Is the Tsunami Warning Over? How Tsunami Alerts Are Cancelled

Direct answer: a tsunami warning is over when the warning centre that issued it cancels it — and it does that only after sea-level measurements confirm that a destructive tsunami will not reach the area, or that the waves have dropped to a level where no further damage is expected. It does not end when the first wave has passed, and it does not end on a timer. Even after a cancellation, stay out of the evacuation zone until your local officials say it is safe to return.

Four kinds of tsunami message, and what each asks of you

The US tsunami warning centres, whose messages also cover much of the Pacific and Caribbean, issue four levels. Their definitions, with what each asks of you:

• Tsunami Warning — a tsunami that could cause widespread flooding inland is imminent, expected or occurring. Move to high ground or inland if you are in a hazard zone.
• Tsunami Advisory — a tsunami could produce strong currents or dangerous waves for people in or very near the water. Stay out of the water and away from beaches and waterways.
• Tsunami Watch — a tsunami may later affect the area. Stay alert for more information.
• Tsunami Information Statement — an earthquake or tsunami has happened that people may want to know about. Most of these say there is no threat.

The first two are the ones that ask you to act. A watch can turn into a warning, or be cancelled. An information statement usually ends the story before it starts.

How a tsunami warning is cancelled

A tsunami is measured, not only predicted. After an undersea earthquake the warning centre issues its first message from the earthquake alone — its size, depth and location — because waiting for the water would waste the minutes that matter. It then watches tide gauges and deep-ocean sensors as the waves cross them.

The warning is cancelled when that water-level data shows one of two things: a destructive tsunami will not reach the area after all, or the waves have diminished to where additional damage is not expected. Sometimes that takes less than an hour. For a large tsunami crossing an ocean, it can take many hours, because the waves reach different coasts at different times.

Why the first wave is not the end

A tsunami is a series of waves, not one. The US National Weather Service puts it plainly: the first wave may not be the last or the largest, and dangerous flooding and strong currents can continue for several hours or even days after the first arrival. Three things follow:

1. A calm sea after the first wave is not a sign it is over. The gap between waves can be long enough to tempt people back to the shore.
2. Strong currents can outlast the flooding. Harbours and river mouths can stay dangerous after the water on land has gone.
3. The all-clear to return is local. Even once the warning centre cancels its message, stay out of the hazard zone until local officials tell you it is safe.

“Is there a tsunami warning right now?” — how to check

• Your national warning service first. Many countries run their own tsunami warnings, and for their own coasts theirs is the one that counts.
• The US centres’ own site, tsunami.gov, for the US, Canada and the international Pacific and Caribbean messages.
• Official phone alerts. A warning for your area is what emergency broadcasts and cell alerts exist for.
• If you feel strong shaking at the coast, or see the sea suddenly pull back, do not wait for any message. Move to high ground.

A message about a different coast is common and easy to misread: a warning for one island group can appear beside an information statement saying another has no threat. Read which area the message names.

How this site shows tsunami alerts

Our tsunami warnings page reads the latest bulletins from the two US warning centres, the National Tsunami Warning Center in Alaska and the Pacific Tsunami Warning Center in Hawaii. An information statement saying there is no threat is not listed — so on most days that page is empty, and that is correct. When a centre cancels a warning, we close it rather than leave it showing as active. This site does not issue warnings; the centres and your local officials do.

Related reading: how tsunami warning systems work, how tsunamis form, how any warning ends and the tsunami safety guide.

Frequently asked questions

Is the tsunami warning over?

Only when the warning centre that issued it cancels it. It does that after sea-level data confirms a destructive tsunami will not reach the area, or that the waves have diminished. Even then, stay out of the evacuation zone until local officials say it is safe.

How long does a tsunami warning last?

There is no fixed length. A warning lasts until water-level data shows the threat has passed, which can be under an hour or many hours for a tsunami crossing an ocean. Dangerous currents can continue for hours or days after the first wave.

What is the difference between a tsunami warning and a tsunami advisory?

A warning means a tsunami that could flood inland is imminent, expected or occurring - move to high ground. An advisory means dangerous currents or waves for people in or near the water - stay out of the water and away from beaches.

Is the first tsunami wave the biggest?

Not necessarily. A tsunami is a series of waves, and the first may not be the last or the largest. A calm sea after the first wave is not a sign that it is over.

Sources

• NOAA/NWS Tsunami Warning Centers - Tsunami message definitions
• US National Weather Service - Tsunami safety: during a tsunami

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