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The Theory of Plate Tectonics (4) Global Seismicity Map  | | Figure source: http://core2.gsfc.nasa.gov/dtam/seismic/ |
Through the modernization of seismic instruments, scientists learned that the majority of earthquakes are concentrated along the oceanic trenches and spreading ridges. By the 1920's, seismologists located prominent earthquake zones at oceanic trenches inclined 40-60 degrees from horizontal and extending several hundred kilometers in to the earth. These seismic zones helped confirm Hess' seafloor-spreading hypothesis by locating the places where oceanic crust is generated (along the ridges) and where oceanic lithosphere is subducted into the mantle (at the trenches).
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