Simultaneous measurements of geomagnetic field with an array of seven proton precession magnetometers along the San Andreas fault show that the most local changes during 1974 were recorded at a site 11 km from a magnitude 5.2 earthquake that occurred on November 28, 1974. A systematic increase in magnetic field of 0.9 &ggr; occurred at this site during the early part of 1974. A more dramatic increase of 1.5 &ggr; occurred about 7 weeks before the earthquake, lasting about 2 weeks. Four weeks prior to the earthquake the magnetic field returned to approximately its initial value and remained at this value through April 1975. These data cannot be explained by ionospheric disturbances or telluric currents. The most probable source is a piezomagnetic effect, which implies that the magnetic field changes represent changes in stress in the rocks nearby the anomalous station.