Deep earthquakes occur at which boundary type?

• A. Divergent boundaries
• B. Transform boundaries
• C. Convergent boundaries
• D. Mid-ocean ridges

Answer: (C)

Deep earthquakes reveal a subduction process where one plate sinks beneath another. At convergent boundaries with subduction, the descending slab remains brittle enough to fracture as it plunges into the mantle, producing earthquakes at great depths. This creates a continuous zone of seismicity that traces the plane of the subducting slab, known as a Wadati-Benioff zone, extending from shallow levels down to about 700 kilometers.

In contrast, divergent boundaries (where plates pull apart) and transform boundaries (where plates slide past one another) generate earthquakes that are typically shallow, near the crust, because the stresses involved are different—extension or shear in the brittle outer shell—rather than the deep, pressure-driven processes of a descending slab. Mid-ocean ridges are a feature of divergent boundaries and are associated with shallow seismicity and volcanic activity at the crest.

Thus, deep earthquakes are characteristic of convergent boundaries with subduction, where the descending plate drives seismic activity to great depths.