Simultaneous measurements of geomagnetic fields have been made at pairs of sites approximately 12 km apart along the Garlock fault. On June 9 and 10, 1974, several minor earthquakes (M = 2.6 to 4.3) occurred near one of these sites 3 weeks after the first measurements were made. Two repeated data sets were taken 2 weeks and 3 months after the earthquakes. The differenced data indicate that, relative to pre-earthquake values, the local magnetic field at this site had increased by about 2γ. A piezomagnetic source for this change implies a stress change that would exceed 10 bars. After 3 months the same form of anomaly still remained although larger in spatial scale. This might indicate dispersion of the stress discontinuities produced by the earthquake.

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