Anomalous tilt preceding the Hollister earthquake of November 28, 1974

The occurrence of a magnitude 5.2 earthquake on November 28, 1974, near Hollister in central California, provided an excellent opportunity to test the potential of a prototype tiltmeter array as a predictive tool. Tilt perturbations on instruments near the epicenter were evident about 36 days before the earthquake and coincided with a magnetic field anomaly reported in a companion paper. The anomalous tilts systematically changed with time, reaching values as great as 7 μrad. These data are interpreted in terms of a preseismic and postseismic slip mechanism involving interaction between the San Andreas and other faults in the region, where slip on one results in a sympathetic slip on others.