Keiiti Aki, 75; USC Expert Refined Ways to Measure Quakes’ Strength

Keiiti Aki, a founder of the Southern California Earthquake Center at USC and arguably the greatest seismologist of the last 50 years, died May 17 on the French island of La Reunion in the Indian Ocean. He was 75.

Aki fell on the street on May 13, according to his wife, Valerie Ferrazzini. The next day, he checked into a hospital, but surgeons were unable to stop bleeding in his brain. He fell into a coma and died three days later.

In his 50-year career in Japan and the United States, Aki “studied a tremendous variety of scientific problems and made substantial contributions in almost all of them,” said seismologist Thomas Jordan of USC, the director of the earthquake center.

Aki “put USC earth sciences on the map,” added USC seismologist Tom Henyey. “He was a giant within the field.”

His first -- and perhaps most important -- contribution was the introduction of a new way to more objectively measure the size of an earthquake.

The Richter scale, developed at Caltech by legendary seismologist Charles Richter, was until then the most commonly used measurement, but its calculation was not very precise and researchers often computed different values for the same earthquake.

In 1966, Aki introduced the seismic moment, a value calculated from geological and seismic measurements.

Based on his studies of the 1964 Niigata earthquake in Japan, the seismic moment represents, in simplest terms, the mathematical product of how far a fault slipped in the event and how long the slip area was.

The values are translated into a number similar to those of the Richter scale.

The seismic moment has since become the most “appropriate single way to characterize” an earthquake’s size, according to seismologist Paul Richards of the Lamont-Doherty Earth Observatory near New York City.

Aki was also a pioneer in using computers to extract the maximum amount of data from seismograms to paint complex pictures not only of the quakes but of the Earth.

Before his era, seismograms were recorded only as the now-familiar complex wiggles on a strip chart. “You could sit and look at it, but you couldn’t do much with it,” Jordan said.

Aki was among the first to record the data digitally, so it could be stored and processed electronically.

By measuring seismic signals transmitted directly from a distant quake and comparing them to signals transmitted through other routes, researchers could perform what is now called seismic tomography to produce a three-dimensional map of the Earth’s interior.

This process is virtually identical to computed tomography using X-rays to produce images of the human body.

In the early 1970s, Aki used data from seismic arrays that were created for monitoring nuclear testing to produce the first images of the Earth underneath the arrays, demonstrating that the concept was viable a decade before the technique became widely used.

Aki also developed basic scaling laws, the mathematical equations that predict how seismic motions -- such as shaking -- vary with earthquake size.

One of the few areas where his touch was not golden was earthquake prediction, a lifelong interest.

One of his first published papers was on the possibility of predicting temblors, and one of the last papers before his death addressed the same issues.

But that is a very tough subject, Jordan noted, and his efforts were largely unrewarded.

Aki was born in 1930 in Yokohama, Japan, and educated at the University of Tokyo.

In the early 1960s, he worked as a postdoctoral fellow with Frank Press at Caltech before returning to the faculty at Tokyo.

While he was at Caltech, he so impressed Richter that in a note to Press, he said Aki “would be welcome here at Caltech at any time on any status from visitor on up to permanent staff member.”

When Press was hired to create a modern, physics-based geology program at the Massachusetts Institute of Technology in 1966, Aki was his first hire.

Aki stayed there for 18 years until he moved to USC in 1984 to be in Southern California, where the earthquakes are. He retired in 2000.

He wrote more than 200 scientific papers and trained more than 60 doctoral and postgraduate students who now hold key scientific positions throughout the world.

And with Richards he wrote Quantitative Seismology, which Jordan calls “the most influential textbook and reference manual in the history of the field.”

Beginning part-time in 1995 and full-time upon his retirement, Aki studied the active volcano Piton de la Fournaise on La Reunion in an effort to understand how seismic tremors could be used to predict volcanic eruptions.

At the time of his death, he was writing his autobiography and a book on earthquake predictions.

Aki was buried in the cemetery of the town of Le Tampon on La Reunion.

He is survived by his wife, sons Shota, of Weare, N.H., and Zenta, of Redondo Beach, from his first marriage; and daughters Kajika and Uka of La Reunion from his marriage to Ferrazzini.