ULTRA-LOW FREQUENCY MAGNETIC-FIELD MEASUREMENTS IN SOUTHERN CALIFORNIA DURING THE NORTHRIDGE EARTHQUAKE OF 17 JANUARY 1994

Measurements of ultra-low frequency (ULF) magnetic field fluctuations by two independent monitoring systems in Southern California were in progress during January 1994 when the moderately-large M6.7 Northridge earthquake occurred on 17 January. Our two measuring systems are located at Table Mountain, on the other side of the San Gabriel mountains and at a distance of 81 km from the epicenter, and at Pinon Flat, south of Palm Springs and at a distance of 206 km from the epicenter. Both systems operated well throughout the month and without interruption due to the earthquake. As a result of the occurrence of a moderate magnetic storm on 11 January, which was followed by a period of enhanced ULF magnetic activity that persisted until after the time of the earthquake, the sensitivity of our measurements throughout California was reduced for roughly a week before the earthquake took place. Nevertheless, no large signals that could be associated with the earthquake were evident at any time, except for the usual co-seismic shaking response of the detectors. Subsequent removal of the upper atmosphere signals from the Table Mountain measurements, using the measurements from the more distant Pinon Flat location as reference, essentially left no significant residual. Thus, assuming that ULF magnetic fields were produced by the earthquake, their amplitudes were too small to produce obvious increases in the ULF background noise at 81 km from the epicenter, which is in agreement with our earlier estimate of a range of about 100 km for the ULF magnetic field fluctuations observed prior to the M7.1 Loma Prieta earthquake. These results imply that a network of conventional magnetic field detectors spaced less than 100 km apart would be required to detect ULF magnetic field fluctuations prior to earthquakes with magnitudes greater than 7. Under the same conditions, superconducting magnetic field gradiometers could offer greater sensitivity and range.