A megathrust earthquake occurs most commonly at which setting?

• A. Transform boundaries.
• B. Mid-ocean ridges.
• C. Major subduction zone boundaries.
• D. Intraplate crust.

Answer: (C)

Megathrust earthquakes occur on the plate interface where a subducting plate dives beneath another plate in major subduction zones. The boundary here can lock and accumulate enormous strain over long periods, and when it ruptures, the slip over a vast area releases immense energy, producing the largest earthquakes and often causing significant vertical movement of the seafloor that can generate tsunamis. This combination of a very large rupture area and strong frictional locking at a convergent plate boundary makes this setting the most common source of megathrust events. In contrast, transform boundaries involve horizontal sliding along faults and typically produce large but different types of earthquakes; mid-ocean ridges are zones of divergent boundaries with mostly smaller, shallow earthquakes from normal faulting; and intraplate earthquakes occur within a single plate away from plate margins and are not driven by the same megathrust mechanics.