When geophysicists started to measure magnetic field anomalies in ocean crust they saw startling patterns emerge - 'stripes'. These stripes run parallel to ridges and their patterns are symmetrical about the ridges. These stripes were explained by sea-floor spreading. As spreading continues, rocks that record the same polarity are split and moved apart from one another. Geophysicists have noted for a long time that earthquakes in the ocean basins have a distinctive pattern.
Earthquakes occur mostly at oceanic ridges and trenches.
The ridge earthquakes are always shallow and occur near the ridge crest. The trench earthquakes may be shallow to very deep, but the shallow earthquakes occur near the trench while the deeper earthquakes towards continents. To balance the growth of new oceanic crust, there must be a place where oceanic crust is consumed. The trenches are locations where the oceanic crust is pushed underneath the continents and consumed at depth. There is quite a lot of recent corroborating evidence in support of sea floor spreading.