Deep earthquakes were discovered in the 1920s, but they remain a subject of contention today. The reason is simple: they aren't supposed to happen. Yet they account for more than 20 percent of all earthquakes.
Earlier this week a magnitude-7.7 earthquake occurred off Sakhalin Island at a depth of 626 kilometers. Visit the US Geological Survey's event page, look for the poster presentation, and see the telltale seismic pattern of the descending plate, called a Wadati-Benioff zone, in the "tectonic setting" map.
Background:
About earthquakes
Deep earthquakes
Measuring earthquake shaking
Comments
When they shouldn’t happen to the actual theories but they are still there, it looks like the theory is not complete regarding this point.