How Donald V. Helmberger inspired me

Robert W. Graves

U.S. Geological Survey, Pasadena, California 91106, USA

I first met Don Helmberger when I visited Caltech in the spring of 1984 as a prospective graduate student.During that visit, I was immediately impressed by Don’s unbridled passion for science, in general, and seismology,in particular. Don made me feel quite at ease as we discussed mantle triplications, core phases and ray theory.Never mind that most of it was well above my head, he patiently continued the discussion while answering my many questions. Then he showed me a paper that literally changed my life:Theory and Application of Synthetic Seismograms(Helmberger, 1983). I was hooked. The notion of creating an earthquake and predicting the resulting ground shaking using a computer was simply amazing to me.

In the fall of 1984, I began my graduate studies at Caltech and was happy to find that I would be working with Don as my research advisor. Eager to start modeling earthquakes, I discovered another of Don’s tenets:modeling is simply a tool to help us better understand and explain observations. Hence, I began my graduate research by hand digitizing numerous seismograms that were stored on microfiche. The goal was to look at multibounce SH waves recorded at sites across North America. While the hand digitizing was tedious, it was also very enlightening. As Don would say, all the bumps and wiggles on those records were telling us something about the Earth. Guided by Don and with help from fellow graduate student Steve Grand, I developed a keen appreciation for identifying phases on the records and seeing how they changed from station to station.Eventually, I guess Don figured I had done enough digitizing, and we finally started on modeling the data.This work culminated in my very first published paper,which focused on the study of laterally varying structure in the upper mantle beneath the Pacific Ocean (Graves and Helmberger, 1988).

Over the next couple of years, I worked on developing new methods for modeling wave propagation in complex 3D media. My interest was the Los Angeles Basin region,spurred on by the pioneering work on the 1971 San Fernando, California earthquake by Vidale and Helmberger(1988). However, I wanted to analyze smaller magnitude events so I could concentrate on the effects of basin structure on the waveforms. But how to get these data?Once again, Don came to the rescue as he excitedly described a new seismic recording system that was being installed in southern California as part of the TerraScope network. These new instruments had high dynamic range,were broadband and fully digital (no more hand digitizing!). Don was like the proverbial kid in a candy shop!

As I neared completion of my Ph.D. thesis, I faced another dilemma: I needed to find a real job. I was discussing this with Don, and I asked him about Woodward Clyde Consultants (WCC), which I knew little about other than they had an office in Pasadena (that Don helped to establish) focused on seismic hazards. The next day I found myself joining Don at a lunch meeting with Larry Burdick and Paul Somerville, who jointly ran the Pasadena WCC office at the time. Shortly after this meeting a job offer from WCC arrived and that is where I spent the next 20 years of my professional life. None of it would have happened without Don.

In 2010, I moved to the USGS Pasadena office and,with closer proximity to the Seismo Lab, rekindled my interactions with Don. As always, one could not come away from a meeting with Don without an increased level of optimism, enthusiasm, and hope. During this time, I was also fortunate to participate in numerous collaborative studies with Don and his students and post-docs, in particular, Shengji Wei, Zhongwen Zhan, and Voon Hui Lai. Voon was Don’s last graduate student, and I am grateful to have had the opportunity to work with her and Don on analyzing and modeling ground motions from small earthquakes in the Los Angeles basin region (Lai et al., 2020). I feel like I have come full circle.

Don, you are truly an inspiration, and I thank you from the bottom of my heart. You will be deeply missed.