Fallout from Cannikin

The Atomic Energy Commission's announcement that it planned to test a multimegaton nuclear device under the Aleutian island of Amchitka last November touched off a shock wave of protest. Some critics charged that the explosion of an H-bomb in a region that was already known to be seismically active could trigger devastating quakes and the great sea waves, known as tsunamis, that often follow them. Environmentalists made dire predictions of a wildlife massacre. Nonetheless, the test took place, and it did not cause serious tremors or lasting environmental damage. Instead, after months of careful analysis, U.S. Government scientists now report that the Cannikin blast may well have provided some highly beneficial information. The fluctuations in the earth's magnetic field that resulted from the blast, they say, could help in the development of an accurate method of predicting major earthquakes.

That conclusion, by Wilfred P. Hasbrouck and Joe H. Allen of the National Oceanic and Atmospheric Administration, is based on readings from magnetometers set up on Amchitka before Cannikin. Sensitive instruments were placed on each side of one of the faults that cross the thin 42-mile-long Aleutian island. A magnetometer on the side where the nuclear device was detonated quickly registered an increase of nine gammas* in the local magnetic field. On the opposite side of the fault from ground zero, the intensity of the magnetic field was found to have dropped by as much as eleven gammas after the test.

Squeezed Field. Hasbrouck and Allen theorize that on the side of the fault where the blast occurred, the rock was compressed. As a result, the magnetic particles in the rock were squeezed closer together, and the magnetic field was intensified. On the opposite side of the fault, the explosion resulted in a stretching force on the rock, pulling the magnetic particles apart and thus reducing the field's intensity.

Scientists have known that the buildup of stresses in the earth before dangerous quakes is often accompanied by slight changes in the magnetism of local rock. But there have been few measurements of that geopiezomagnetic effect before or during actual quakes. With more data like those gathered during Cannikin, Hasbrouck and Allen hope, scientists should be able to determine accurately the relationship between accumulated stress and the magnetic changes in an earthquake zone. Then, by monitoring the magnetic field, they may well be able to forecast serious upheavals.

* Units of magnetic intensity.

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