Tsunamis are series of ocean waves generated by large, sudden disturbances that displace massive amounts of seawater — typically undersea earthquakes, but also submarine landslides, volcanic eruptions, and even large meteorite impacts. In the deep ocean, tsunami waves travel at the speed of a jetliner (up to 500 mph) but are imperceptible at the surface. As they approach shore and the seafloor shallows, they slow down and grow dramatically in height, sometimes reaching 100 feet or more. In the United States, tsunami risk is highest along the coasts of Alaska, Hawaii, California, Oregon, and Washington. Ready.gov’s Tsunami Hazard Information Sheet provides the essential guidance that can save your life if you live in or visit a coastal area.
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Understanding Tsunami Generation
The vast majority of damaging tsunamis are caused by large undersea earthquakes (magnitude 7.0 or greater) that involve vertical seafloor displacement along subduction zones. The Pacific Ring of Fire contains most of the world’s subduction zones and generates the most tsunamis. The U.S. Pacific Northwest faces the threat of a major tsunami from the Cascadia Subduction Zone, running from northern California to southern British Columbia. Scientists believe a Cascadia megathrust earthquake (potentially magnitude 9.0 or greater) could generate a tsunami that would inundate low-lying coastal areas within 15–30 minutes of the quake — leaving little time for official warnings.
Natural Warning Signs
Ready.gov emphasizes that nature itself provides warning signs that a tsunami may be approaching:
- Strong or prolonged ground shaking. If you are in a coastal area and feel an earthquake strong enough to make it difficult to stand, or that lasts more than 20 seconds, a tsunami may follow within minutes. Do not wait for an official warning — move to high ground immediately.
- Unusual sea behavior. The sea may recede dramatically before a tsunami arrives, exposing the seafloor. This is NOT a sign to walk out and look — it is a sign to run to high ground. The water that receded will return as a massive wave.
- Loud ocean roar. A roaring sound from the ocean similar to a train or airplane engine can indicate an approaching tsunami.
Before a Tsunami: Preparation
- Know your tsunami hazard zone. Your state emergency management agency and local government publish tsunami inundation maps. Look up your home, workplace, and favorite beach locations.
- Know your evacuation route. Identify routes to high ground (at least 100 feet above sea level or several miles inland) from every coastal location you frequent. Walk the route before you need it.
- Post tsunami evacuation maps in your home, hotel room, and business if you are in a coastal area.
- Sign up for emergency alerts. The National Tsunami Warning Center (NTWC) and Pacific Tsunami Warning Center (PTWC) issue tsunami watches and warnings through NOAA and the National Weather Service.
- Prepare an emergency kit for evacuation: water, food, medications, important documents, cash, and a battery-powered radio.
During a Tsunami
- Move immediately to high ground. Do not wait for official confirmation. Do not try to watch the tsunami arrive — by the time it is visible, escape may be impossible.
- Go on foot if possible. Vehicles can be caught in traffic gridlock. Tsunami evacuation routes are typically walkable to high ground.
- Do not return to the coast until officially authorized. Tsunamis come in series of waves — the first wave is often not the largest.
After a Tsunami
- Stay away from flooded areas — water may still be receding or more waves may arrive
- Avoid entering damaged buildings — check for structural stability first
- Be aware of hazards in floodwater: contamination, debris, electrical hazards
- Document damage, follow official guidance, and register with disaster assistance if your area receives a federal disaster declaration
How PubSafe Supports Coastal Tsunami Preparedness
In a near-field tsunami event generated by a local earthquake, official warning systems may not have time to alert coastal residents before the first wave arrives. PubSafe helps coastal communities by supporting real-time community communication about earthquake activity and sea behavior observations, enabling rapid welfare checks after a tsunami to identify survivors needing rescue in inundated areas, helping coordinate evacuation logistics matching people who need transport to high ground with those who have vehicles, and connecting visitors with local residents who know the evacuation routes — critical in high-tourism coastal areas where many people are unfamiliar with local evacuation infrastructure.
In the 2011 Tohoku tsunami which killed nearly 20,000 people, communities with strong pre-existing social networks and clear evacuation culture performed significantly better. PubSafe is the digital infrastructure for building that community connection. Download it now if you live in or visit coastal areas.
Resources
- Ready.gov Tsunamis page
- National Tsunami Warning Center: tsunami.gov
- Pacific Tsunami Warning Center: ptwc.weather.gov
Download the Ready.gov Tsunami Hazard Information Sheet, walk your evacuation route, and connect your coastal community on PubSafe.
The Cascadia Subduction Zone: America’s Greatest Tsunami Threat
The Cascadia Subduction Zone (CSZ) is a 700-mile-long fault system running offshore from northern California through Oregon and Washington and into southern British Columbia. It is a convergent plate boundary where the Juan de Fuca oceanic plate is being pushed beneath the North American plate at a rate of about 4 centimeters per year. Unlike many subduction zones that release accumulated stress through frequent moderate earthquakes, the Cascadia fault appears to store energy over centuries and release it in catastrophic “megathrust” earthquakes — events that scientists believe reach magnitudes of 8.0–9.2 or greater.
Geological evidence from tree rings, sediment cores, and coastal subsidence patterns indicates that the last major Cascadia earthquake occurred on January 26, 1700 — a date that Japanese historical records helped scientists pin down precisely, because the tsunami from that earthquake reached Japan and was recorded in Japanese historical documents. The 1700 earthquake is estimated to have been magnitude 9.0 or greater and generated a tsunami that devastated coastal communities throughout the Pacific Northwest and crossed the Pacific Ocean.
Scientists believe that Cascadia megathrust earthquakes occur with an average return period of 200–500 years, with some evidence of more frequent smaller events on individual fault segments. The Pacific Northwest is currently about 325 years past the last full-margin rupture. FEMA and Oregon Emergency Management have modeled a major Cascadia event and project that it would be the most destructive natural disaster in the history of the United States — potentially killing thousands in the tsunami inundation zone and causing tens of billions of dollars in damage.
For residents of coastal Oregon, Washington, and Northern California, this is not a theoretical risk — it is a matter of when, not if. Know your evacuation zone, walk your evacuation route, and connect your coastal community on PubSafe so that your neighborhood is organized and communicating when the ground starts shaking.
Distant-Source Tsunamis: Pacific Warning System
While near-field tsunamis generated by local earthquakes provide the least warning time, tsunamis generated by distant earthquakes — in Alaska, Japan, Chile, and other Pacific Rim locations — provide hours of advance warning before waves reach U.S. coastlines. This warning time is managed by the Pacific Tsunami Warning Center (PTWC) in Ewa Beach, Hawaii, and the National Tsunami Warning Center (NTWC) in Palmer, Alaska. These centers monitor seismic activity around the Pacific and issue tsunami watches, advisories, and warnings based on the detected earthquake’s magnitude, location, and depth.
The 2011 Tohoku, Japan earthquake and tsunami provided several hours of warning for U.S. Pacific coastlines. Coastal communities from Alaska to Hawaii to California and Oregon received warnings and activated evacuations. The response was generally successful, though there were injuries and property damage in some communities (particularly in harbors, where tsunami currents caused significant damage to boats and marina infrastructure).
For distant-source tsunamis, the warning system is effective — but it requires that communities have pre-planned evacuation procedures, that residents know and heed the warnings, and that evacuation routes can handle the traffic loads expected during a major evacuation. These factors require ongoing community attention, practice, and investment. Annual Tsunami Preparedness Week (typically held in March in Oregon and Washington) provides opportunities for communities to practice evacuation routes, review emergency plans, and connect residents with preparedness resources. Check with your state emergency management agency for local tsunami preparedness events and resources.
