Arthur L. Day Medal
Donald W. Forsyth
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Presented to Donald W. Forsyth
Citation by Sean C. Solomon
Don Forsyth is an extraordinarily innovative geophysicist who has made seminal contributions to an unusually diverse sweep of topics. Don has a knack for clarifying a problem, distilling its elements into logical ideas, and marshalling the observations needed to distinguish among those ideas.
More than any other individual, Don has defined the seismic structure of oceanic lithosphere, its variation with seafloor age, and the anisotropic characteristics of oceanic mantle and their implications for plate formation and evolution. With Frank Press, Don showed how petrological models for oceanic lithosphere could be distinguished on the basis of a comparison of predicted and observed seismic properties versus age. He then applied age-binned Rayleigh and Love wave phase velocity measurements to improve the resolution of models for the shear-wave velocity structure of oceanic lithosphere. His work, later joined by his student Clyde Nishimura, established the age dependence of both the isotropic and anisotropic shear wave structure, limited models for the thickening of the lithosphere with age, and elucidated patterns of azimuthal and polarization anisotropy versus age that constrain the spreading process at ridges and the pattern of asthenospheric flow. In the course of that work he developed novel analysis methods for multi-mode surface waves, and he perfected appropriate inversion methods.
Not content with data that could be gathered from land seismic stations, Don led the first modern marine seismic experiment to deploy a network of ocean-bottom seismometers to measure the shear-wave velocity structure of young oceanic crust and mantle. The MELT (Mantle ELectromagnetic and Tomography) experiment was a pioneering effort in marine seismology, because it demonstrated that seafloor seismic instruments deployed from a ship could recover long-period seismic waves suitable for the analysis of surface wave dispersion, shear wave splitting, receiver functions, and mantle tomographic imaging. Again, Don had to develop a new inversion method to accommodate multipathing in the recorded surface waves, a method subsequently utilized by many others. The MELT experiment provided what is now the type structure for a region near a super-fast spreading ridge, demonstrated that the fast direction of shear wave propagation is parallel to the spreading direction in such a setting, and documented an asymmetry in seismic structure across the East Pacific Rise axis interpreted convincingly by Don as a signature of enhanced asthenospheric return flow from the region of the South Pacific Superswell. Equally importantly, MELT opened the door to other long-term deployments of ocean-bottom seismometer experiments to address mantle dynamical problems in diverse oceanic settings.
Don led one of those follow-on experiments (GLIMPSE) to tackle the origin of linear chains of seamounts and volcanic centers aligned with the spreading direction on young portions of the Pacific plate. Through a combination of seismic measurements, gravity anomalies, and bathymetry, Don discounted both of the hypotheses previously advanced to account for such volcanic chains — small-scale convection in rolls aligned by plate shear, and lithospheric extension perpendicular to the spreading direction. Don instead advanced the new idea, based on laboratory measurements of two-component fluids, that the volcanic chains are signatures of the viscous interfingering of enriched material upwelling and diverging from the Superswell region with normal oceanic asthenosphere.
Don’s influential contributions extend in many other directions. With Seiya Uyeda Don carried out the first formal inversion of tectonic plate speeds for the forces that contribute to plate motions. That work demonstrated clearly that plate speeds are controlled by plate subduction, and that the driving force of sinking lithospheric slabs is nearly fully opposed by resisting forces at overthrust plate boundaries and viscous tractions on the subducting plate.
With Bill Chapple and later alone, Don elucidated the nature of lithospheric flexure at trench outer rises. Whereas others had noted the flexural nature of outer rise topography and gravity anomalies, Don shrewdly turned to what could be learned from the mechanisms and focal depths of outer rise earthquakes. He showed persuasively that earthquakes above and below the lithospheric neutral plane divide by focal mechanism (normal faults at shallow depth, thrust faults at greater depth) in precisely the manner expected for a flexed plate, and he derived new constraints on the thickness and properties of the mechanical lithosphere from those observations.
With several colleagues Don explored the three-dimensional structure of oceanic crust along spreading ridge-transform boundaries and the implications of those structures for patterns of mantle upwelling, melt generation, and melt delivery to crustal levels. His work involved a masterful synthesis of geophysical and geochemical data with numerical models of three-dimensional flow and melting. Don was the first to calculate the mantle Bouguer gravity anomaly for mid-ocean ridges, to strip the effects of constant crustal thickness from the Bouguer gravity field. He discovered bull’s-eye lows centered on ridge segments that he attributed to along-axis variations in crustal thickness and mantle density. Don’s methodology has since been widely adopted in other studies of spreading center systems.
On top of those contributions, Don invented the spectral coherence method for inferring the thickness of the elastic lithosphere in continental regions where variable erosion has rendered the admittance method inapplicable; he developed techniques for combining results from multiple fault-plane solutions in a region to derive rigorous estimates of principal stress directions; and he elucidated constraints on the vertical extent of mantle partial melting beneath mid-ocean ridges from crustal thickness and major element chemistry of erupted magmas.
It is my pleasure to introduce Don Forsyth as the 2005 Arthur Day Medalist.
2005 Day Medal - Response by Donald W. Forsyth
Thank you Sean for the generous citation and for taking time out of your busy schedule to attend this ceremony. As a more senior graduate student, Sean was one of my role models when I first arrived at MIT after graduating from a small college in the Iowa cornfields. By example, he taught me how to stay up all night before a conference or an abstract deadline and how to prepare figures with a balky Leroy ink pen. Thank goodness those days are gone, at least the dealing with clogged pens or ink spills. Now we get to struggle with printers that cut off the last 6 inches of your poster.
In reading over the list of previous winners of the Day Medal, I have noticed another change that has taken place over the years. It used to be that winners were giants that strode the earth, heroes to a young scientist like me. Medalists included my thesis advisor, Frank Press, and pioneers like Ewing, Birch, Hubbert and Bullard. Now the winners are simply colleagues and friends. What’s changed of course is that now I can add the human dimension, like my memories of exchanging elbows with Dennis Kent on the basketball court, to the names and accomplishments. The human dimension diminishes the awe, but not the respect, and it increases the affection and appreciation. Like the time that medalist Rick O’Connell was my host when I tried to give a lecture at Harvard despite my having the flu — the predictable end result of that misjudgment definitely demonstrated my human side and his graciousness!
My wanderings through geophysics and geology in search of how the earth works have been delightful, greatly enhanced by the people who have joined me on parts of the journey. Maybe I wouldn’t say this if I was a geochemist, but I have found that most of the people in this field are really nice. My early mentors, Joe Phillips, Frank Press and Seiya Uyeda, gently guided me toward important problems. Equally important in those early days were fellow grad students, including Sean, Keith Louden, Norm Sleep and Randy Richardson. After a stimulating three years in the seismology group at Lamont, I moved on to Brown University, where I have been for the last 29 years. Brown has been a wonderful place to work, with many bright students and congenial colleagues. I’ve worked most closely with Marc Parmentier and Karen Fischer, who share my interests in understanding convection, magma generation, and plate tectonic processes, but it is the atmosphere of the whole department that makes it a perfect fit for me. Shared lunches, a drive to explore fundamental scientific questions, a passion for excellence in teaching, and genuine caring for the well-being of the department and each other are part of that atmosphere.
Perhaps the most important companions on the journey have been my students, both undergraduates and graduates. They have shared struggles with faulty or inadequate data and a commitment to excellence, they have taught me how to get the printer to print out that last six inches of the poster, they have tolerated my nit-picking about words in manuscripts, and they have been willing to play with me. I can’t mention them all, so I’ll just say that they are all above average and that my most recent three grad students, Dayanthie Weeraratne, Yingjie Yang, and Nick Harmon, have been to sea with me and yet still have been willing to play volleyball with me on Friday afternoons.
One of my greatest sources of delight is the joy of exploration. I love finding out new things about the earth through analyzing measurements; what I think of as letting the data tell its story. But there is nothing more exciting than actual exploration. I’ve been lucky enough to participate in cruises to all the major oceans, putting down and recovering ocean bottom seismometers, but also just plain mapping the bathymetry, gravity and magnetic anomalies of previously unstudied parts of the mid-ocean ridge system. My wonderful companion on several of the cruises has been Dan Scheirer, a master of underway geophysical measurements who also loves exploring new territory.
Of course, a journey isn’t much fun if you don’t enjoy walking. There is a lot of seeming drudgery involved, removing glitches from data, or trying for the nth time to get a figure or calculation just right. Here it helps to have an aesthetic appreciation for the observations. I may have been brain-damaged from too many years of hand-digitizing analog seismograms, but to me, there is nothing more beautiful than a nicely dispersed Rayleigh wave.
So, I thank my friends for nominating me and the GSA for honoring me for what amounts to a walk in a fascinating park. I have been extraordinarily fortunate, but we are all lucky to live in an era and in a society that supports such endeavors.