What happens is that seismic events on or below the seafloor generate energy that is transmitted into the ocean water column. The graph below shows how sound speed varies as a function of water depth, and shows a sound speed minimum in the depth range of between 700 m and 1200 m below the sea surface. Hydrophones are now used to record T-waves from earthquakes generated throughout the ocean basins. NOAA, Canadian, and academic research vessels investigated the site of this first event. This effort was remarkably successful, and the nature and consequences of a magmatic seafloor spreading episode were thoroughly studied for the first time.
Figure shows the earthquake locations as white circles plotted on the bathymetry of the ridge crest.
The earthquakes migrated down the slope of ridge indicating that a dike had been intruded along the ridge. Red and orange areas mark the location of new lava flows that were mapped after the seismic events were recorded. Apparently magma within the dike that intruded along the ridge axis erupted on the seafloor to form new lava flows. Because the magnitude of the events was so small, none of them were detected by land-based seismometers in the Pacific Northwest. Union, 229-232, 1940.