Hawaii is highly susceptible to tsunamis due to its isolated location in the middle of the Pacific basin. These waves, a series of ocean disturbances, are typically caused by underwater events like large earthquakes, volcanic eruptions, or landslides. The islands sit within the Pacific “Ring of Fire,” a vast, seismically active zone that generates most of the world’s tsunamis, making this natural hazard a significant concern.
Primary Sources of Tsunami Generation
Tsunamis affecting Hawaii are categorized as either distant-source or local-source events. Distant-source tsunamis originate more than 600 miles away, typically from the Pacific Ring of Fire. Major subduction zone earthquakes in regions like Alaska, Chile, or Japan displace the seafloor, generating powerful waves that travel across the entire ocean.
These distant waves travel at high speeds in the deep ocean, but the vast distances provide several hours for warning and evacuation. For example, a tsunami from Alaska may arrive in about five hours, while one originating from Chile could take up to 15 hours.
Local-source tsunamis are much rarer but pose a greater danger due to minimal warning time, sometimes only a few minutes. These events are generated close to the islands, often from large earthquakes on the flanks of volcanoes like Kīlauea or Mauna Loa. The rapid movement of the island’s south flank, known as a basal-slip earthquake, can immediately displace ocean water.
Other potential local causes include submarine landslides or the flank collapse of a volcano, where a massive section slides into the ocean. If a strong local earthquake makes it difficult to stand, the shaking acts as a natural warning, requiring immediate movement to higher ground without waiting for official alerts.
Noteworthy Historical Tsunami Events
Hawaii’s tsunami history is marked by two destructive distant-source events that spurred the creation of the modern warning system. The “April Fools’ Day Tsunami” of April 1, 1946, was generated by a magnitude 8.6 earthquake in the Aleutian Islands, Alaska. It arrived in Hawaii in under five hours, surprising the population because no formal warning system existed.
This Pacific-wide wave killed 159 people and caused $26 million in damage, primarily devastating the waterfront area of Hilo. The largest wave run-up was recorded at 17 meters (55 feet) in Pololū Valley. The severity of this event led directly to the establishment of the Seismic Sea Wave Warning System, which later became the Pacific Tsunami Warning Center (PTWC).
Another devastating event was the 1960 Chile Tsunami, caused by the largest recorded earthquake in history (magnitude 9.4–9.6). The tsunami struck Hawaii about 15 hours later. While wave heights were generally 3 to 17 feet across most of the islands, Hilo Bay experienced a localized amplification effect.
The third wave that struck Hilo transformed into a powerful bore, reaching a run-up height of 10.7 meters (35 feet), destroying over 500 homes and businesses. This event resulted in the loss of 61 lives in Hilo alone, reinforcing that a tsunami is a series of waves and that local topography can dramatically amplify the hazard.
Hawaii’s Tsunami Warning Infrastructure
Hawaii’s preparedness relies on a sophisticated, multi-layered system providing timely alerts for both distant and local threats. The Pacific Tsunami Warning Center (PTWC), headquartered in Hawaii, is the primary authority monitoring earthquakes and issuing alerts for the entire Pacific basin. The PTWC uses seismic data to quickly determine the potential for a tsunamigenic earthquake.
A network of Deep-ocean Assessment and Reporting of Tsunami (DART) buoys is deployed across the deep ocean, functioning as a real-time reporting system. These buoys use bottom pressure recorders to detect and measure tiny changes in sea level indicating a passing tsunami wave. The data is relayed via satellite to the PTWC for analysis, allowing for the confirmation or cancellation of a threat.
The statewide Outdoor Warning Siren System is the most visible alert component, consisting of over 400 towers across the islands. The sirens emit a distinct, steady “Attention Alert Signal” to notify people of an impending hazard, including tsunamis. This system is integrated with the Hawaii Statewide Alert and Warning System, which also utilizes the Emergency Alert System (EAS) and Wireless Emergency Alerts (WEA) via mobile phones.
Personal Safety and Evacuation Protocols
The state issues distinct alert levels that demand specific actions from the public. A Tsunami Watch means a potentially tsunamigenic distant earthquake has occurred and officials are monitoring the situation. A Tsunami Advisory is issued when a tsunami is expected to generate strong currents or dangerous waves, requiring people to stay out of coastal waterways.
The most serious alert is a Tsunami Warning, meaning a destructive tsunami is imminent or expected. This requires immediate evacuation from coastal low-lying areas. Residents and visitors in designated evacuation zones must move inland or to high ground, ideally at least 100 feet above sea level. If you feel a strong earthquake near the shore that makes it difficult to stand, do not wait for official sirens, as this signals a local tsunami arriving in minutes.
Other natural warning signs include a sudden receding of the ocean water or a loud, unusual roar coming from the sea. If these signs are observed, immediate self-evacuation is necessary. Once safely evacuated, stay put and monitor local media for the official “all clear” signal from civil defense authorities. Since a tsunami is a series of waves, the danger can persist for many hours.