Seismology: Toward Better Quakecasting

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California's politicians have long boasted that their state is on the move. Scientists agree. On opposite sides of a 600-mile line called the San Andreas fault, the coastal strip of California is slowly but inexorably moving to the northwest while the remainder of the state is shifting toward the southeast.

This strange mobility of terrain is of more than academic interest. It produces the earthquakes that suddenly and with out warning jolt areas of California, occasionally with catastrophic results.

Although scientists are powerless to prevent earthquakes, they have high hopes that they can some day forecast them with reasonable accuracy. That day may not be far off. By carefully measuring movements along the San Andreas and nearby smaller faults, Seismologist Renner Hofmann says, he has successfully predicted recent California earthquakes. To prove that he is not merely displaying scientific hindsight, Hofmann has issued a new U.S. quake-cast. Within the next 18 months, he predicts, earthquakes of at least moderate intensity will rock areas near Santa Cruz and south of Bakersfield.

Between Two Points. A team led by Hofmann developed the quakecast method hile investigating fault zones for the California Department of Water Resources, which is understandably co cerned about the effects of earth slip page and quakes on its vast system of pumping plants, dams and aqueducts.

To measure the earth movements in the fault zones, they established over 90 fixed observation points along the faults.

Using an instrument called the geodimeter, Hofmann aimed a beam of in tense light from a site on one side of a fault at a reflector set up on the other side, between twelve and 20 miles away.

By measuring the time required for the light to travel to the reflector and back to the geodimeter, he calculated the precise distance between the two points.

By repeating the measurement annually, he was able to determine with precision the amount and direction of movement that had taken place since the last measurement. Slowdowns & Reversals. At many observation points, along the San Andreas fault, the scientists found that California's coastal strip was moving to the northwest at a rate of two inches per year. In some areas, however, friction between the sliding masses of rock caused the movement to slow and even to stop. "When the fault sticks," Hofmann says, "the movement is transferred to smaller, adjacent faults that can stand only a limited amount of movement. When these smaller faults reach their limit, the forces increase until the main fault breaks loose again. This sudden breaking loose is the earthquake."

By observing slowdowns — and even apparent reversals—in the fault movements at certain points, and by correlating them with subsequent earthquakes in the same areas, Hofmann gradually developed a rule-of-thumb system for quake prediction. His technique is far from foolproof; although he has correctly forecast eight recent earthquakes of significant size, 17 other quakes that his method predicted have failed to materialize. But Hofmann believes that more frequent monitoring of an even larger system of observation points will make his technique more reliable. He is convinced that the future of earthquake forecasting lies in being diligent to a fault.