Magnetic disturbances preceding the 1964 Alaska earthquake

THROUGH a fortunate circumstance, a recording magnetometer was operating in the city of Kodiak, 30 km north-west of the surface trace of a fault zone along which movement occurred at the time when the earthquake occurred in Alaska on March 27, 1964. Fortunately, too, the instrument was on such high ground that it was not reached by the subsequent seismic sea wave which virtually destroyed the city. The magnetometer recorded the fact that the largest of several magnetic disturbances briefly increased the intensity of the Earth's magnetic field by 100γ at Kodiak, 1 h 6 min before the earthquake (Fig. 1).