Tsunamis are a series of large ocean waves generated by major earthquakes beneath the ocean floor or major landslides into the ocean. When the waves enter shallow water, they may rise to several feet or, in rare cases, tens of feet, striking the coast with devastating force. People on the beach or in low coastal areas need to be aware that a tsunami could arrive within minutes after a severe earthquake. The tsunami danger period can continue for many hours after a major earthquake. A tsunami can occur during any season of the year and at any time, day or night.
If you are in a tsunami risk area, learn how to protect yourself, your family, and your property. Be familiar with the tsunami warning signs. A strong earthquake lasting 20 seconds or more near the coast may generate a tsunami. A noticeable rapid rise or fall in coastal waters is also a sign that a tsunami is approaching.
The West Coast/Alaska Tsunami Warning Center (WC/ATWC) and The Pacific Tsunami Warning Center (PTWC) may issue the following bulletins:
- WARNING: A tsunami was or may have been generated, which could cause damage; therefore, people in the warned area are strongly advised to evacuate.
- WATCH: A tsunami was or may have been generated, but is at least two hours travel time to the area in watch status. Local officials should prepare for possible evacuation if their area is upgraded to a warning.
- ADVISORY: An earthquake has occurred in the Pacific basin, which might generate a tsunami. WC/ATWC and PTWC will issue hourly bulletins advising of the situation.
- INFORMATION: A message with information about an earthquake that is not expected to generate a tsunami. Usually only one bulletin is issued.