Deep earthquakes exceeding 20 kilometers in depth occur at which tectonic setting?

• A. Transform boundaries
• B. Divergent boundaries
• C. Convergent boundaries
• D. Hot spots

Answer: (C)

Deep earthquakes are a hallmark of subduction zones, where one plate dives beneath another. In a convergent boundary, the cold, rigid slab sinks into the mantle and can break or slip along faults at great depths, creating a dipping seismic zone that tracks the descending slab. This Wadati-Benioff zone shows earthquakes from shallow levels down to hundreds of kilometers deep, something that only subduction zones routinely produce because they continually force rock into the high-pressure mantle environment where brittle failure can still occur.

By contrast, transform boundaries move plates side by side, so earthquakes there are typically shallow and localized to faults that accommodate horizontal motion. Divergent boundaries pull the crust apart and generate normal faults near the surface as magma rises and the crust thins, also giving mostly shallow earthquakes. Hot spots produce volcanism away from plate boundaries, and the earthquakes associated with them are usually shallow too, not the deep, slab-related events seen in subduction zones.

So the setting that best explains deep earthquakes is the convergent boundary with subducting slabs.