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Keiiti Aki: Seismological Polymath

Like the seismic waves he studies, Keiiti Aki’s pioneering work on the basic tenets of seismology reaches across the planet. In recognition of his global impact, the American Geophysical Union (AGU) and European Geosciences Union have awarded Aki their highest honors: the William Bowie and Beno Gutenberg medals, respectively.

Keiiti Aki, a groundbreaking seismologist, was awarded the Bowie Medal at the American Geophysical Union’s annual meeting last December. He will receive the European Geosciences Union’s Gutenberg Medal in April. Courtesy of Tom Jordan, SCEC.

Aki “has created new understanding in almost every aspect” of seismology, said Tom Jordan of the University of Southern California in his citation at the AGU annual meeting in San Francisco last December. Jordan, who followed in Aki’s footsteps as head of the Southern California Earthquake Center (SCEC), listed many of Aki’s accomplishments, from groundbreaking work in processing seismic data, to inferring Earth’s inner structure, to measuring earthquake sizes.

Determining the key physical attribute of earthquake magnitude is one of Aki’s more important breakthroughs, says Paul Richards, a seismologist at the Lamont-Doherty Earth Observatory. In a landmark paper published in 1966, Aki determined the so-called seismic moment of the 1964 Niigata earthquake in Japan, showing how it can be measured on seismograms. He also showed that it is the product of how far a fault slipped and the area of the fault on which the slip occurred. The concept of seismic moment and the magnitude derived from it quickly replaced the empirical Richter magnitude scale as the most “appropriate single way to characterize” an earthquake’s size, Richards says.

Born in Japan, Aki received his bachelor’s degree in 1952 and Ph.D. in 1958 at the University of Tokyo. He conducted research until 1960 at the university’s Earthquake Research Institute, after which he made his first foray to the United States, to work with Frank Press at Caltech’s Seismological Laboratory in Pasadena, Calif., for several years.

As an undergraduate student deciding to pursue seismology, Aki intended to become an “earthquake predictor,” he says, but he realized that he needed to think about deep Earth first.

At the time, seismology was booming because of the need to improve the tracking of nuclear test explosions. Press introduced Aki to long-period earthquake waves that travel through Earth, says Bill Ellsworth, head of the U.S. Geological Survey (USGS) seismology group and Aki’s former student. Such waves are key to understanding both quakes and Earth’s deep structure from afar.

After returning to teach at the University of Tokyo for several years, Aki’s second visit to the United States coincided with the 1966 Parkfield earthquake. He met with USGS seismologists at Menlo Park, Calif., who introduced him to coda waves, the train of scattered waves that occur after an earthquake’s major identifiable waves. Aki “developed a passion for using those waves to investigate Earth,” Ellsworth says. “He came from Japan as a statistically oriented seismologist, but he was not afraid to transform himself.”

“I prefer risk to security,” Aki says, if a risky path may open up greater possibilities. Nevertheless, in his work, he says, “when I faced some difficulty, I tried to avoid it by choosing easier paths,” a seemingly contradictory mix he attributes to his cultural roots and admiration of the philosopher Spinoza.

Press invited Aki to join him at MIT in 1966. He taught geophysics there until 1984, when he moved back to California to work at the University of Southern California. There, Aki established the SCEC, a leading seismology research center focused on characterizing (and potentially forecasting) Southern California earthquakes.

“What Kei Aki did,” Jordan says, “was create a master model for Southern California,” much like a global climate model that tracks the planet’s atmosphere. Aki’s whole-systems viewpoint, Jordan says, is “very visionary” for seismology.

Ellsworth describes Aki as an “extraordinarily gifted” mathematician, who sees solutions in creative ways and has a deep-seated desire to get to those solutions first. “He has a number of themes that have sustained him through his career,” Ellsworth says, plus “whole areas of investigation that he’s created or selected to pursue,” including tomography, or the 3-D imaging of Earth’s innards.

“His style of work is to be admired,” says Richards, who co-authored Quantitative Seismology, one of the premier textbooks on seismologic methods, with Aki (first released in 1980 and revised in 2003). Richards says that Aki also has had “a huge impact through his students,” of which he has mentored more than 50 over his career.

Aki’s collaborative and original work at SCEC and elsewhere is similar to a form of poetry called Renga, Jordan says. “One person comes up with a line and other people add lines.” And like the poetry, Aki “very much is interested in understanding the harmony of the world,” he says.

Aki currently lives on the French volcanic island of La Réunion, which serves as a small-scale laboratory for earthquakes in volcanic systems. Recently, he published a conceptual model of the Piton de la Fournaise volcano — a small seismological study with implications for earthquake prediction in Southern California.

Naomi Lubick

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