Ultralow‐Frequency Magnetic Fields Preceding Large Earthquakes

The Great Alaska Earthquake ( M 9.2) of 27 March 1964 was the largest earthquake ever to strike the United States in modern times and one of the largest ever recorded anywhere. Later that year, Moore [1964], in a surprisingly rarely cited paper, reported the occurrence of strong ultralow‐frequency (ULF; ≤10 hertz) magnetic field disturbances at Kodiak, Alaska, in the 1–2 hours before the earthquake. That report has since been followed by others [ Fraser‐Smith et al. , 1990; Kopytenko et al. , 1993; Hayakawa et al. , 1996; see also Molchanov et al. , 1992] similarly describing the occurrence of large‐amplitude ULF magnetic field fluctuations before other large earthquakes (“large” describes earthquakes with magnitudes M ∼7 or greater). These reports involving four separate, large earthquakes were made by four different groups and the results were published in well‐known, refereed scientific journals, so there is no doubt that there is evidence for the existence of comparatively large ULF magnetic field fluctuations preceding large earthquakes.