Keiiti Aki

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Keiiti Aki
Keiiti Aki headshot.gif
PhD university Tokyo University

Keiiti Aki (1930-2005) was a leader in the field of Earthquake Seismology.

Memorial [1]

by Michael Fehler

Keiiti Aki (1930-2005), known to most of us simply as Kei, died in an accident in mid May 2005 on the island of La Reunion in the Indian Ocean. Kei touched the lives of many seismologists whether they had the privilege of working directly with him or whether they benefited from the extraordinarily large number of areas in which he worked and made valuable contributions. His work also had a significant impact on a society that is becoming increasingly aware of the risks of earthquakes and interested in understanding their mechanisms. To those who knew and worked with him, Kei was a teacher, colleague, friend, and a mentor who brought a human touch to science. He was generous with his time and enthusiastic about new ideas. Through example, he taught those around him always to be excited about the act of discovery and to pursue new ideas with vigor. While he possessed enormous analytic ability and understanding of the intricacies of the theoretical aspects of wave propagation, he eagerly sought out data as the fundamental essence of science. His work paved new roads in seismology and increased our understanding of earthquakes enormously.

He developed seismologic capabilities to quantify many important parameters through his amazing insight and ability to combine theory and measurement. I will never forget his admonition that I must be “optimistic about data.” He proposed new ideas about the quantification of earthquake size and how earthquakes scale with size. He developed methods for the analysis of surface waves to determine earthquake depths and their use to infer crustal structure. He initiated the concept of seismic coda and proposed empirical and theory-based methodologies for its analysis. The list goes on. Many people are aware of only a small portion of Kei’s contributions because they were spread across so many subdisciplines of geophysics. The book that he coauthored with Paul Richards, Quantitative Seismology Theory and Methods, has been used by a large number of people who likely have little knowledge of the breadth of Kei’s own research nor the breakthroughs that he provided the field.

While he did not directly work in the area of exploration seismology, he performed work throughout his career that has had a lasting impact on the field. In 1969, he published papers with Jerry Ware on an inverse scattering approach applied to a stratified elastic medium. He introduced the concept of seismic traveltime tomography to seismology in the early 1970s and developed approaches for doing tomography using teleseismic data, regional earthquake data, and local earthquake data where the earthquake locations were also determined. He participated in initial interpretation of seismicity induced by hydraulic fracturing in a geothermal reservoir in the 1970s. He helped design acquisition for, collect, and analyze some of the first high tracedensity cross well seismic data ever collected. He used crosswell data and induced seismicity data to make inferences about the fracture system in a hydraulically-fractured reservoir. With Ken Larner, he developed the so-called Aki-Larner method for doing numerical wave equation modeling in media with slowly varying interface topography. He later worked with Michel Bouchon in the extension of this method to complicated layered media and what is now called the discrete wavenumber method for modeling. He mentored Ru-Shan Wu in the development of several analytic approaches for modeling wave propagation in heterogeneous media and their extension to making quantitative measurements of medium heterogeneity and its relation to seismic attenuation.

Kei grew up in Japan during World War II. He attended Tokyo University, where he received his PhD. He held a postdoctoral position at the Seismological Laboratory at CalTech, then returned to Japan. When Frank Press invited him to join the faculty at MIT, he accepted and spent many years in Cambridge. During this time, he mentored a large number of PhD candidates pursuing a wide variety of research topics: surface wave analysis, earthquake source modeling, numerical wave propagation modeling, volcano observations and modeling of volcanic tremor, scattering observation and theory, and seismic tomography. As his student during his stay at MIT, I marveled at how many topics he could discuss with his students in one day and maintain his ability to provide beneficial insight. Many of these topics were of sufficient magnitude to be the foundation for a career. Not being one to stay put forever, Kei ventured again into earthquake country by joining the faculty at the University of Southern California. He moved to Los Angeles and continued with the broad scope of work and his mentoring of students. Many of us were impressed with his enthusiasm for the “California lifestyle” including his taking up the sport of boogie boarding. He remarked how impressed he was with the diversity of trees on the USC campus and his enjoyment of them. Kei was the leader of the Southern California Earthquake Center, SCEC, and provided considerable intellectual foundation for the center. After several years in California, Kei moved again, this time to the French island of La Reunion, where he focused his research efforts almost entirely on volcano seismology.

Kei was a member of the U.S. National Academy of Sciences, and a fellow of the American Academy of Arts and Sciences. He won many awards from professional societies including the Harry Fielding Reid Medal of the Seismological Society of America, the Thorarinsson Medal of the International Association of Volcanology and Chemistry of the Earth’s Interior, William Bowie Medal from the American Geophysical Union, and the Beno Gutenberg Medal from the European Geosciences Union—all the highest awards of these societies. Kei was consistent in his enjoyment of working with and mentoring young scientists. He was always positive and his enthusiasm was easily conveyed to them. As he became widely recognized as a leading seismologist, he understood the importance that his encouragement could have for people early in their careers. He was generous with his praise. He listened to them explain their work and willingly offered suggestions for future direction. He constantly had a smile on his face and he was excited when he or those around them learned something new.

We will all miss Kei. However, his legacy will live on through the ideas he generated, the new approaches he developed, the insight he provided us through his work, and the inspiration he has given us.


  1. The leading edge, August 2005, Vol. 24, No. 8, P. 854.