Donald V. Helmberger memoir — exceptional seismologist, teacher, mentor, friend

Douglas Dreger

University of California, Berkeley, CA 94720, USA

Donald V. Helmberger was an unparalleled observational seismologist, and theoretician. His pioneering work in seismic waveform modeling, and the numerical methods he developed to simulate seismograms to study the structure of the Earth and the nature of earthquake sources have become techniques that are now commonplace. But,that wasn’t always the case. He once confided in me that when he was first proposing the modeling of whole waveforms there were not many who believed him that it was possible, and was ‘laughed at.’ I don’t know if that later part is true or not, but perhaps it was his way of teaching a young scientist about humility, and to give him confidence that the current scientific struggle will succeed with tenacity and hard work.

I first met Don while visiting Caltech’s Seismo Lab at coffee break. The topic was the core-mantle boundary —to me it was mysterious, fascinating and inspiring, though for a prospective student the coffee break dynamic was daunting. At one point in the conversation I was asked what I had been working on. As I described the shallow applied geophysics work I was doing at the time I was feeling it wasn’t very significant given what was just being discussed. However, Don took great interest in what I shared, particularly my description of how I was able to see the structures I imaged, the reflectors/refractors(various weathering horizons) and scatterers (hard rock knockers) after the bulldozers stripped away the soft material, allowing me to make the connection between seismic observation and the actual materials. His unassuming nature was welcoming and we had a very nice discussion. It was during this conversation that he said‘arrival times are not the only information that one can measure from seismic experiments’ and he introduced me to the idea that entire waveforms can be used to learn much more about the nature of seismic wave propagation,the structures imaged by the traveling waves, and the earthquake source that generated them. That began a wonderful 5-year journey with Don exploring the mysteries of the seismograms stored at Kresge and the new digital data from Terrascope, a brand new network of broadband seismic stations in southern California.

At the time, Terrascope was just getting underway, and there were numerous small-to-moderate earthquakes occurring throughout southern California. Don and I spent countless hours examining the waveforms, learning how to best filter them, numerically modeling the complete records, and untangling the complex local and regional distance seismograms, revealing both the source and propagation. It seemed that each earthquake had a new secret to discover. I was in awe of his ability to read seismograms and identify the often-times very subtle features that would lead to the greatest insight into the problem being tackled. It was as if he could invert a seismogram in his head, taking a bulldozer, stripping away the upper layers, and revealing the complexity of the Moho discontinuity or other such structure. With his initial assessment in hand, I would then go off and attempt to model the waveforms using a variety of tools we developed. Many times I would find a result often supporting his intuitive hypothesis, but sometimes not,leading to wonderful brainstorming sessions punctuated with reflective, wondering gazes outside his office window towards Mt. Wilson. Of course, Don’s intuition was rooted by vast observational experience reading seismic waveforms as well as synthetic seismograms, allowing him to make the connection between the complex waveforms and Earth structure. Together we found PL type waves traveling in the upper weathered layer of the crust (Dreger and Helmberger, 1991a, 1993) and that S-wave multiple arrivals enabled reliable determination of earthquake source depth as well as providing information to constrain upper crustal structure (Dreger and Helmberger, 1990).Figure 1 illustrates the broadband S-wave modeling for the 1988 Upland earthquake from Dreger and Helmberger(1990). We also demonstrated the feasibility of inverting three-component records from a single station for the earthquake source mechanism, beneficial under the very sparse recording situations that was the norm at the time(Dreger and Helmberber, 1991b, 1993). This work ultimately led to the development of automated source determination methods that became part of the operational system shared between UC Berkeley, Caltech and the U.S.Geological Survey (e.g., Pasyanos et al., 1996; Dreger et al., 1998), and led to the use of regional moment tensor methods in source-type discrimination of nuclear explosions (Ford et al., 2009). The mentoring and inspiration Don provided to me was ultimately the driving force in many of the works by his ‘grand-students.’

Figure 1. Originally from Dreger and Helmberger (1990),tangential component synthetics (left- and right-most columns),and data (interior columns) for the 1988 M4.6 Upland mainshock and M3.7 aftershock are compared. Broadband displacement waveforms are shown in the first row, and longperiod (wa.lp) and short-period (wa.sp) Wood-Anderson waveforms are shown in rows 2 and 3, respectively. The synthetics were computed using Generalized Ray Theory(Helmberger, 1974). The first two negative S-wave pulses are direct S and the first near-surface multiple, which is strongly source depth dependent. The long-period response that follows is the result of evanescent reverberations generating the Love wave.

In addition to being a great scientist, and exceptional mentor, he could also be your friend. Don was always interested in hearing about your life, and sharing his. For us the connection was football. Don, originally coming from Minnesota, was a Vikings fan, and me a 49ers fan. In his office reading seismograms, discussing our various projects, gazing at Mt. Wilson and contemplating the data,we would also discuss football. On occasion we would watch games together. But Saturday was the blowing-offsteam day. His students and postdocs (and those of other professors) would meet up Saturday morning for football scrimmage. It was great fun, building a strong camaraderie among the students, postdocs, and visitors – his team.Afterward we would go to Continental Burger on Lake Street for lunch, a pitcher of Shandy and great conversation. When I think back to the time I spent with Don Helmberger, I realize that it was perhaps the best time of my life, and I am forever grateful for his friendship,generosity, mentorship, and the hand he had shaping my career.