How deep do strong earthquakes start?

Try this

Select 3 Deep, 300 km or more in the depth bands. The map collapses to a few places where one plate dives far beneath another, led by the Fiji Islands and Tonga, then Indonesia and Japan.
Look at the spike at 10 km in the depth histogram. Those events are not all exactly 10 km deep: when a depth can't be pinned down, the catalogue fixes it at 10 km.
Select Tonga in the area list and look at the depth histogram: Tonga's earthquakes run the whole range of depths.

Depth is hard to measure. A fixed default depth is a convention, not a measurement, so treat the 10 km spike as "shallow, depth unknown".