TECTONOMAGNETIC ANOMALY DURING THE SOUTHERN-CALIFORNIA DOWNWARP

Resurveys of the local geomagnetic field in southern California, using magnetometers in a differential mode, indicate the development of an anomalous field of more than 10 gamma that appears to correspond to the partial collapse of the southern California uplift. The 10-gammar field increase occurred within a 30-km fault segment near the junction of the San Andreas and San Jacinto faults. It took place episodically in time, 80% accumulated between late 1973 and the end of 1974 and the remaining 20% between 1974 and 1976. Changes of smaller maximum amplitude and opposite sense occurred to the southeast along the San Jacinto fault. Swarm seismicity started at the northwest end of the zone of maximum anomaly in late 1976 but ceased in late 1977. The anomaly amplitude has slowly decreased with time since its peak in 1976. Tectonomagnetic models imply localized stress changes at depth of at least 10 bars in the vicinity of the magnetometer sites near the various faults, although details concerning the geometry and dimensions of the region responsible for the magnetic field changes are poorly resolved. -Authors