The Mechanism of Earthquakes

Earthquakes seem to happen at random; they seldom give advance warning, and cannot be predicted with any certainty. Last week Professor Hugo Benioff of CalTech explained his theory that all great shallow earthquakes (originating in rock slippage less-than 45 miles below the surface) have a common cause and are released by a single "mechanism."

Geophysicists have long thought that earthquakes are caused by a sudden release of strain (distortion) in the earth's crust. Dr. Benioff studied the records of all major earthquakes since 1904 and made a chart of their "strain-rebound" characteristics. Somewhat to his surprise, the chart made a regular, sawtooth pattern, with the teeth getting smaller and lower as the diagram approached 1950.

Such regularity in a chart usually means that an overall law is operating. Dr. Benioff studied more records, made more charts, and found evidence that the earth generates earthquake-producing strain at a constant rate. When the strain is not released in earthquakes, it accumulates at crustal weak points until something has to give. Then comes a series of earthquakes, followed by a period of quiet until more strain has accumulated.

In 1904, says Dr. Benioff, there was a great burst of earthquakes that lasted three years.* This period produced a tall jog on Dr. Benioff's chart. Since then the jogs have been smaller. Today the earth is having continual, mild earthquake activity. This means in Dr. Benioff's theory that the strain in the crust is being released as fast as it is generated. By the same reasoning, a period of no earthquake activity ought to be followed by a proportionately violent flare-up.

Dr. Benioff does not try to guess what generates the strain, but he speculates on how it is released. Great earthquakes, he says, are associated with major faults (cracks) in the crust. When the faults are locked tightly together, the strain accumulates without producing earthquakes. When the faults relax, the rocks slip and shake the earth.

What locks the faults? There is astronomical evidence, says Dr. Benioff, that the earth's radius changes slightly, for an unknown reason. When its radius is smaller than normal, the faults may be locked tightly. When the earth swells up again, even a tiny bit, the faults may loosen enough to set off earthquakes.

* The period of the San Francisco earthquake (April 18, 1906), caused by slippage of the San Andreas fault. With the fire that followed, it destroyed four square miles of the city, killed hundreds of people and did some $500 million damage.

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