Deep earthquakes occur in the transition zone between the upper mantle and lower mantle, from 400 to 700 kilometers below the surface. (sciencedaily.com)
Deep earthquakes do not define a descending slab; in fact, do not even occur in most places along the trenches. (creationscience.com)
Such deep earthquakes usually don't cause enough shaking on the surface to be hazardous, but scientifically they are of great interest. (sciencedaily.com)