Continuous tilt, strain, and magnetic field measurements near four earthquakes (ML = 3.6 to 3.8) on the San Andreas fault, California

Four moderate earthquakes (M = 3.6 to 3.8) have occurred on the San Andreas fault in central California since October, 1977. These earthquakes are the first since 1974 to occur at this magnitude level within the array of tilt, strain, and magnetic instruments between Chalome (35.726N, 12.249 W) and San Francisco (37.79N, 122.23W) shown in Figure 1. They offer, therefore, one of the few opportunities to search for indications of precursive ground deformation. Furthermore, several other geophysical parameters such as resistivity, creek, radon, seismicity, geodetic strain, etc, are also monitored and allow comparisons for some of these events. This note reports comparative data and some possible implications in continuous strain, tilt, magnetic field and other measurements obtained from instruments within 10 km of the epicenters.