The long-period (> 1 day) behavior of the seafloor electromagnetic fields during EMSLAB is considered in detail with an emphasis on interpretation in terms of oceanic motions. The study begins with a summary of the physics of motional electromgnetic induction, in which the seawater conductivity-weighted, vertically integrated velocity measured by the horizontal electric field is emphasized. Using frequency-domain methods, it is shown that seafloor and terrestrial magnetic variations have similar spectral shapes, indicating a common origin, but the seafloor electric field is not consistent with either at periods longer than 4 days, suggesting an oceanic source. The magnetic field variations are highly coherent across the EMSLAB array at periods shorter than 9 days but the similarity decreases at longer periods, probably due to long-term instrumental drift. The seafloor horizontal electric field data exhibit the broadband coherence characteristic of inospheric sources only at periods shorter than 1--2 days and are essentially incoherent between 4 days and a week except in association with certain propagating wave phenomena. The distinction between the depth-averaged velocity inferred from the horizontal electric field and its point measurement by the veritcal electric field is demonstrated with data. Some specific properties of the seafloor electric fields are then considered in detail. The first of these is a 4-day wave observed to propagate from north-to-south along the east flank of the Juan de Fuca Ridge. The wavelength, propagation sense, and association with ridge are all consistent with topographically trapped Rossby wave behavior at a zero in the group velocity. A second north-to-south propagating wave is seen at periods of 8--12 days in the middle of the EMSLAB area with a probable wavelength of order 1000 km. Unresolved low-frequency structures in the electric field are also mentioned. These observations clearly demonstrate the power of electromagnetic array methods for the study of long period oceanic behavior. ¿ American Geophysical Union 1989