California Institute of Technology, Pasadena
Donald Helmberger was awarded the Inge Lehmann Medal at the AGU Fall Meeting Honors Ceremony, which was held on December 10, 1997, in San Francisco, California. 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. The citation and response are given here.
“One century ago, on June 12, 1897, the great Assam earthquake occurred. It was thoroughly investigated by R. D. Oldham, who was the head of the Geological Survey of India. Oldham used recordings of the earthquake to identify clearly for the first time the S wave and to prepare the first travel time tables for P waves and S waves. Thus began the use of seismological data to infer the internal structure of the Earth. Subsequently, in 1906, Oldham deduced the existence of the fluid core. Gutenberg published his determination of its radius in 1915 and Lehmann discovered the inner core in 1935. Her discovery is the primary basis for the establishment of the award in her name.
“In the late twentieth century it was discovered that the inner core is solid, that it is anisotropic, and that it has a dissipative zone just beneath the inner core boundary. Furthermore, it seems to rotate slightly faster than the rest of the Earth. For the outer core and the base of the mantle it has been discovered that there are patches of very low shear modulus just above the outer core boundary and that the outer core is very nearly a perfect fluid. The cores of the Earth play the major role in the generation of the Earth’s magnetic field and contribute prominently to variations in the length of day. The study of the Earth’s cores has become a major branch of geophysics, the seismological components of the study being dominated by Don Helmberger and his students and postdoctoral fellows. His contributions are manifold, quantitative, and durable. For all that he has accomplished he has been chosen to be the first recipient of the Inge Lehmann Medal of the American Geophysical Union.
“It is usually considered to be bad luck to be a younger member of a large family. One’s older siblings tend to smother one’s individuality. There are exceptions, though. One was Benjamin Franklin, who was preceded by 14 brothers and sisters. Another is Don Helmberger, the youngest of a baker’s dozen. Don grew up in a Catholic family in Minnesota and received his undergraduate education there.
“As a Ph.D. student at the Scripps Institution of Oceanography, Don was supervised by Russell Raitt and me. Don’s performance was a pleasure to us. He used the excellent marine seismic data collected by Russ and carefully archived at Scripps and the method of generalized ray theory for computing synthetic seismograms to produce a Ph.D. thesis that, in retrospect, was considerably ahead of the times. Don, more than anyone else, pioneered quantitative seismology: the comparison of realistic, computed seismograms with observed data to infer deep Earth structure.
“Don’s tenure at the California Institute of Technology has been marked by a sequence of major accomplishments, and he is being honored here for those that relate to the Earth’s cores. Yet it is very important to emphasize that Don is a superb teacher and trainer of research students, as well. His graduates have populated the faculties of the best universities in the country and a handful of them have won the AGU Macelwane Medal. I know that Don is quietly proud of them, just as I am proud of him.
“If Don’s bibliography were restricted only to his contributions to the structure of the Earth’s cores, it would be a tribute to any geophysicist. Those of us who know Don are aware that there is much more than that restricted list. His work on the structure of the crust and upper mantle and his studies of earthquake source mechanisms, very important to the comprehensive test ban treaty, attest to the range of this gifted scientist. To tell more now would be to exceed the scope of this citation. So, we must await another occasion, another time, perhaps another award, to learn more about the career of Donald Vincent Helmberger, Inge Lehmann Medalist.”
—J. FREEMAN GILBERT, University of California, San Diego
“Thank you, Freeman, for your generous remarks and to all of you who have earned this award for me. I am delighted to receive this medal on behalf of all of us wiggly line lovers.
“It is traditional for medalists to describe the particular case of circumstances and good luck leading to their careers. In my case, it is serendipity cubed, having been raised on the shores of Lake Wilbegon (about halfway between Fargo and Brainerd). Recently, I have discovered that Gary Glazmeier was born on the opposite shore, and he too must have observed the Northern Lights and other strange optical phenomenon associated with very cold climates. Perhaps, we both dreamed of studying such stuff, which leads one to think of a rotating core? Or maybe, just some other place that is warmer than Minnesota? I don’t remember, except dreaming is what I do best.
“In the summer of 1960, a cruise to the North Pacific and the Bering Sea, called Leapfrog, provided my first big opportunity to see the Earth without snow on it. George Shor and his wife, Betty, also introduced me to Scripps hospitality at its finest, something totally different than the culture at a 30K university. He and Russ Raitt (a remarkable man) slipped me through the back door of University of California at San Diego as one of their first graduate students. Russ did his Ph.D. work under Millikan at the California Institute of Technology and approached marine geophysics quite differently than other people in the exploration business. He conducted research in the ocean as if it were a laboratory, no easy task as I quickly learned (100-knot winds, etc.). The pressure history of each shot was measured and logged, the recording system calibrated in absolute strengths, and all assembled nicely into operator form. After taking Freeman’s course in theoretical seismology, it was easy to construct synthetics by simply performing convolutions, since they did all work.
“My next good fortune was meeting Frank Press, who offered me a postdoc at the Massachusetts Institute of Technology. There, Nafi Toksoz introduced me to real seismograms and deep Earth structure. Ralph Wiggins and I started modeling upper mantle triplications with this data, which seems to have set the course of my career.
“After a year at Princeton, I joined the Seismology Lab at Caltech. Team teaching with Dave Harkrider, looking at seismograms with Hiroo Kanamori, and trying to keep up with Don Anderson’s triple puns have been most interesting, but working with talented graduate students has been the most rewarding.
“Thank you all for sending your best students to Caltech. Keep it up, and I promise to use the new broadband data to sharpen some of our crude images and perhaps, to image a plume all the way from the core-mantle-boundary to the surface (dream).”
—DONALD HELMBERGER, California Institute of Technology, Pasadena