A large-scale earthquake lasting for more than twenty seconds can be the precursor for a tsunami. When a region is hit by an earthquake of large magnitude, you should be aware that coastal regions located near the earthquake’s epicenter could be hit by a tsunami.
A more immediate warning sign of a tsunami is the rapid and unexpected decline of water level, below what is normal during low tie. This can happen for a few minutes before the coast is hit by a tsunami and is the only sign along coasts that are located too far away from the epicenter for the earthquake to be felt.
A tsunami can just as often occur without any warning signs.
The Seismological Society of America monitors these events at any time of day. When a tsunami does occur, they try to give the earliest possible warning to the media in regions that are likely to be affected.
The Communication and Traffic Services of the U.S. Coast Guard will also release maritime warnings in case of a tsunami.
When you become aware of a tsunami warning, seek higher ground if possible.