Tsunamis occur when underwater earthquakes, submarine volcanoes, or massive underwater landslides suddenly push up seawater. The most common cause is subduction-zone seafloor earthquakes.
Sudden seafloor uplift pushes all water above it upward. This entire water layer transmits outward as waves while pulled by gravity. Offshore, tsunami heights are about 1 meter but wavelengths exceed 100 km.
Tsunamis travel very fast in deep ocean. Speed is proportional to the square root of water depth, so in 4000m deep ocean they travel at nearly 800 km/h. As they approach shore, shallowing water slows them while increasing height.
Tsunami warning systems consist of seismometers on the seafloor and buoys across the Pacific. When earthquakes are detected, warning systems issue real-time alerts to some coastal areas. However, near epicenters tsunamis may arrive before warnings, making self-judgment evacuation important when feeling earthquakes.