A tsunami is a series of ocean waves generated by the sudden, large-scale displacement of water, typically caused by underwater earthquakes, landslides, or volcanic activity. These waves travel rapidly across the deep ocean and build to destructive heights as they enter shallow coastal waters. The United States is vulnerable to these hazards, but the risk is highly localized. The greatest threat is concentrated along the Pacific basin coasts due to active tectonic plate boundaries, where warning times can range from minutes for a local quake to hours for a distant seismic event.
The Highest Risk Zone: The Pacific Coast (Cascadia)
The greatest potential for a catastrophic local tsunami in the contiguous United States lies along the Cascadia Subduction Zone (CSZ). This 700-mile fault runs offshore from northern California up to British Columbia. The CSZ is the boundary where the Juan de Fuca tectonic plate slides beneath the North American plate, building immense stress. Geologic evidence shows this fault has a history of producing magnitude 9.0 or greater megathrust earthquakes, the last of which occurred in January 1700.
A full rupture of the CSZ would generate a powerful, localized tsunami, striking the coasts of Washington, Oregon, and northern California with virtually no warning. Since the epicenter is close to the shore, initial waves are modeled to arrive in as little as 7 to 30 minutes after the ground shaking begins. This short arrival time means the only reliable alert for coastal residents is the natural warning sign: a strong earthquake that makes standing difficult.
The massive displacement of the seafloor during such an event would also cause coastal land along the fault to permanently subside by several feet. This land sinkage would expand the tsunami’s reach by lowering the elevation of beaches and infrastructure, increasing the inundation zone. The CSZ represents the most significant local tsunami threat to the lower 48 states due to the anticipated magnitude and short notice.
Unique Threats of Alaska and Hawaii
Alaska and Hawaii face distinct tsunami risks due to their positions within the Pacific Ring of Fire. Alaska’s threat primarily comes from the Aleutian Subduction Zone, one of the world’s most seismically active regions. This zone generates both local and trans-Pacific tsunamis. Local waves can be generated by tectonic displacement or, uniquely in Alaska, by earthquake-triggered submarine and subaerial landslides within steep-walled fjords and bays.
Landslide-generated tsunamis are dangerous because they arrive almost immediately after ground shaking, leaving no time for evacuation. Aleutian earthquakes also routinely generate distant tsunamis that propagate across the Pacific basin, posing a threat to the US West Coast and Hawaii. Studies of the eastern Aleutian Subduction Zone suggest that even previously less active sections of the fault could produce major waves traveling toward the central Pacific.
Hawaii is situated near the center of the Pacific Ocean, making it vulnerable to distant-source tsunamis from nearly anywhere around the Pacific Rim, including Japan, Chile, and Alaska. The island chain’s geography offers little natural protection from these deep-ocean waves that travel for hours before impact. The Big Island of Hawaii also faces a rare, local threat from the massive, unstable flanks of active shield volcanoes, such as Kīlauea. Catastrophic flank collapses could generate extremely large, immediate-impact waves.
Lesser Known Vulnerabilities: Atlantic and Gulf Coasts
The Atlantic and Gulf Coasts are not near major tectonic subduction zones, but they are vulnerable to non-seismic sources. The primary threat is massive submarine landslides along the continental shelf. The movement of sediment masses on the ocean floor, potentially triggered by smaller earthquakes or destabilized by gas hydrates, can displace enough water to generate a tsunami.
A distant seismic threat exists for the Atlantic coast from the Puerto Rico Trench, an active plate boundary in the Caribbean. Modeling indicates that a large earthquake there could send destructive waves to the East Coast, particularly Florida and the Carolinas. The remote possibility of a flank collapse of a volcanic island in the North Atlantic, such as the Canary Islands, has also been studied as a source for trans-oceanic waves.
The Gulf of Mexico is primarily at risk from submarine landslides, with evidence pointing to large, prehistoric slides near the Mississippi River delta. Although seismic activity is minimal, the heavily urbanized and industrialized nature of the Gulf and Atlantic margins means that even a moderate, non-seismic tsunami poses a substantial risk to life and infrastructure. The wide and shallow continental shelf in both regions may help dissipate wave energy before it reaches the shore.
Historical Precedent: Past US Tsunami Events
The United States has a history of devastating tsunami impacts. The most destructive event in North American history was the M9.2 Great Alaska Earthquake of March 1964, which generated both local and distant tsunamis. The resulting waves caused widespread death and damage across Alaska, with fatalities recorded as far south as Oregon and California.
The 1946 Aleutian Islands earthquake generated a massive, distant tsunami that traveled across the Pacific to Hawaii. This wave devastated Hilo, claiming 159 lives and causing millions in damage, despite originating nearly 2,400 miles away. The devastation of the 1946 event led directly to the establishment of the US Seismic Sea Wave Warning System, the predecessor to the modern Tsunami Warning Centers.
More recently, the 2011 M9.1 Tohoku earthquake off Japan caused a trans-Pacific tsunami that reached the US West Coast. Although the waves were smaller than the 1964 event, they caused significant damage to harbors and boats, particularly in Crescent City, California. This highlights the ongoing threat posed by major seismic activity anywhere in the Pacific basin.