Natural magnetic disturbance fields, not precursors, preceding the Loma Prieta earthquake

Available records of the magnetic indices Dst and ap together with standard observatory recordings of 1‐min field levels were examined for the period preceding the earthquake of October 1989, centered near Loma Prieta, California. The magnetic records showed that the Fraser‐Smith et al. (1990) report claiming the existence of a 100‐s (ultralow frequency) geomagnetic field precursor signal at Corralitos, California, foretelling a nearby earthquake is not valid. My study shows that the Stanford ULF signal was not local but rather widespread throughout the western United States and, therefore, expected to be due to a coincidental geomagnetic solar‐terrestrial disturbance field.