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Benioff, Victor Hugo

BENIOFF, VICTOR HUGO

(b. Los Angeles, California, 14 September 1899; d. Mendocino, California, 29 February 1968)

geology, seismology.

Benioff, who used his middle name, played an important role in the development of seismology, first by creating instruments of greater sensitivity than had been used before, and second by developing ideas about the relationships between earthquakes, rock deformation, and tectonics.

Benioff was raised by his mother, Frieda (née Widerquist) after her brief marriage to Simon Benioff. Originally Hugo had planned to become an astronomer, and worked summers doing solar observing at Mt. Wilson Observatory while attending Pomona College, from which he graduated in 1921; he then did graduate study at the California Institute of Technology (Caltech) while working as an assistant at Lick Observatory. However, he found it impossible to observe at night and sleep during the day, and so abandoned this career in 1924 for the newly formed seismology program that the Carnegie Institution of Washington had begun in 1921 under the leadership of H. O. Wood. He remained there as the program merged with Caltech, becoming a member of the faculty there in 1937. He stayed at Caltech until his retirement in 1965, though he had little involvement with students, working primarily with the other scientists and engineering staff at the Seismological Laboratory a few miles from campus.

Development of Instruments. The initial assignment given to Benioff was to develop a system that would allow seismic signals recorded at different stations to be accurately timed to the nearest tenth of a second: not an easy task. To drive the recording drums, he developed a motor driven by a tuning fork, to give uniform speed; time marks were put on the records from marine chronometers driving relays, with the whole system checked by twice-daily radio time signals—all run automatically.

Benioff’s next contribution, in 1930, was to design a seismometer of much higher sensitivity than any so far built—an activity he pursued in various ways over the next three decades. At the time the highest sensitivity came either from very large masses (up to 20 tons) driving mechanical systems, or from electromagnetic systems, in which the motion of a coil in a magnetic field generated a voltage to drive a galvanometer. The sensitivity of these systems was limited by the low field strength available from permanent magnets. Benioff adapted a transducer that used changes in the flux through a magnetic circuit with a variable air gap (termed the reluctance) to generate a higher voltage for a given motion. Applying this variable-reluctance sensor to vertical and horizontal seismometers gave magnifications at periods of around one second up to 100 times larger than had been available before, especially in the vertical component. These instruments were first installed at the stations of the southern California network, where they not only recorded local earthquakes but gave high-quality records of distant earthquakes, showing seismic waves not previously observed and providing especially good records of deep earthquakes.

With slight redesigns, these instruments remained one of the best available seismic sensors for many years; they were used in the World-Wide Standardized Seismic Network, which operated from 1961 into the 1980s.

The next application of the variable-reluctance transducer, in 1932, was to a strain seismograph, which used a steel rod attached to the ground at one end, with the other end using the transducer to measure the distance from the end of the rod to another attached point, so that the whole system measured the stretching of the ground from seismic waves. Originally designed to give good records at high frequencies, this turned out to be most useful at lower ones, especially for recording long-period surface waves from distant earthquakes.

After 1946 Benioff combined instrument development with studies of earthquake mechanism and the relationship between seismicity and tectonics. Using the recent results of his colleagues Beno Gutenberg and Charles F. Richter on the relationship between earthquake magnitude and energy in seismic waves, he equated this energy to the energy released by the elastic rebound of rocks around an earthquake fault, and from this derived the average strain in the earthquake region. For a series of earthquakes, plotting the cumulative strain against time showed patterns that could be related to experimental studies on the creep of rocks under stress. Benioff’s first application of these ideas, in 1949, was to argue that the pattern of strain release for deep earthquakes in a particular area showed that they all occurred on a common structure, which he took to be a large inclined fault. In 1952 he extended this concept to zones of deep earthquakes all around the Pacific Ocean, making many geologists aware of these large and deep structures; when plate tectonics explained these regions of deep seismicity as locations of subduction, they came to be called Benioff zones. While Benioff used his strain-release methodology to display other patterns of earthquake occurrence, this approach was not much pursued by other seismologists.

Benioff, in his instrumental work in the 1950s, continued to pursue higher sensitivity at longer periods. One stimulus for this was the observation on the strainmeter, following a great earthquake in 1952, of signals with about a one-hour period, which could be interpreted as free vibrations of the whole Earth, a phenomenon known from theory but never observed. Benioff improved the performance of his strainmeter, and built new instruments in quieter locations in California and (as part of the International Geophysical Year in 1957–1958) in Peru. When the largest earthquake of the twentieth century occurred in Chile in 1960, these instruments gave clear records of free vibrations at many frequencies, inaugurating a new branch of seismology. For his accomplishments Benioff was elected to the National Academy of Sciences in 1953, and received two awards, the Arthur L. Day Medal of the Geological Society of America in 1957 and the William Bowie Medal of the American Geophysical Union in 1965.

Benioff had a lifelong interest in acoustics and music, which led him to develop novel musical instruments and to experiment with listening to sped-up seismograms to see what the ear might detect. He put this interest to more direct use during World War II, when he and his engineering staff worked on radar and acoustics for the Submarine Signal Company.

Benioff married Alice Silverman in 1929; they had three children and divorced in 1953, after which he married Mildred Lent, with whom he had one child.

BIBLIOGRAPHY

WORKS BY BENIOFF

“Seismic Evidence for the Fault Origin of Oceanic Deeps.” Bulletin of the Geological Society of America 60 (1949): 1837–1856.

“Earthquakes and Rock Creep.” Bulletin of the Seismological Society of America 41 (1951): 31–62.

“Earthquake Seismographs and Associated Instruments.” In Advances in Geophysics, vol. 2. New York: Academic Press, 1955.

With Frank Press and Stewart W. Smith. “Excitation of the Free Oscillations of the Earth by Earthquakes.” Journal of Geophysical Research 66 (1961): 605–619.

OTHER SOURCES

Goodstein, Judith R. “Waves in the Earth: Seismology Comes to Southern California.” Historical Studies in the Physical and Biological Sciences 14 (1984): 201–230.

Press, Frank. “Victor Hugo Benioff.” Biographical Memoirs, vol. 43. Washington, DC: National Academy of Sciences, 1978.

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