Tsunamis: Causes, Warning Signs, and Safety

A tsunami is a series of ocean waves generated by a sudden, large displacement of water — most often caused by an underwater earthquake. Tsunamis can travel across entire ocean basins at the speed of a commercial aircraft and inundate coastlines with walls of water tens of metres high.

How Earthquakes Generate Tsunamis

Not every earthquake generates a tsunami. The conditions required are:

• A large magnitude earthquake (generally M7.0 or above)
• Located under or near the ocean
• A shallow hypocenter (usually less than 70 km depth)
• Vertical displacement of the seafloor — thrusting or faulting that moves water up or down

When a section of the seafloor suddenly moves upward (as in a megathrust subduction earthquake), it displaces a massive column of water, generating waves that radiate outward in all directions. These waves are barely noticeable in the open ocean — often less than a metre high — but as they approach shallow coastal water, they slow down and their energy compresses into towering waves.

How Fast Does a Tsunami Travel?

In deep water, tsunami waves travel at up to 800 km/h — as fast as a commercial jet. They can cross the Pacific Ocean in under 24 hours. This speed is what enables warning systems to alert distant coastlines before a tsunami arrives.

Tsunami Warning Systems

The main international tsunami warning centres are:

• Pacific Tsunami Warning Center (PTWC) — based in Hawaii, covers the Pacific and Indian Oceans
• National Tsunami Warning Center (NTWC) — covers the US and Canadian Pacific and Atlantic coasts
• Japan Meteorological Agency — issues warnings for Japan within 3 minutes of a large earthquake

Warning systems use a network of seismographs to detect earthquakes and DART buoys (Deep-ocean Assessment and Reporting of Tsunamis) to confirm whether a tsunami has actually formed.

Natural Warning Signs

If you are on a coast and experience any of the following, move immediately to high ground:

• Strong or prolonged earthquake shaking (strong enough to make standing difficult)
• The sea suddenly recedes dramatically, exposing the seafloor
• A loud roaring sound coming from the ocean

Do not wait for an official warning. If you feel the earthquake, that IS your warning. Move inland and upward immediately.

What to Do in a Tsunami

1. Move immediately to high ground or inland — aim for at least 30 metres elevation if possible
2. Walk quickly; do not stop to gather belongings
3. Stay away from the coast until authorities declare it safe — subsequent waves can arrive hours after the first
4. Do not go to the beach to watch — the first wave is not always the largest
5. Follow official evacuation routes if available