On 6 June 1961, a storm of microseisms with periods of about 27 seconds and a duration of about 8 hours was detected by long period seismographs throughout the world. At Palisades, ultra-sensitive seismographs detected the storm for an interval of about 2 days during which the periods of the waves decreased from about 28 to about 20 seconds. The seismic waves appear to be largely of the Rayleigh type and seem to originate in the southern or equatorial Atlantic Ocean.
The favored hypothesis on the nature of the source mechanism suggests that the seismic waves were generated by dispersed ocean waves striking the coast of the Gulf of Guinea. A second hypothesis suggests that the microseisms are a form of harmonic tremor associated with magmatic activity beneath the South Atlantic Ocean.