A tsunami is a powerful series of ocean waves generated by the sudden displacement of a large volume of water. This disturbance is most often caused by a powerful seafloor earthquake, but it can also result from underwater landslides or volcanic eruptions. When a tsunami approaches the shore, the ocean surface does not always rise first. Instead, a counterintuitive phenomenon known as drawdown occurs, where the sea level drops rapidly and the water recedes far from the coastline. This unexpected retreat is a natural, urgent warning sign that the main wall of water is rapidly approaching.
The Generation and Physics of a Tsunami Wave
Tsunamis are fundamentally different from typical wind-generated waves because they involve the movement of the entire water column, from the ocean floor to the surface. This massive scale allows the wave to carry immense energy across vast distances with minimal dissipation. The sudden vertical shift of the seafloor, often during a subduction zone earthquake, displaces this water column, creating a series of waves with extremely long wavelengths. In the deep ocean, where depths average 4,000 meters, a tsunami can travel up to 700 kilometers per hour, comparable to a jet airliner, yet the wave height is generally less than one meter and nearly imperceptible to ships.
The Preceding Trough and Water Drawdown
Water recession is rooted in the geometry of the tsunami wave train, which is a succession of peaks (crests) and valleys (troughs). If the seafloor movement that generates the tsunami is a drop or subsidence, the initial wave radiating outward will be a trough, representing the lowest part of the wave cycle. As this leading trough approaches shallow coastal water, it slows down significantly due to friction with the seabed. The water is effectively pulled seaward to fill this depression, acting like a massive, prolonged low tide. This process causes coastal water to be sucked away from the shore, exposing normally submerged areas of the seabed.
Observable Characteristics of the Recession
The drawdown is an unmistakable visual cue of danger, manifesting as a rapid, unsettling drop in sea level. The distance the water recedes can be dramatic, sometimes pulling back hundreds of meters and exposing large stretches of the ocean floor, reefs, and tidal pools. The time between the maximum recession and the arrival of the first crest varies widely, depending on the distance of the earthquake source and local seabed topography. For distant tsunamis, the water may stay receded for several minutes or even an hour before the wave arrives. A local tsunami, however, may offer only a few minutes of warning after the recession is noticed.
Immediate Action When the Water Retreats
The sight of the ocean receding is a definitive natural warning sign that must be heeded immediately without waiting for an official alert. The exposed seabed is a dangerous indicator that a catastrophic wave is imminent, not an invitation to explore. Anyone who witnesses the water receding must immediately evacuate the area and move to higher ground or well inland. The safest course of action is to seek ground at least 30 meters (100 feet) above sea level or move at least three kilometers (two miles) inland, if terrain allows. Since a tsunami is a series of waves, the danger remains until authorities confirm that the threat has fully passed, as subsequent waves can arrive minutes or hours later.