What defines a megathrust earthquake and where are they most at risk?

• A. An enormous shallow earthquake at a subduction interface; high risk in the Pacific Ring of Fire
• B. A deep focus earthquake in ocean crust
• C. A small crustal earthquake along transform faults
• D. An earthquake triggered by volcanic eruption

Answer: (A)

Megathrust earthquakes involve rupture along the boundary where a subducting plate dives beneath another plate, producing enormous slip on a fault at shallow to intermediate depths. This makes them incredibly powerful and capable of generating large tsunamis, since the rupture occurs at the interface between plates that are being forcibly pushed together. They occur most where subduction zones are active, with the Pacific Ring of Fire hosting many of these zones—along coasts of Japan, western North and South America, Alaska, Indonesia, and parts of New Zealand. The other scenarios don’t fit: deep focus earthquakes happen deep within the subducting slab and aren’t the boundary-wide ruptures characteristic of megathrusts; small crustal earthquakes along transform faults involve horizontal sliding on faults and not the large plate-boundary thrusts; volcanic eruption–triggered quakes are related to magmatic and volcanic processes rather than subduction-zone plate boundary rupture.