Keiiti Aki, 1930–2005, American seismologist, b. Yokohama, Japan, Ph.D. Univ. of Tokyo, 1958. Associated with the Univ. of Tokyo 's Earthquake Research Institute from 1963, Aki joined the faculty of the Massachusetts Institute of Technology in 1966 and the Univ. of Southern California in 1984. He is best known for his studies of the behavior and origins of earthquakes, which contributed to a deeper understanding of natural disasters. While studying the consequences of a massive earthquake that struck Niigata, Japan, in 1964, Aki originated the concept of the seismic moment as a measure of the energy radiated by an earthquake; it is now used along with the moment magnitude scale as the standard measurement of the size of an earthquake. A groundbreaking seismologist, Aki contributed to the development of seismic tomography, which uses a network of seismometers spread over the continents to provide digital images of the interior of the earth. With P. G. Richards he wrote Quantitative Seismology (1980, 2d ed. 2002), a standard work on theoretical seismology.