Adam Dziewonski

I am a geophysicist who has spent most of his professional career mapping the deep interior of the Earth. Nearly twenty years ago I developed a method which would allow us to see in three dimensions features that are as deep as 1000 miles beneath the Earth's surface. This approach is now called seismic tomography', because of the analogy with medical tomography. Mapping of regions that are anomalously hot or cold is likely to tell us the origin of the driving forces of plate tectonics. The field of seismic tomography has expanded rapidly; some of the developments with which I have been involved are outlined below. To interpret these records I entered into co-operation with Freeman Gilbert of the University of California, San Diego, and this started a very fruitful alliance. Using the data set of nearly 100 digitized seismograms of the Alaskan earthquake, we were able to detect and measure frequencies of many previously unidentified modes of free vibrations of the earth, correct some of the earlier mis-identifications and, generally, significantly improve the overall accuracy of the entire data set. An immediate reward was the ability to present a satisfactory proof that the inner core of the earth is solid (Dziewonski and Gilbert, 1971). A more subtle development was tightening of the bounds on allowable earth models and, in particular, demonstration that the changes in the density as a function of radius in the lower mantle and the core could not be far from adiabatic (Dziewonski, 1971; Dziewonski and Gilbert, 1972). This implies that convection in both regions is possible and highly probable. At about the same time as this work was being completed, Harvard offered me a faculty position and I moved there in 1972. As a part of start-up resources, I was provided with state-of-the-art computerized digitizing equipment, which made the conversion of analog records much easier and more accurate. Even though the study of the records of the Alaskan earthquake was a success, it was clear that much more could be learned about the average structure of the Earth if we could measure higher frequency overtones of the free oscillations of the earth. A giant deep earthquake under Colombia in 1970 presented an unusual opportunity. The analysis of some -- two or three, as digital seismology was still in the early stages of development -- digital records of this earthquake revealed that it was very rich in overtone energy. Even though the digital recordings were clearly superior, the number of available analog records made them, at that time, a much more important source of data. A year-long effort yielded 165 digitized recordings (each of some 16-20 hours duration, on average) of the Colombian earthquake. These were subjected to novel procedures of phase equalization, which allowed us to measure the frequencies of overtones even though their amplitude in individual recordings was much below the noise level (Gilbert and Dziewonski, 1975) and expanded the list of modes with measured frequencies by a factor of three or four, to over 1,000. Yet, the most important result was not originally anticipated. In the course of this work, which required knowledge of the equivalent forces acting at the focus, it became clear that a much more general approach to retrieving information about the earthquake mechanism and its time history was possible. Rather than assuming, as had been the usual practice, that an earthquake is a failure of the material on one of the two perpendicular planes (double-couple mechanism), it is possible to retrieve source parameters in the form of a moment tensor, which allows for the presence of forms of stress release other than plain shear. This seemed particularly important to find out for deep earthquakes, whose origin was, and still is, poorly understood. Inversion of the Colombian data for the moment rate tensor yielded a surprise: a slow compressional event precursory to the main shear failure (Dziewonski and Gilbert,