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