Describe the Cascadia subduction zone hazard profile and why it is significant for North American risk.

• A. A major intraplate boundary causing modest earthquakes.
• B. A major locked subduction plate boundary capable of Mw 9+ megathrust earthquakes with tsunamis in the Pacific Northwest; long recurrence intervals; important for regional planning.
• C. A volcanic arc with frequent eruptions.
• D. An isolated small quake zone with no tsunamis.

Answer: (B)

The essential idea is that the Cascadia zone is a long, locked subduction boundary where the Juan de Fuca plate slides beneath North America. This configuration stores enormous elastic strain in the overlying plate, which is released during very large earthquakes known as megathrust events. When such a rupture occurs, it can produce shaking of magnitude around 9 or greater and simultaneously deform the seafloor, driving tsunamis that can affect the Pacific Northwest and even reach distant shores.

Recurrence of these gigantic quakes is infrequent, measured in centuries, but the consequences when they happen are massive. The last major event is dated to around 1700, so the region lives with a long interval since the previous rupture, which translates into high consequence if and when another event occurs. The risk is not confined to a single town or coastline; it spans coastal towns, major cities like those along the coast and across Puget Sound and the inland corridors, and critical infrastructure such as roads, bridges, power and gas networks, and ports.

Because of its nature as a subduction boundary capable of Mw 9+ megathrust earthquakes with accompanying tsunamis, the Cascadia zone represents a significant regional hazard for North America. This combination of extreme ground shaking, destructive tsunamis, and long return periods makes regional planning, preparedness, and resilient design essential.