What Is a Tsunami? Warning Signs & Prep Guide

What Is a Tsunami?

A tsunami is not just a big wave. It is a series of waves generated by the rapid displacement of a large volume of water — typically ocean water — caused by an underwater earthquake, landslide, or volcanic eruption. Unlike a normal ocean wave driven by wind on the surface, a tsunami involves the entire water column, from ocean floor to surface, moving as a mass.

That distinction explains why tsunamis are so destructive. A typical ocean wave is surface energy — it breaks in shallow water and dissipates. A tsunami is kinetic energy moving through the full depth of the ocean. When it reaches shallow coastal water and the seafloor rises, that energy has nowhere to go but up. The wave slows, compresses, and amplifies into a wall of water — sometimes tens of feet or even over 100 feet high — with the momentum of the entire water column behind it.

The 2004 Indian Ocean tsunami, triggered by a magnitude 9.1 earthquake off Sumatra, killed approximately 227,000 people across 14 countries. The 2011 Tohoku tsunami in Japan reached heights over 128 feet above sea level in some locations and penetrated more than 6 miles inland. These are not rare extremes — they are what tsunamis do when the conditions align.

How Tsunamis Form

The Physics of Wave Formation

A tsunami begins when a sudden geological event displaces a massive volume of water vertically. In the case of a submarine earthquake, the seafloor along a subduction zone can drop or rise by 10 to 30 feet across a rupture zone hundreds of miles long — in seconds. That vertical displacement transfers directly into the water above it, creating a series of radiating waves.

Think of dropping a large flat object into a bathtub. The splash radiates outward in all directions. A tsunami works the same way, except the “drop” is the seafloor, and the “bathtub” is the ocean.

How Tsunamis Travel Across Oceans

In deep open water, a tsunami travels at roughly 500 to 600 mph — approximately the cruising speed of a commercial jetliner. At that speed, a tsunami generated off the Alaskan coast reaches Hawaii in approximately 5 hours and the US West Coast in 4 to 6 hours.

The wave height in open ocean is deceptive — typically under 3 feet, with a wavelength (crest to crest) of 60 to 300 miles. Ships at sea may not feel a tsunami at all. The danger is entirely at the coast.

Coastal Amplification (Shoaling)

As a tsunami enters shallow water — typically under 600 feet depth — its speed drops dramatically, from 500 mph to 20 to 45 mph. But the energy does not dissipate. It compresses vertically. The wave front “piles up,” and wave height increases in a process called shoaling.

This is why a wave under 3 feet in open ocean can become a 30-foot or 100-foot wall of water at the coast. Flat, low-lying coastlines concentrate wave energy and allow deep inland penetration. Steep coastal terrain limits horizontal reach but can still see wave heights exceeding cliff faces in localized areas.

What Causes a Tsunami?

Submarine Earthquakes (Most Common)

Approximately 80 percent of all tsunamis are generated by underwater earthquakes along subduction zones — areas where one tectonic plate slides beneath another. When the subducting plate snags and then releases, it can displace the overlying seafloor vertically across hundreds of miles in an instant.

Key factors that determine whether an earthquake generates a dangerous tsunami:

Magnitude: Earthquakes below magnitude 7.0 rarely generate significant tsunamis. Magnitude 7.5 and above with vertical seafloor displacement is the real threshold.
Focal depth: Shallow earthquakes — under 30 miles depth — displace more seafloor and generate larger tsunamis than deep earthquakes of the same magnitude.
Fault type: Thrust faults with vertical movement are the primary tsunami generator. Strike-slip faults with horizontal movement rarely produce dangerous tsunamis.

The Cascadia Subduction Zone off the Pacific Northwest coast is the most significant US tsunami threat. Geological evidence — preserved in sediment layers, “ghost forests” of drowned trees, and oral histories of coastal Indigenous peoples — shows it has produced magnitude 9.0 and above earthquakes with catastrophic tsunamis approximately every 200 to 500 years. The last rupture was January 26, 1700. The USGS considers another major rupture overdue.

Submarine Landslides

Underwater landslides on the continental shelf can displace water and generate localized but extremely intense tsunamis. The 1958 Lituya Bay megatsunami in Alaska — caused by a rockslide into the bay following an earthquake — stripped vegetation from hillsides to a height of 1,720 feet. It is the tallest recorded tsunami run-up in modern history.

Submarine landslides can also occur without an accompanying earthquake, triggered by slope instability or sediment failure — meaning they may arrive with no seismic warning at all.

Volcanic Eruptions

Volcanic activity can generate tsunamis through caldera collapse, explosive eruptions that displace ocean water, and volcanic flank collapses. The 2022 Hunga Tonga eruption generated a tsunami felt globally — pressure waves crossed every ocean. The 1883 Krakatoa eruption generated waves over 100 feet high that killed approximately 36,000 people.

Volcanic tsunamis are less predictable than earthquake-generated ones. Warning times can be even shorter.

Warning Signs of an Incoming Tsunami

Natural Warning Signs

When you are near the coast, these physical signals may precede a tsunami and may arrive before any official alert activates:

Strong or prolonged earthquake near the coast Any earthquake you can feel while standing near the coast — especially one lasting 20 seconds or more — should be treated as a potential tsunami trigger. You do not need to know the magnitude. You do not need to wait for a report. If you feel it, start moving toward high ground immediately. This is your primary warning in a Cascadia-type event.

Rapid recession of the ocean One of the most documented pre-tsunami phenomena is a sudden, dramatic withdrawal of the ocean — exposing seafloor, stranding fish, revealing rocks normally well below the tide line. This occurs because the trough of the incoming wave arrives at the coast before the crest. If you see the ocean pulling back rapidly and unusually far, you have seconds to minutes before the first wave arrives. Do not approach the waterline to watch or collect stranded fish. Run.

Roaring sound from the direction of the ocean Survivors frequently describe hearing a loud roaring or hissing sound — like a freight train, jet engine, or continuous waterfall — coming from the ocean before a tsunami arrives. This is the sound of the approaching wave. It may be the last warning you receive.

The Critical Limitation of Official Warnings

Official alerts require time to issue — typically 5 to 15 minutes minimum from when an earthquake is detected to when a warning is broadcast. For a Cascadia Subduction Zone rupture, the tsunami arrives at coastal Washington and Oregon beaches in 15 to 30 minutes. That leaves essentially no margin. Natural warning signs are your primary alert in this scenario.

The Tsunami Warning System

NOAA operates two tsunami warning centers in the United States:

National Tsunami Warning Center (NTWC) — Based in Palmer, Alaska. Covers the US West Coast, Alaska, British Columbia, and all contiguous US coastlines. Issues warnings for tsunamis that could affect any US Pacific or Atlantic coast.

Pacific Tsunami Warning Center (PTWC) — Based in Ewa Beach, Hawaii. Serves Hawaii and the international Pacific Ocean region, including US territories in the Pacific.

Warning levels issued by these centers:

Tsunami Warning: Destructive waves expected. Immediate evacuation of the coast is required.
Tsunami Advisory: Hazardous currents and strong wave action expected near shore, but not necessarily large destructive waves. Stay away from the shoreline.
Tsunami Watch: Conditions exist that could produce a tsunami; threat is being evaluated. Be ready to evacuate.
Tsunami Information Statement: An earthquake has occurred; tsunami potential is under assessment. Monitor updates.

These alerts reach the public via NOAA Weather Radio, the Emergency Alert System on TV and radio, Wireless Emergency Alerts (WEA) sent to cell phones in the affected area, and local coastal siren systems installed in high-risk communities.

NOAA’s tsunami.gov provides real-time warnings, watches, and regional resources updated around the clock.

Tsunami Zones: Which US Coasts Are at Highest Risk

Tsunami risk in the United States is not uniformly distributed. NOAA and state emergency management agencies have mapped inundation zones for all high-risk coastal areas.

Region	Risk Level	Primary Threat
Alaska	Very High	Alaska-Aleutian Subduction Zone
Hawaii	Very High	Pacific-wide tsunamis, local volcanic activity
Pacific Northwest (WA, OR, N. CA)	Very High — catastrophic risk	Cascadia Subduction Zone
Northern California	High	Cascadia + offshore fault systems
Southern California	Moderate	Remote Pacific sources + local faults
Gulf Coast (TX, LA, MS, FL)	Low to Moderate	Atlantic earthquake sources, landslides
Puerto Rico / US Virgin Islands	Moderate to High	Caribbean seismic zone
East Coast	Low	Atlantic margin landslides, remote sources

The Cascadia worst-case scenario: A full-margin Cascadia Subduction Zone rupture would generate a magnitude 9.0 to 9.2 earthquake felt throughout the Pacific Northwest, followed by a tsunami with wave heights of 50 to 100 feet striking low-lying coastal communities within 15 to 30 minutes. FEMA’s Cascadia Rising exercise (2016) projected over 13,000 fatalities, 27,000 injuries, and more than 1 million displaced persons across Washington, Oregon, and California.

To find your specific zone, search “[your state] tsunami inundation map” — Oregon, Washington, California, Alaska, and Hawaii all publish detailed inundation maps by community.

How Long After an Earthquake Before a Tsunami Hits?

The answer depends entirely on where the earthquake originates relative to your location.

Local source (earthquake offshore, same coast): 15 to 30 minutes. This is the Cascadia scenario for the Pacific Northwest coast. Some areas may have under 15 minutes. Natural warning signs — the shaking itself — are your only reliable alert.

Regional source (earthquake in same ocean basin, hundreds to thousands of miles away): 1 to 4 hours. The 1964 Alaska Good Friday earthquake sent waves into Crescent City, California roughly 3.5 hours later.

Distant source (trans-Pacific, opposite side of the ocean): 5 to 24 hours. A tsunami generated off Japan reaches Hawaii in approximately 8 hours and the US West Coast in 10 to 12 hours. Official warning systems can be effective in this scenario.

The general rule: the farther the source, the more warning time you have. For local tsunamis, assume you have under 20 minutes and act on natural signs immediately.

Tsunami Evacuation Planning

Know Your Zone Before You Need It

Every person living or regularly spending time near a tsunami-risk coast should know three things:

Whether they are in a designated tsunami inundation zone — available from state emergency management agencies and NOAA.
The designated tsunami evacuation routes for their area — marked with blue-and-white tsunami hazard zone signs along coastal roads.
The nearest high ground — the shortest route from their home, workplace, hotel, or beach access point to ground above 50 feet in elevation or clearly outside the inundation zone.

Find your zone now, not the day before a trip.

How High Is High Enough?

The minimum elevation target for evacuation is 50 feet above sea level, but this is a floor, not a ceiling. For areas near the Cascadia Subduction Zone, USGS modeling suggests local wave heights of 50 to 100 feet in exposed coastal communities. If your high ground destination is at 50 feet, continue moving inland and uphill as conditions allow.

Look for terrain features: hillsides, ridges, and elevated parks outside the mapped inundation zone. Many Pacific Northwest communities have designated vertical evacuation structures — tall reinforced concrete buildings engineered to withstand both the earthquake and the wave — marked on local hazard maps.

How Fast Do You Need to Move?

Walking speed is approximately 3 to 4 miles per hour. Running is 6 to 8 mph or more. For a local Cascadia-type tsunami, you have roughly 15 to 30 minutes to reach high ground.

That means:

At a brisk walk, you can cover 0.75 to 1.5 miles
At a jog, you can cover 1.5 to 2 miles

If your nearest high ground is more than a mile away and you are on foot, you need to identify closer options — including vertical evacuation structures — or account for this in your planning.

Do not plan to drive as your only option. A magnitude 9.0 earthquake will damage roads, collapse bridges, and create gridlock within minutes. Many tsunami evacuation routes in coastal communities are designated specifically because they are walkable to high ground.

Building Your Tsunami Go-Bag

A tsunami scenario requires your go-bag to be ready for immediate departure — within 30 seconds of the shaking stopping. Key additions beyond a standard 72-hour kit:

Sturdy footwear at the door — The earthquake precedes the tsunami. There will be broken glass and debris. Shoes you can put on in five seconds, staged at the exit, are not optional.
NOAA Weather Radio — A hand-crank or battery-powered unit tuned to your local station with SAME codes configured for your county. Receive official all-clears and wave arrival updates.
Whistle — Three blasts is the universal distress signal. If you are caught in debris or water, a whistle carries farther and lasts longer than your voice.
Waterproof document pouch — IDs, insurance cards, prescriptions, and emergency contacts in a sealed, waterproof bag. Post-tsunami recovery involves insurance claims and identity verification.
Cash — ATMs and card readers will be offline for days after a major event.

For the full kit framework, see our 72-hour emergency kit guide.

Walk Your Routes Before You Need Them

Identify at least two evacuation routes on foot from your home, your workplace, and any beach or coastal area you visit regularly. Walk them. Note which routes run inland versus parallel to the coast — avoid any route that keeps you adjacent to the shoreline.

Bring your family. Children who have physically walked the route once are far more likely to navigate it correctly under stress than those who have only been told about it.

What to Do During a Tsunami

If you feel a strong earthquake near the coast: Do not wait for a siren. Do not wait for an official warning. Do not wait to see if the ocean recedes. Move immediately toward your pre-identified high ground. The earthquake is your warning.

If you are driving: Drive inland and uphill via designated evacuation routes. If roads are blocked or impassable, abandon the vehicle and move on foot. Do not stop to watch the ocean from a coastal overlook.

If you are in a building and cannot evacuate: Move to the highest floor of the tallest available reinforced concrete structure. Upper floors of a well-built multi-story building may provide survival odds when outside evacuation is impossible — but this is a last resort, not a primary plan.

If you are caught in the water: Do not try to swim against the current. Grab the largest, most buoyant object available and stay on top of it. Debris rides the surface; currents below are unpredictable and you cannot outswim a tsunami surge. Conserve energy. Signal for rescue once the water stabilizes.

Do not:

Stop to gather belongings
Watch the ocean from shore as the waves arrive
Drive through flooded inundation zones once water has begun rising
Return to the coast because the first wave appears to have passed

What to Do After a Tsunami

Multiple Waves

A tsunami is a series of waves, not a single event. The second or third wave is often larger than the first. The gap between waves can range from 5 minutes to over an hour — long enough for survivors to begin returning to shore, where they are struck by the next wave.

Do not return to the coast, the inundation zone, or low-lying areas until you receive an official all-clear from NOAA or local emergency management. Historical data shows secondary-wave casualties are significant and preventable.

Contaminated Water

Tsunami floodwater is not clean water. It carries sewage, fuel, industrial chemicals, heavy debris, and pathogens. Assume all floodwater is contaminated. Wear rubber boots and waterproof gloves during any cleanup work. Assume tap water is unsafe until your local water utility confirms otherwise.

Buildings that have been inundated may have compromised structural integrity. Mold development begins within 24 to 48 hours in water-saturated walls.

Infrastructure Damage

A major local earthquake preceding the tsunami will have damaged roads, bridges, water mains, gas lines, and power infrastructure simultaneously. Treat the post-tsunami environment as a combined earthquake-plus-flood disaster. Do not assume any road is intact, any utility is functional, or any structure is safe to enter without inspection.

Bringing It Together: Your Tsunami Readiness Checklist

Understanding the threat is the starting point. For a broader look at how tsunamis fit into the full spectrum of natural disaster readiness — including hurricanes, floods, and wildfires — see our natural disaster preparedness guide. And if you are weighing whether to evacuate versus shelter in your home in a coastal emergency, our bug out vs shelter in place guide works through the decision framework in detail.

For a complete review of what belongs in any emergency kit — coastal or inland — start with our earthquake preparedness guide to understand how to build your physical readiness layer before the next event.

Sources: NOAA National Tsunami Warning Center (tsunami.gov); USGS Earthquake Hazards Program; FEMA Cascadia Rising Exercise After-Action Report (2016); Pacific Northwest Seismic Network (PNSN); Oregon Office of Emergency Management tsunami inundation mapping program; Journal of Geophysical Research — 2004 Sumatra-Andaman and 2011 Tohoku event analyses.

Frequently Asked Questions

What causes a tsunami?

Most tsunamis — roughly 80 percent — are caused by underwater earthquakes along subduction zones where one tectonic plate slides beneath another. When the seafloor shifts vertically, it displaces the entire water column above it. Submarine landslides and volcanic eruptions can also trigger tsunamis, sometimes with even less warning time than earthquake-generated waves.

How do you know a tsunami is coming?

Three natural warning signs precede a tsunami: a strong or prolonged earthquake felt while near the coast, a sudden rapid recession of the ocean exposing normally submerged seafloor, and a loud roaring or jet-engine sound coming from the direction of the ocean. Any one of these signals warrants immediate movement to high ground — do not wait for an official alert, especially on the Pacific Northwest coast where a Cascadia quake could send waves ashore in under 20 minutes.