Keiiti Aki

2004 William Bowie Medal Winner

Keiiti Aki was awarded the 2004 William Bowie Medal at the AGU Fall Meeting Honors Ceremony, which was held on 15 December 2004, in San Francisco, California. The medal recognizes outstanding contributions to fundamental geophysics and for unselfish cooperation in research.


Kei Aki’s scientific research has expanded the frontiers of seismology for 50 years. He pioneered the electronic processing of seismic data to infer Earth structure and properties of the earthquake source. Much of what we know about large earthquakes follows from his work.

He was the first to measure seismic moment (for the 1964 Niigata earthquake); Aki moment has since replaced Richter magnitude as the fundamental measure of earthquake size. He discovered the fundamental scaling laws for seismic spectra that resolved inconsistencies among different magnitude scales, and he proposed physical models of earthquakes for heterogeneous fault rupture. This work contributed significantly to both the basic understanding of the rupture process and the practical understanding of strong ground motions.

He elucidated the scattering and attenuation processes that govern the propagation of high-frequency seismic waves, from which he derived new methods for measuring earthquake size, scattering distributions, and intrinsic attenuation parameters. He demonstrated, for instance, that the intrinsic attenuation must decrease at high frequencies.

He developed many novel approaches for describing aspects of seismic waves, including free-oscillation splitting, Gaussian beams, and boundary integral methods. In each case, he and his students successfully used these techniques to extract new types of information from seismograms.

He pioneered travel-time tomography as a means to study lithospheric structure beneath dense seismic arrays, publishing seminal papers almost a decade ahead of its widespread application to global seismology. He established new seismological perspectives on volcanic processes, including the relationship of seismicity and harmonic tremor to magma injection and eruption. He detected fault-zone guided waves and used their propagation characteristics to constrain the width and elasticity of the damage zones around faults.

As impressive as this abbreviated list might sound (more topics could easily be added), it fails to communicate Kei’s true impact within the geoscience community. He literally wrote the book Quantitative Seismology, the most influential textbook and reference manual in the history of the field, which he coauthored with Paul Richards in 1980.

As a teacher and mentor, he entrained many bright students in his quest to understand the active Earth, producing over 50 Ph.D.s who now occupy key positions in seismology worldwide. His success in guiding young scientists stems in part from the depth of his understanding, but also from his remarkable personal qualities-charm, wit, and a deep respect for the harmony and poetry of the natural world.

Kei’s quiet leadership in seismology has demonstrated the subtlety and power of unselfish cooperation in research. He has held many positions: president of the Seismology Section of AGU, president of the Seismological Society of America, chair of the NAS Committee on Seismology.

But his greatest leadership achievement was the creation of the Southern California Earthquake Center in 1991. As its founding director, Kei articulated a vision for SCEC in which the investigations by disciplinary working groups would be woven together into a system-level ‘master model’ for earthquake hazard and risk in Southern California. The master-model concept led to many advances in seismic hazard analysis, such as the incorporation of GPS data into long-term earthquake forecasting, and it continues to guide the growing SCEC collaboration.

Some scientists loom so large in their fields that we must mark their impact with special honors. Kei Aki, the 2004 recipient of the William Bowie Medal, is one of our giants.

—THOMAS H. JORDON, University of Southern California, Los Angeles


William Bowie, after whom the Bowie medal is named, was upset by the finding of seismologists in the early 1920s that earthquakes may be occurring at depths of nearly 1,000 km, because he had been living in a harmonious world of isostasy, in which the Earth’s interior is in hydrostatic equilibrium below the 100-km thick crust.

In spite of that, we have a long list of seismologists who were awarded his medal. Jeffreys and Gutenberg received it in 1952 and 1953, respectively. They together completed the classic seismology to such perfection that I felt there was nothing left in seismology to do as a graduate student.

Then the wave-theoretical approach was opened in Earthquake Seismology by Ewing, Benioff, and Press, who were awarded the medal between 1957 and 1979. They started with the long-period waves to which relatively simple deterministic models of the Earth and earthquakes are applicable.

Long-period seismology flourished under the leadership of Anderson, Gilbert, and Dziewonski, who were awarded the medal between 1991 and 2002. It is now called broadband seismology in which the upper bound of its applicable frequency range has been pushed steadily upward.

In my first decade as a seismologist, after being overwhelmed by Jeffreys and Gutenberg, and before Frank Press opened my eyes to the possibility of the deterministic approaches in earthquake seismology, I was trying to introduce stochastic modeling into seismology, motivated by Norbert Wiener’s Cybernetics, published when I was an undergraduate student at Tokyo University. In fact, Japanese seismologists used to call me a statistical seismologist.

The familiarity with both statistical and deterministic approaches, together with the recognition that seismology is at a contact point between two broader fields of knowledge regarding the solid Earth, namely, geology and civil engineering, gave me a vast area to work in a style that matched well with the very open system of doing science in this country. It also helped me to attract numerous talented students and colleagues throughout the world to work together. The prestigious William Bowie Medal given to me surely recognizes the sum of their contributions.

—Keiiti Aki