Comparison of ultra-low frequency electromagnetic signals with aftershock activity during the 1989 Loma Prieta earthquake sequence

Ultra-low frequency (0.01 to 10.0 Hz) magnetic field fluctuations near the epicenter of the 1989 Loma Prieta earthquake rose sharply immediately before the earthquake following indications of increased disturbance during the previous 12 days. The magnetic activity remained much higher than the pre-earthquake background level for 6 weeks following the mainshock. These observations suggest a causal relationship between the earthquake failure process and the magnetic signals. A search for similar precursory electromagnetic signals associated with aftershocks of this earthquake yields negative results. Specifically, no correlation appears to exist between the amplitude of the electromagnetic activity and the frequency or magnitude of aftershocks following the mainshock. Either a “threshold” earthquake magnitude larger, in this case, than ML 5.5, may be necessary to generate precursory electromagnetic signals or the continued generation of magnetic signals related to the mainshock may have masked signals generated by the larger aftershocks.