Tsunamis: The Long-Wave Threat to Coastal Communities

Tsunamis are among the rarest but most geographically extensive natural hazards, capable of causing catastrophic destruction across entire ocean basins. Understanding the warning signs and appropriate response can mean the difference between life and death for coastal residents and visitors.

What Causes a Tsunami?

A tsunami is a series of long, high-energy ocean waves triggered by sudden, large-scale displacement of water. The most common cause is a submarine earthquake (subduction zone megathrust earthquakes are particularly efficient tsunami generators), but tsunamis can also be generated by submarine landslides, volcanic island collapses, and, very rarely, large meteorite impacts. The energy in a tsunami wave travels as a disturbance in the water column from surface to seafloor, giving these waves fundamentally different dynamics than wind-generated waves.

In the open ocean, tsunami waves may be only 1-3 feet high but travel at speeds of 500-600 mph - comparable to a jet aircraft. As the waves approach shallow coastal water, they slow dramatically and their height increases - a process called shoaling - potentially reaching tens of meters in height at the coast.

Warning Systems and Lead Time

The Pacific Tsunami Warning Center (PTWC) and West Coast and Alaska Tsunami Warning Center operate 24/7, monitoring global seismic networks for earthquakes with tsunami potential. For distant tsunamis - those originating thousands of miles away - warning times of 30 minutes to several hours are often achievable. However, for locally generated tsunamis where the earthquake source is near the coast, warning times may be only minutes. In such cases, the earthquake itself serves as the warning: if you feel strong, prolonged ground shaking in a coastal area, move to high ground immediately without waiting for an official alert.

U.S. Tsunami Risk

The most significant U.S. tsunami risk is associated with the Cascadia Subduction Zone, which poses the threat of both a catastrophic local tsunami affecting the coasts of northern California, Oregon, Washington, and British Columbia, and a transoceanic tsunami affecting Hawaii and other Pacific regions. Alaska, Hawaii, Puerto Rico, and U.S. Pacific territories also face significant tsunami risk. All U.S. Pacific coastal communities have designated tsunami inundation zones and evacuation routes that residents should familiarize themselves with.

How to Respond

• Know your evacuation routes: Identify the nearest high ground and the official evacuation routes in your community before a tsunami threatens.
• Recognize natural warnings: Strong prolonged earthquake shaking near the coast, unusual receding of the ocean ('drawback'), or a loud roaring sound from the ocean are all natural warning signs. Do not wait for official notification - move to high ground immediately.
• Go far and high: Inundation can extend far inland. Move at least 100 feet above sea level or at least 2 miles inland - whichever is most quickly achievable.
• Do not return until officials say it is safe: Tsunamis arrive in multiple waves; the first wave is not always the largest. The hazard may persist for many hours.