Graduate studies with Donald V. Helmberger

John E. Vidale

Department of Earth Sciences, University of Southern California, CA 90089, USA

Arriving at the Seismo Lab in 1981, I first worked with Hiroo Kanamori on the seismotectonics of the New Hebrides (Vidale and Kanamori, 1983) (I don’t remember why). Then with Rob Clayton I worked on finitedifference methods of modeling seismic waves, specifically how to bump second-order methods to fourth-order and encode the inner loop in assembly language to make it run twice as fast (Vidale, 1990). My studies progressed to wandering the third-floor hall in search of chatty mentors who would have insight into the random real and synthetic seismograms I carried, which increasingly meant Don,although Rob and Hiroo also were often interrupted. By graduation at the end of 1986, I’d concocted some numerical methods (Vidale, 1986, 1988, 1990; Vidale et al., 1985) and explored LA basin resonance (Vidale and Helmberger, 1988; Helmberger and Vidale, 1988),subducting slab structure (Vidale, 1987), nuclear-explosionpowered near-field Rayleigh waves in the Aleutians(Vidale and Helmberger, 1987), and abandoned numerous less successful forays.

It’s remarkable, at least in my memory, that Don gave few imperatives or barriers to my aimless progress.Fortunately, Heidi Houston was more incisive. Perhaps due to fellowships, I was left free to dabble with faint reflections of mantle discontinuities–at least until Terry Wallace asked the obvious question how minuscule would they be compared to the overwhelming noise level. I was searching for subtle waveform distortions of subducting slabs (Vidale, 1987), until we realized some early MIT slab anomaly models were erroneously strong. There was something unresolved about stresses in slabs (I warned my qualifying exam committee not to ask about my vaporware third proposition), so I tried refining mantle discontinuity triplications with numerical magic until I learned one cannot get something from nothing. I still don’t know why I tried to re-code the Zoeppritz equations or make a gridbased travel time calculator (Vidale, 1988) and a complex polarization filter (Vidale, 1986). Don was a gentle guide,as were Rob and Hiroo.

Don was generous. He overlooked the homework I didn’t do for his and Dave Harkrider’s classes, settling for having me work as TA (teaching assistant) for both classes the next year, with re-adjustment of the grade (unlike my equally-earned and more permanent D in Brad Hager’s geodynamics) and added the TA pay to my fellowship. He hired me with double salary as a post-doc for a month or two when I defended my thesis sooner than I’d projected.

But mostly Don trained immersion in the data until the wiggles could be plausibly explained by synthetics. We first laid out the figures, then I filled in the rest of the paper. The seismograms, and the cartoons explaining how they were derived, and the models they required to be true are the core; the words are secondary. If the story is not convincing, go back to get more data and calculate more and better synthetics. Entertain as many ideas as one could,but don’t get too attached, and the geology was much less interesting to us than the seismic velocity structure. He was adept, as he knew the influence of many levels and scales of structure, as well as the peculiarities of earthquakes and foibles of data and synthetics, and places to find more data. I’ve striven to follow his path for better and worse.

Many of the stories of Don have documented his contributions to understanding the crust, mantle, core,earthquakes, and explosions. One prescient paper that he never mentioned to me struck me as potentially seminal and underappreciated. So I’ll offer an interpretation. The 1973 paper (Allen and Helmberger, 1973) in a Stanford volume with Clarence Allen cleanly showed no changes of the sort that would match highly publicized claims about the temporal velocity changes associated with the“Palmdale Bulge.” A several-inch-high bulge across the Mojave Desert was claimed to be a symptom of dilatancydiffusion along the San Andreas fault. This was a fashionable theory of the time that suggested earthquakes would soon be predictable, and that Los Angeles in particular had reason to be alarmed (therefore seismologists needed a funding boost, which showed up as the National Earthquake Hazards Reduction Program in 1976).The geodetic bump turned out a few years later to probably be optical leveling survey errors, not a real bulge. Don cleanly demonstrated there were no resolvable velocity changes to be seen and moved on, although it took several years for many prominent seismologists, some down the hall, to follow. Don understood data.

One of my more oblique interactions with Don came through his proposals. Any cutting edge seismologist recognized Don’s wide ranging contributions, despite an inexplicable rivalry between those using Don’s approximations and purists, usually in San Diego or on the East Coast, who denigrated less than an exact analytic solution with formal error bars. But Don’s proposals were famously quirky. More than once, I found myself on a proposal review panel mulling in puzzlement at Don’s verbiage. Don had plenty of accomplishments from past funding, but the proposed plan was inscrutable to any but his former students, who were fortunately legion. Upon translation into English and straightforward thought patterns, the proposals were impressive, but translation was often required.

A last anecdote: Don featured prominently in an encounter with grizzlies in Denali National Park in 1984.Grad students Steve Grand, Heidi Houston, and I, plus Don and recently-graduated Thorne Lay, unwisely chose to hike across the tundra, and noticed increasingly fresh bear tracks as we walked. After a while, we spotted a mother grizzly with her two cubs on a hillside about 100 m away. Don must have been bored - he loudly clanked a rock against a culvert pipe, proclaiming “If the bear comes this way, I don’t have to be the fastest, just not the slowest to climb into the culvert.” The bear abruptly turned and stared, but fortunately was not hungry. Despite their claim to be full, we forced our way onto the next bus that went down the road.

With Don, one usually found a path to the next adventure.