A tsunami is a series of ocean waves caused by the sudden displacement of a large volume of water. This displacement is most often triggered by powerful underwater earthquakes, but it can also result from massive landslides, volcanic eruptions, or meteorite impacts. Tsunami waves involve the entire water column, moving at great speeds across the deep ocean. The factors that generate these waves vary significantly across the United States, creating a diverse risk profile for coastal states.
The State with the Highest Tsunami Frequency
Alaska has experienced the highest frequency of tsunamis in the United States, a consequence of its location along one of the world’s most active tectonic boundaries. The state’s extensive coastline and proximity to the Aleutian Trench place it directly in the path of numerous seismic events, generating both destructive trans-Pacific tsunamis and smaller, localized waves. The Aleutian Subduction Zone is the primary engine for this activity. Major events, such as the magnitude 9.2 Great Alaska Earthquake in 1964, demonstrated the region’s capacity to generate waves that cause widespread devastation both locally and across the entire Pacific basin. Furthermore, Alaska’s coastal geography includes many steep-sided fjords and bays, making it vulnerable to landslides that can create extremely high, localized waves, such as the 1,720-foot run-up recorded in Lituya Bay in 1958.
Hawaii, while experiencing fewer locally-generated tsunamis, is the most frequently impacted by distant-source tsunamis, making it second in overall risk. The islands sit near the center of the Pacific Ocean, which positions them to intercept waves generated from nearly any active zone around the Pacific Ring of Fire. For example, the 1946 Aleutian earthquake generated a devastating tsunami that killed 159 people in Hawaii, highlighting the islands’ vulnerability to far-field events.
Geological Mechanisms Driving US Tsunami Activity
The vast majority of US tsunami activity is driven by the dynamic geology of the Pacific basin, particularly the system of subduction zones known as the Pacific Ring of Fire. A subduction zone is a convergent boundary where one tectonic plate is forced beneath another, a process that builds immense strain over time. When this strain is suddenly released, it results in a megathrust earthquake, which vertically displaces the seafloor and the overlying water column, generating a tsunami.
Two major subduction zones directly affect US coastlines: the Aleutian Subduction Zone and the Cascadia Subduction Zone (CSZ). The Aleutian zone, extending from the Gulf of Alaska southwest along the Aleutian Islands, is responsible for the high frequency of tsunamis impacting Alaska and sending waves across the Pacific. In contrast, the Cascadia Subduction Zone stretches for 700 miles offshore from Northern California to Vancouver Island, British Columbia. The Cascadia zone is capable of producing magnitude 9.0 earthquakes, which last occurred in 1700, sending a massive “orphan tsunami” across the Pacific to Japan. This megathrust fault is currently locked, building strain that will eventually release and cause a massive earthquake and tsunami that will directly impact the Pacific Northwest coastlines of Washington, Oregon, and Northern California. Beyond these primary seismic causes, secondary mechanisms like underwater landslides, often triggered by smaller earthquakes, are also significant. These landslides displace large amounts of water.
Distinguishing Local and Distant Tsunami Sources
The origin of a tsunami relative to the coast dictates the warning time. Tsunamis are categorized as either local (near-field) or distant (far-field) based on their source location. A local tsunami is generated by an event near the coast, arriving in under an hour, sometimes as quickly as a few minutes.
The danger posed by local tsunamis is magnified because the ground shaking from the causative earthquake serves as the only immediate warning, potentially arriving before any official notification can be issued. This is the primary risk for the Pacific Northwest, where a Cascadia Subduction Zone rupture would send a wave to Washington, Oregon, and California shores within 15 to 30 minutes. Consequently, preparedness focuses on immediate self-evacuation: if strong shaking lasts 20 seconds or more, people must move immediately to higher ground.
A distant tsunami, conversely, originates from thousands of miles away, often on the other side of an ocean basin. These waves can take many hours to reach distant shores, such as the 13 hours it takes a Chilean-sourced tsunami to reach Southern California. This extended travel time is why the US Tsunami Warning System is effective for distant events, providing sufficient time for authorities to issue warnings, forecast wave heights, and organize evacuations in places like Hawaii and the US West Coast. A single event, such as a major earthquake off Alaska, can be a local tsunami for Alaskan communities but a distant tsunami for California and Hawaii.