No. 36: Nov-Dec 1984
We reported above that the earth's magnetic field "jerked" in 1969; that is, it suddenly accelerated its westward drift. The earth's core, which through dynamo action reputedly generates the magnetic field we detect at the surface, apparently does not keep pace with the outer crust. It is this sluggishness that produces the observed westward drift of the magnetic field of about 1 meter per hour. While most geophysicists acknowledge that something significant happened to the core in 1969, the geographical extent of the "jerk" is unclear. The acceleration of the field was clearcut in Europe but obscure or undetectable over much of North America. If the jerk was geographically limited, the core perturbation probably was, too. The earth's core may, in fact, eddy and swirl like the planet's atmosphere.
Going over past records, geophysicists think they have spotted another jerk in 1912; only that time the field decelerated.
(Kerr, Richard A.; "Magnetic 'Jerk" Gaining Wider Acceptance," Science, 225:1135, 1984.)