretty exciting discovery, which really proved that the Earth was very strongly varying in the upper mantle.”

Digging a little deeper, Helmberger, working with Thorne Lay, also found somewhat unexpectedly that the lower mantle was quite varied as well. “We were just looking at the core–mantle boundary, trying to understand it,” he says, “then we noticed in the record section that there was something organized in the records that didn’t belong there.” Those observations led to the discovery of another boundary that occurs approximately 300 km above the core–mantle boundary, although only in certain regions (5). More recently, Helmberger investigated deeper still, and, together with Xiaodong Song, a former student, provided evidence that the Earth’s solid inner core is also layered (6).

In his PNAS Inaugural Article (1), Helmberger, with the help of new broadband systems allowing more detailed Earth imaging, shed light on the regional discontinuities in the deep mantle that he and Lay first identified more than 20 years ago. “It took us a long time to figure out what we were looking at,” he says, “to put it all together where it occurred on Earth and where it didn’t. There’s a lot of other stuff going on down there; this discontinuity is not the only story.” Helmberger now thinks that his and Lay’s observations represent a phase change in mineral content, going from perovskite to postperovskite. “That turns out to be very important, because that can tell us both about the minerals that exist at that depth and the temperature and pressure at which they’re operating. I think that’s going to be a pretty important subject down the line,” he says.

Science for a Cause

One of Helmberger’s most recent forays involves CalTech’s Tectonic Observatory, a project funded by the Gordon and Betty Moore Foundation (San Francisco, CA), which, among other goals, focuses on the role of earthquakes in plate tectonics. “This group is leading the way on studying the great big earthquakes, like the recent ones in Sumatra and Pakistan. We’re trying to understand the physics behind these earthquakes. How deep are they? How fast do they rupture? Why are they where they are?” says Helmberger.

“These big earthquakes are one of those incredible tragedies that scientists can help a little bit, I think,” he adds. Helmberger is leveraging his longstanding association with DARPA, where he currently serves on an Air Force Technical Applications Center panel, to involve the government more in studying catastrophic earthquakes. “The Air Force operates seismic equipment globally that’s very high-quality. They have the capability to know where an earthquake started to within 5 km, which even the people in the area don’t know,” explains Helmberger.

Helmberger and other scientists have convinced the Air Force to work with the U.S. Geological Survey to develop a program to help earthquake victims. “It’s beginning to happen, but it’s not going as fast as I would like it,” he says. He remains optimistic for the future, perhaps because he knows that DARPA scientists have succeeded in the past, such as with the nuclear test ban treaty between the United States and the Soviet Union. “The seismologists were really key in pushing forward the test ban treaty. We discovered that both countries weren’t pressing for a test ban treaty as hard as they could, so we started some groups that eventually led to exchanging equipment between the U.S. and Soviet Union,” says Helmberger. “So just by working with fellow scientists, we can tell if the other guy is testing or not and if we should push for a treaty. Even now, we still monitor the Earth to make sure no one is cheating.”