ULF/ELF polarimetry: Observations of anomalous ULF signals preceding the Northridge earthquake of January 17, 1994

Continuous ULF background data are recorded at the Low Frequency Observatory in San Diego since 1990. Some 18 days preceding the large Northridge earthquake of January 17, 1994. Anomalous signals of approximately twice the background level were recorded. The timing of the anomalous signals, the return to normal levels after the quake and a delay in the onset of higher frequency anomalous signals all suggest a strong correlation of the anomalous signals with the events at Northridge. A simple model of migrating micro-fractures is used to explain the temporal characteristics of the signals.