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      2. Donald V. Helmberger (1938 - 2020)

         Donald v. Helmberger (1938 - 2020)

        Donald V. Helmberger, PhD and Smits Family Professor of Geophysics Emeritus at Caltech, and one of the most impactful seismologists to have lived, died on 13 August 2020. Don was born 23 January 1938, the youngest child of 13, in the small rural town of Perham, Minnesota. He completed his bachelor’s degree in physics at the University of Minnesota in 1961. That summer he participated in a cruise involving seismic imaging of the oceanic crust in the Bering Sea and was inspired by the challenge of interpreting the recorded seismic waveforms – the beginning of a lifelong passion. That experience drew him to a new focus on geophysics and he enrolled in the graduate program at UC San Diego, where he completed a master’s degree in 1965 and a Ph.D. in 1967. His graduate work was strongly influenced by Russell Raitt and Freeman Gilbert. After a two-year appointment as a research associate at MIT, working with Frank Press and M. Nafi Toks?z, he joined the faculty for one year at Princeton University, and in 1970 moved to the Seismological Laboratory at Caltech where he was to spend the rest of his career. From 1998 to 2003 he served as Director of the Seismo Lab, one of the few intervals in which he let an administrative role divert him from seeking to understand the wiggles in seismograms. He became emeritus in 2017.

        In his Ph.D. work, Don pulled together source and wave propagation theory to develop new synthetic waveform modeling capabilities. This was initially applied to active source seismic signals and nuclear test signals, for which the sources could be analytically prescribed. However, the recorded shaking from shallow earthquakes required a more representation of a complex source embedded in a varying velocity medium. The 1971 San Fernando earthquake spurred his interests in earthquake signals, and he focused on applying the powerful analytic techniques of Cagniard and de Hoop to develop quantitative waveform modeling capabilities in collaborations with colleagues such as Ralph Wiggins and graduate students Charles Langston and Thomas Heaton. This truly,was the birth of quantitative synthetic waveform calculations for remote and local distances, and provided the foundation for the next 40 years of strong-motion and teleseismic waveform modeling, finite-fault source inversion, detailed waveform modeling of regional waveguides, upper mantle structure, lower mantle structure, and core structure.

        In concert with a host of talented Caltech graduate students, major discoveries ensued, including space-time models of faulting for large earthquakes, imaging of strong lithospheric lateral gradients in velocity structure, details of transition zone velocity discontinuities, discovery of lower mantle seismic discontinuities, discovery of ultra-low velocity zones at the core-mantle boundary, existence of strong lateral gradients on the margins of large low shear velocity provinces in the deep mantle, anomalous gradients in seismic velocity above the inner core, faulting triggered by nuclear explosions in the form of tectonic release, quantitative explosion yield estimation from waveform modeling at regional and teleseismic distances, and a vast number of other applications. The common denominator through the many studies has been quantitative prediction of recorded seismic waveforms with flexible physical source representations. Don’s enthusiasm for application of generalized ray theory to address modeling of high quality waveforms for just about any Earth problem was extraordinary and infectious for those of us fortunate enough to work with him. His impact was felt well beyond basic Earth Sciences, as it resonated with the nuclear testing treaty monitoring community and with the seismic hazard assessment community. He received many honors in recognition of his contributions including being selected as the first recipient of the American Geophysical Union Lehmann Medal in 1997 and being elected to the U.S. National Academy of Science in 2004. Throughout his career he was unassuming and generous, never seeking the limelight despite his profound contributions and creativity. And he was wicked fast, darn hard to catch on a football field. Truly, a giant of the golden age of seismic waveform modeling, and his impact will sustain for generations.

        Thorne Lay, based on various internet sources and personal recollections.

        Interview with Donald V. Helmberger: https://oralhistories.library.caltech.edu/155/

        PNAS Profile of Don Helmberger: https://www.pnas.org/content/103/7/2009

        Caltech Memorial: https://www.caltech.edu/about/news/donald-v-helmberger-19382020

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