James Freeman Gilbert

USA

1990 Balzan Prize for Geophysics (solid earth)

Balzan Prize Awards Ceremony 1990
Rome, Accademia dei Lincei, 16 November 1990


Mr. President,
Ladies and Gentiemen:

Both my profession and I are honored today by the award of a Balzan Prize. My profession is geophysics, which many think began here in Italy in the days of the Roman Empire. Pliny the Elder (Gaius Plinius Secundus) was killed in the eruption of Mt. Vesuvius in 79 AD. His nephew, Pliny the Younger (Publius Caecilius Secundus, later known as Gaius Plinius Caecilius Secundus), wrote a report about the eruption and his uncle’s death, a report that may well have been the very first geophysical paper. My own contributions are more modest and more recent.
Although the subject of quantitative seismology, my branch of geophysics, began slightly more than a century ago, the discipline of low frequency seismology is only 30 years old. Prior to 1960 there were a few theoretical studies and even fewer computational studies, but in that year Nature made the earth speak to us as never before. On May 22, 1960, a gigantic earthquake occurred in Chile. The rupture in the crust of the earth ran for more than 1,000 km.

For the people of Chile and surrounding lands, the earthquake was a natural disaster, an Act of God. However, out of the tragedy emerged new information that led to improved understanding about the structure of the earth and about the process of the generation of earthquakes. The earthquake vibrations were recorded around the world by thousands of conventional seismographs, and, fortunately, on a few special instruments, recently developed, in Europe, South America and North America.
The recordings of the special instruments lasted for days after the main shock of the earthquake. They were analyzed by the use of digital computers, which had barely enough capacity and speed in 1960 to meet the task. Also, the computational methods were quite inefficient. Better methods, technically termed the fast Fourier transform (fFT), were known to a few scientists in fields such as radio astronomy, but were unknown and, therefore, unused by the large majority of scientists in 1960. Subsequently, and ironically, it was learned that the fFt was developed by C.F. Gauss early in the l9th century.

The analysis of the great Chilean earthquake showed clearly for the first time that the earth resonates at very low frequencies after a large earthquake. The frequencies of resonance are far below the audible, with periods of oscillation typically measured in hundreds of seconds. The amplitudes of ground acceleration are tiny, about ten parts in one trillion of the acceleration of gravity. Yet, these new data, the frequencies and amplitudes of the low frequency oscillations of the earth, have enabled our improved understanding of the earth and have given rise to a new scientific discipline.
A second earthquake, about 650 km below Colombia on July 31, 1970, led to another important advance. This time Nature did not shout so loudly to get our attention but the instrumentation and computational facilities were much improved.
The spectrum of oscillations excited by the Colombian earthquake was very rich in signals that are sensitive to the very deepest parts of the earth, and made us realize for the first time that gigantic, catastrophic earthquakes are not the only source of low frequency data. It was the analysis of the Colombian earthquake that motivated the development of global arrays of digital seismographs devoted to low frequency seismology. These early arrays were the forerunners of today’s growing, international network of digital seismographic stations.
Today, we stand in a position where the theoretical, observational and computational achievements of the past 30 years, in all branches of geophysics, encourage us to believe that we are dose to a more detailed understanding of the structure of the earth and of the process of the generation of earthquakes. Let us hope that our belief is not far from the truth.

It is true that my professional life has been enriched by collaborating with many colleagues. There are too many for me to acknowledge all of them, but four must be mentioned. They are George Backus, Jon Berger, Adam Dziewonski and Guy Masters. My collaboration with each of them was and is enjoyable, and, I think, fruitful. Last, and best, has been nearly one third of a century of sharing life with my wife, Sally.
In closing I am pleased to say that the Balzan Prize is gratefully received as a recognition that understanding our earth is a worthwhile, intellectual endeavor.