Francis A. Dahlen, Jr. was awarded the Lehmann Medal at the AGU Fall Meeting Honors Ceremony, which was held on 10 December 2003, in San Francisco, California. The medal honors “outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core.”  

“I feel honored and pleased to cite my friend and Princeton colleague Tony Dahlen for the Inge Lehmann Medal. Given Tony’s wide range of important contributions, there is actually a choice of AGU honors one might cite him for; his influence extends well beyond those fields that are primarily associated with the Lehmann Medal.

“Tony started his scientific journey as an undergraduate at Caltech. By the time he moved on to graduate studies with George Backus and Freeman Gilbert at Scripps he was already applying his many talents to geophysics. He soon pioneered a series of papers on normal modes that represent the first substantial step away from Earth’s spherical symmetry. In fact, all of the current research on the use of low-frequency seismic data for the determination of the Earth’s three-dimensional structure is based on this early work, its extension to an inverse problem, and subsequent research with Martin Smith and John Woodhouse. His interest in the theory of global tomography has survived until this day: recently, he developed a very elegant and efficient theory to include the frequency-dependent effects of diffraction into body wave tomography, a theoretical improvement that was almost immediately rewarded by the imaging of a large number of mantle plumes. These represent the first concrete seismological evidence that many hot spots originate deep in the mantle, confirming Jason Morgan’s long-standing hypothesis.

“Tony’s research into low-frequency seismology led him to investigate the rotation of the Earth; he discovered the excitation mechanism for the Chandler wobble and he quantified the influence of the oceans on rotational variations of the Earth, enabling us to identify those variations that find their cause in the deep interior. In the 1970s, he incorporated prestress, rotation, and self-gravitation into dislocation theory. His research in this area has made him the preeminent scholar in the theory of the free oscillations of the Earth, and resulted in the definitive treatise ‘Theoretical Global Seismology,’ written with Jeroen Tromp.

“It would be wrong, through, to see Tony as a seismologist per se, since he has also made major contributions to geology. With Dan Davis and John Suppe he developed the concept of critical-taper wedge mechanics in the 1980s to explain fold-and-thrust belts and accretionary wedges. This work is arguably one of the most important contributions to the mechanics of mountain belts in the twentieth century and of great relevance for the understanding of earthquakes in such areas. He resolved a petrological controversy regarding equilibrium conditions for a metamorphic reaction under deviatoric stress, showed that the energy dissipation in accretionary wedges is divided roughly equally between basal friction and work against gravity, and was the first to model the role of erosion in the dynamics and thermal evolution of mountain belts, showing it to be a dominant process, a subject of much current interest.

“Tony’s leading position as a scientist was recognized by memberships in the American Academy of Arts and Sciences and the National Academy of Sciences. He was the recipient of a Guggenheim Fellowship and has served the scientific community in too many capacities to list here. No one deserves this year’s Lehmann Medal as much as he does.”

—GUUST NOLET, Princeton University, N. J.

“It is a great honor to accept this 2003 Inge Lehmann Medal from the AGU, and it is a particular pleasure to receive such a flattering citation from my longtime colleague and close friend, Guust Nolet. The Lehmann Medal is given in recognition of ‘outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core.’ My contribution to this understanding has been modest and indirect: Together with John Woodhouse, the 2001 Lehmann Medalist, I helped to develop a few theoretical tools which John and other seismologists have used to obtain remarkably detailed 3-D images of the Earth’s interior. “I had a Tom Sawyer childhood in Winslow, Arizona, and was enabled to attend one of our nation’s finest educational institutions, Caltech, by the award of a generous 4-year scholarship—which I assure you was not granted on the basis of my dubious ability as a football player. “At Caltech, I was exposed to the excitement of forefront geophysical research by legendary but readily available professors, including Jerry Wasserburg, who was prudent enough not to let me near the glassware in his lab. As a graduate student at Scripps, I had the great good fortune to pursue my Ph.D. studies under the supervision of two of the world’s preeminent geophysicists: George Backus, the 1986 Fleming Medalist, and Freeman Gilbert, the 1999 Bowie Medalist. It has been my privilege to work for my entire career at a third great institution, Princeton, where I have witnessed the transformation of a distinguished classical geology department into a modern department of geosciences, encompassing all of the solid-Earth and fluid-Earth disciplines in the AGU. “During my 33 years at Princeton, I have been blessed with many exceptional graduate students, including Martin Smith, Gordon Shudofsky, Richard Strelitz, Ivan Henson, Peter Davis, Fred Pollitz, Zheng Wang, Li Zhao, Jeroen Tromp, Junho Um, and Henk Keers. I owe a special debt of gratitude to Jeroen, whose shared passion for theoretical global seismology led us to write a 1000-page, 10,000-equation opus on the topic. “In 1980, I pointed out a minor algebraic error in a preprint on the mechanics of mountain building by Dan Davis and John Suppe, and they graciously invited me to be a coauthor on what is, by far, my most frequently cited paper. I spent most of the next decade elaborating upon the critical Coulomb wedge model, together with Wu-Ling Zhao, Hongbin Xiao, and Terence Barr. “For the past 7 years, Guust and I have been seeking to improve the theoretical foundations, resolution, and fidelity of seismic tomography, in collaboration with Henk Marquering, Shu-Huei Hung, Adam Baig, Ying Zhou, Raffaella Montelli, and Guy Masters. “I am well aware that honors such as the Lehmann Medal generally come in the twilight of one’s career, but I am determined to keep laboring in the theoretical trenches and attending AGU meetings, in order to share in the ongoing evolution of our ‘understanding of the structure, composition, and dynamics of the Earth’s mantle and core.’” —FRANCIS A. DAHLEN, JR., Princeton University, N.J.