The piezomagnetic properties of rock suggest that a change in subsurface stress will manifest itself as a change in the magnetic susceptibility and remanent magnetization and hence the local geomagnetic field. A differential array of magnetometers has been operating since late 1965 on the San Andreas fault in the search for piezomagnetic signals under conditions involving active fault stress. Local changes in the geomagnetic field have been observed near Hollister, California, some tens of hours preceding the onset of abrupt creep displacement on the San Andreas fault.
On July 9, 1962, shortly after 0900 UT, the United States detonated a nuclear device in the megaton range at an altitude of over 200 miles near Johnston Island in the Pacific Ocean. The rocketborne device was designed, among other reasons, to test the effects of such a detonation on electromagnetic phenomena in the high atmosphere and the resultant disruption of communication bands. As expected, the shot caused a very large perturbation on the normal magnetic field of the earth for at least six minutes immediately after the detonation. This change was recorded by two model X‐4936 rubidium vapor magnetometers in Palo Alto, California. One was located at site 501, Stanford University, and the other at Varian Associates, less than a mile away. The chart speed at the Stanford site was 6 inches per hour with a selected full‐scale sensitivity of 20 γ; the Varian system recorded at 2 inches per minute and 2 γ full scale. Unfortunately, the magnitude and frequency of the initial oscillations were much larger than expected and, though the magnetometer sensing head is capable of following millisecond field variations, the recorder response time was such that the precise amplitude and frequency of the initial oscillations were not accurately measured. By detailed comparison and analysis of both records, however, the character and general magnitude of these oscillations were determined and are reproduced in Figure 1.
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