Tsunami Risk in the US: How FEMA Scores It and How to Prepare

Written by Dvir · Data: FEMA National Risk Index v1.20 · Updated August 4, 2026

Tsunami is the rarest hazard in the National Risk Index — only 65 US counties record measurable risk, the fewest of any of the 18 hazards. It is also the one with the most extreme gap between “typical year” and “the event”: nothing happens for decades, and then a subduction-zone earthquake delivers a series of waves that can erase coastal towns. The US holds roughly $720 million in tsunami expected annual loss (hazard code TSUN), almost all of it pressed against a few hundred miles of Pacific coastline.

What counts as tsunami risk

A tsunami is a series of ocean waves — often an hour apart, with the first rarely the largest — generated when something abruptly displaces the water column: usually a great undersea earthquake, occasionally a landslide or volcanic collapse. In deep ocean the waves travel at jetliner speeds and pass unnoticed; at the shore they arrive not as surfable walls but as violent, debris-laden floods that surge inland for 15 minutes or more, then drain with equal force.

US risk divides into two very different problems. Distant tsunamis (from Alaska, Japan, Chile) give hours of official warning. Local tsunamis — the dominant threat — arrive 10 to 30 minutes after the earthquake that caused them, faster than any official system can move a population. The Cascadia Subduction Zone off Washington, Oregon, and Northern California produces magnitude-9 earthquakes on a centuries-long cycle; the last, in January 1700, sent a damaging tsunami across the Pacific to Japan. The 1964 Alaska earthquake’s tsunami killed people from Alaska to Crescent City, California — which has recorded more tsunami damage than any other town in the lower 48.

How the NRI measures it

FEMA combines each coastal county’s exposure inside mapped tsunami inundation zones, annualized frequency from historical and geological records, and historic loss ratios into an Expected Annual Loss scored 0–100 against all counties. With events this rare, geological evidence — buried marsh soils, tsunami sand sheets — does much of the frequency work.

Where the risk concentrates

The ranking is a Cascadia roll call. Grays Harbor County, WA leads the nation (score 100, ~$121M expected annual loss — low-lying towns like Ocean Shores and Westport sit squarely in the inundation zone), followed by Del Norte County, CA (Crescent City, ~$114M), Clatsop County, OR (Astoria and Seaside, ~$100M), Pacific County, WA, and Humboldt County, CA. Beyond Cascadia, meaningful risk covers Alaska’s coast (the highest US frequency), Hawaii (hit repeatedly from distant sources — Hilo most tragically in 1946 and 1960), and Puerto Rico, which has its own local subduction source.

How to read your county’s score

Tsunami risk is binary at the parcel level in a way no other hazard matches: you are inside the mapped inundation zone or you are not, and every at-risk state publishes those maps street by street. If your county scores high, the questions are: Is your home, workplace, or child’s school in the zone? What is your evacuation route on foot — and how many minutes does it take? Do you know the high ground or, where flat terrain makes it unreachable (Ocean Shores, Seaside), the designated vertical evacuation structure? Washington has begun building these — the Ocosta school in Westport was North America’s first.

What actually reduces the risk

  • The earthquake IS the warning. On the Pacific coast, if shaking is strong or lasts longer than ~20 seconds, evacuate inland or uphill immediately when the shaking stops — do not wait for sirens, alerts, or confirmation. A Cascadia tsunami can arrive in 15 minutes.
  • Know the natural signs: strong or long shaking, the ocean receding abnormally (the classic drawback), or a loud ocean roar. Any one means go, now.
  • Go on foot, go up. Roads jam and bridges may be down after the quake; plan a walking route to high ground (aim for 100+ feet elevation or 2 miles inland where terrain allows) and practice it — communities that drill, survive.
  • Stay out until officials clear it — tsunamis are wave trains; the third or fifth wave, hours later, is often the largest. Returning between waves is a recurring cause of death.
  • Boaters: in port with time short, abandon the boat and evacuate; with a distant-source warning and enough time, deep water (100+ fathoms) is the safe harbor.
  • Distant warnings come via NOAA’s tsunami warning centers, Wireless Emergency Alerts, and coastal sirens — take official evacuation orders literally even under sunny skies; the 1946 Hilo dead had no earthquake to warn them.

The tsunamis that built the warning system

Hilo, Hawaii, April 1, 1946. A magnitude 8.6 earthquake in the Aleutian Islands sent waves across the Pacific. They arrived in Hawaii about five hours later, under a clear sky, with no earthquake felt and no warning of any kind. The waves killed 159 people, many of them along Hilo’s bayfront and at Laupāhoehoe, where a schoolhouse stood at sea level. The response was the creation of what became the Pacific Tsunami Warning Center — the first tsunami warning system in the world, and the reason distant-source tsunamis are now a solvable problem.

Hilo again, May 1960. The magnitude 9.5 Chile earthquake — the largest ever instrumentally recorded — sent waves that reached Hawaii roughly 15 hours later. A warning was issued. Sixty-one people died anyway, largely because some residents did not evacuate or returned too early. The 1960 event is the enduring lesson that a warning system only works if the population acts on it, and it drove the modern emphasis on evacuation drills and clearly marked routes.

Alaska, March 27, 1964. The magnitude 9.2 Good Friday earthquake generated both a Pacific-wide tsunami and devastating local waves and submarine-landslide surges. It destroyed the town of Chenega, wiped out Valdez’s waterfront, and killed people as far away as Crescent City, California, which has recorded more tsunami damage than any other town in the contiguous US. The event led directly to the establishment of the Alaska (now National) Tsunami Warning Center.

The 1700 Cascadia earthquake. No Americans wrote it down, but the evidence is unambiguous: buried marsh soils and sand sheets along the Northwest coast, drowned “ghost forests” of red cedar, Indigenous oral histories describing a night the ground shook and the sea came in, and Japanese records of an “orphan tsunami” that struck the coast of Honshu on the night of January 27–28, 1700 with no earthquake felt. Working backward from the Japanese arrival times gave the rupture a date and a magnitude near 9. Every Cascadia preparedness program in Washington, Oregon, and Northern California descends from that piece of detective work.

Indian Ocean 2004 and Japan 2011. Neither struck the US mainland, but both transformed American practice: 2004 drove the global expansion of deep-ocean DART buoys and the extension of US warning coverage to the Atlantic, Gulf, and Caribbean; 2011 provided high-definition documentation of how a modern, well-prepared coastal society still loses tens of thousands of people when a local tsunami arrives in under 30 minutes — and it validated the case for vertical evacuation structures where high ground is unreachable.

The two problems: distant and local

Tsunami risk in the US divides cleanly into two hazards that require opposite responses.

Distant tsunamis originate thousands of miles away — Alaska, Japan, Chile, Kamchatka. They give hours of warning. NOAA’s warning centers detect the source earthquake, confirm wave generation with DART buoys, and issue watches and warnings through Wireless Emergency Alerts, NOAA Weather Radio, coastal sirens, and local emergency management. This is the system that works. The failure mode is human: people who do not evacuate because the sky is clear and nothing has happened, or who return between waves.

Local tsunamis originate on your own coastline and arrive in 10 to 30 minutes. No official warning system can move a population that fast — the alert may well arrive after the wave. For Cascadia, the Alaskan coast, and Puerto Rico, the entire response rests on individuals recognizing the natural warning and self-evacuating without instruction. This is the scenario that drives the numbers in this hazard.

Seasonality does not apply. Subduction earthquakes have no season, no time of day preference, and no forecast. What varies is who is on the beach — summer daytime hours in a tourist town represent a fundamentally different exposure than a February night.

Geographically, the Cascadia coast of Washington, Oregon, and Northern California carries the highest expected annual losses, because low-lying towns sit directly in mapped inundation zones with limited high ground. Alaska has the highest event frequency, Hawaii is exposed to sources from every direction around the Pacific rim, and Puerto Rico and the US Virgin Islands face a local subduction source in the Puerto Rico Trench with arrival times comparable to Cascadia’s. The Atlantic and Gulf coasts carry small but non-zero risk from distant sources and submarine landslides.

Structures, routes, and community measures

You cannot harden a house against tsunami inundation in any practical sense. Mitigation for this hazard is about getting people out of the zone and, secondarily, about designing the few structures that must survive.

  • Know the inundation map for your address. Every at-risk state publishes them street by street — Washington, Oregon, California, Alaska, Hawaii, and Puerto Rico all have detailed modeled inundation and evacuation mapping. This hazard is binary at the parcel level in a way no other hazard in this index is: you are in the zone or you are not.
  • Time your walking route. Roads will be damaged, bridges may be down, and traffic will not move after a magnitude-9 earthquake. Walk your route, with children and the least mobile member of the household, and time it. Aim for high ground of roughly 100 feet elevation or two miles inland where terrain allows. If the route takes longer than the arrival time, you need a different plan, not a faster walk.
  • Vertical evacuation structures exist where high ground is unreachable — the flat coastal plains of Ocean Shores, Westport, Long Beach, and Seaside. Washington built North America’s first at the Ocosta Elementary School in Westport, and more have followed. Find out whether your community has one, where it is, and how long it takes to reach on foot.
  • Structural design for the zone. Where building in the inundation zone is unavoidable, current codes (ASCE 7 includes tsunami design provisions for critical facilities in the highest-risk states) call for elevated construction on strong columns with breakaway lower walls, so water passes through rather than pushing the structure over. Debris impact, not water pressure, destroys most buildings.
  • Community programs matter more here than personal gear. The NWS TsunamiReady program certifies communities on warning reception, evacuation mapping, signage, and public education. Signage and drills are what actually save people when the ground stops shaking and nobody knows what to do.
  • Secure the earthquake side first. For a local tsunami, the earthquake comes first and may injure you, block your door, or destroy your stairs. Everything in the earthquake guide — bolting furniture, keeping shoes and a flashlight by the bed, structural retrofit — is also tsunami preparation, because you must be uninjured and mobile in the first minutes.

Insurance and financial exposure

  • Tsunami damage is flood damage for insurance purposes. Standard homeowners policies exclude it; NFIP or private flood insurance is the coverage that applies, with the usual 30-day waiting period.
  • NFIP limits are $250,000 building and $100,000 contents for residential policies — often well short of rebuild cost in coastal Pacific communities. Excess flood coverage from the private market fills the gap.
  • The earthquake and the tsunami are separate claims. Shake damage requires a separate earthquake policy (excluded from homeowners everywhere); inundation damage requires flood coverage. A Cascadia event will produce both, and a household with only one of the two policies will be substantially uninsured.
  • Vehicles are not covered by flood insurance — comprehensive auto coverage handles them, if there is anything left to claim.
  • Business interruption in coastal tourist economies is the large uninsured exposure, and it lasts far longer than the physical cleanup.
  • Document now. Video the house and contents and store it off-site or in the cloud. In a full inundation there is nothing left to photograph afterward.

Before, during, and after

Before. Look up your address on the state inundation map. Walk the evacuation route and time it. Identify where each member of the household will be during a normal day — school, work, daycare — and confirm that each of those places has a tsunami plan, because you will not be able to reach them and should not try. Agree in advance that everyone evacuates independently and reunites at a designated high-ground meeting point; the single most common cause of death in local tsunamis is people going back for someone. Keep a grab-and-go bag and sturdy shoes accessible, since you will be walking, possibly for hours, possibly in the rain, and services will be gone for days to weeks.

During. The earthquake is the warning. On the Pacific coast, if shaking is strong, or if it lasts longer than about 20 seconds, evacuate inland or uphill the moment the shaking stops — do not wait for a siren, an alert, an official order, or confirmation from anyone. Also treat these natural signs as immediate evacuation orders: the ocean receding abnormally and exposing seafloor, or a loud roar from the water. Go on foot. Do not go to the beach to look, do not stop to secure property, and do not drive unless the person you are moving cannot walk.

For a distant-source warning, follow the official evacuation order literally even under a clear sky and calm sea. That is precisely the scenario that killed people in 1946 and 1960.

After. Stay out until officials declare it over. A tsunami is a wave train that continues for hours, and the first wave is frequently not the largest — the third, fifth, or eighth may be. Returning between waves is a recurring and entirely avoidable cause of death. Expect that the water carries debris, vehicles, fuel, and sewage, and that the outflow is as violent as the inflow. After a local Cascadia event, plan on being self-sufficient for at least two weeks: roads, bridges, water systems, and fuel supply across the region will be severed simultaneously, and outside help will take a long time to reach the coast.

Boaters: with only minutes, abandon the vessel and evacuate on foot. With a distant-source warning and hours of lead time, taking the boat to deep water — well offshore, in at least 100 fathoms — is the standard practice, since tsunamis are harmless in deep ocean and destructive in harbors.

Sources

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