Whose observations first proved that some earthquakes occur inside subducting plates far below the surface?

Japanese seismologist Kiyoo Wadati and American geophysicist Hugo Benioff independently studied earthquake depths and recognized inclined zones of seismicity beneath ocean trenches. These Wadati-Benioff zones are now known to mark subducting tectonic plates descending into the mantle. The pattern was crucial evidence for plate tectonics and subduction. Their observations linked earthquake distribution directly to large-scale plate motion.