Whose idea first explained deep-focus earthquakes in subduction zones?

Japanese seismologist Kiyoo Wadati found that earthquakes in some regions could occur far below the surface, not just near it. His observations in the 1920s and 1930s helped reveal a narrow zone of deep seismicity beneath subduction zones, now often called the Wadati-Benioff zone. These earthquakes happen as a cold oceanic plate sinks into the mantle. The discovery was important because it showed that tectonic plates can generate earthquakes at great depth. It remains one of the clearest signatures of subduction on Earth.