Tsunami Warning Signs: What to Watch For After an Earthquake
Tsunamis are among the most destructive forces on Earth. Generated primarily by large undersea earthquakes, these massive waves can cross entire oceans and devastate coastal communities thousands of kilometers from the source. Understanding warning signs and response procedures is critical for anyone living near or visiting the coast.
How Tsunamis Form
Most tsunamis are caused by large earthquakes (M7.5+) that occur on reverse or thrust faults beneath the ocean floor. When the fault ruptures, it displaces a massive volume of water vertically. This displacement creates waves that radiate outward in all directions from the epicenter.
In the deep ocean, tsunami waves are only about 30-60 cm tall but travel at 700-800 km/h (the speed of a jet aircraft). As they approach shallow coastal waters, they slow down dramatically but grow in height. A wave that was barely noticeable in open water can become 10-30 meters tall when it reaches shore.
Not all large earthquakes produce tsunamis. The key factor is vertical displacement of the seafloor. Strike-slip earthquakes (where plates move horizontally past each other) rarely generate significant tsunamis because they do not displace water vertically.
Natural Warning Signs
Several natural indicators can warn of an approaching tsunami, sometimes hours before official alerts:
- Strong or long earthquake shaking: If you feel a powerful earthquake near the coast, assume a tsunami may follow. The longer and stronger the shaking, the greater the risk.
- Rapid water recession: If the ocean water suddenly pulls back far beyond normal low tide, exposing the seafloor, a tsunami is likely imminent. This happens because the wave trough arrives before the crest.
- Loud roaring from the ocean: Many survivors report hearing a sound like a freight train or jet engine coming from the sea before the wave arrives.
- Unusual ocean behavior: Water rising rapidly, unusual currents, or changes in water color can indicate an approaching tsunami.
Official Warning Systems
The Pacific Tsunami Warning Center (PTWC) in Hawaii and the National Tsunami Warning Center (NTWC) in Alaska monitor seismic activity around the clock. When a large earthquake occurs beneath the ocean, they analyze seismic data to determine if a tsunami was generated.
DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys measure pressure changes on the ocean floor, confirming whether a tsunami exists and measuring its actual size. This network, combined with seismic data, allows forecasters to predict arrival times and wave heights for specific coastlines.
Warning levels include: Advisory (strong currents likely, stay off the beach), Watch (tsunami possible, monitor situation), Warning (dangerous tsunami expected, evacuate coastal zones).
What to Do
If you are near the coast and feel a strong earthquake, do not wait for an official warning. Move inland and uphill immediately. Go at least 1.5 km inland or 30 meters above sea level. If you cannot reach high ground, go to the upper floors of a reinforced concrete building (3+ stories).
Never go to the beach to watch a tsunami arrive. If you can see the wave, you are too close to escape it. Tsunami waves come in series, often with 15-60 minutes between waves. The first wave is often not the largest. Stay away from the coast until authorities give the all-clear.
Track seismic events that could trigger tsunamis on our significant earthquakes page.