The term “tidal wave” is commonly used to describe immense, destructive ocean waves, but this phrase is misleading and inaccurate within the scientific community. It incorrectly links catastrophic events to the predictable, daily processes of Earth’s tides. This article clarifies the true nature of waves related to astronomical tides and explains the distinct mechanics of tsunamis.
Understanding Waves Driven by Astronomical Tides
Astronomical tides are the rhythmic, long-period changes in sea level caused by the gravitational forces exerted by the Moon and, to a lesser extent, the Sun on the Earth’s oceans. The Moon’s gravitational pull creates two bulges of water on Earth: one on the side closest to it and one on the opposite side due to inertia. As the Earth rotates, coastal areas move through these bulges, experiencing the daily cycle of high and low tides.
This movement of the water column is effectively a wave with an extremely long wavelength, potentially spanning thousands of miles across an ocean basin. Because of this vast length and low amplitude in the open ocean, the wave is essentially imperceptible to a mariner at sea. Its effects are observed as a slow, predictable rise and fall of the coastline over a cycle of approximately 12 hours and 25 minutes.
The closest phenomenon to a true, visible “tidal wave” is a tidal bore, which occurs in relatively few locations worldwide. A tidal bore forms when the incoming high tide is funneled from a broad bay into a shallow, narrowing river channel or estuary. This constriction causes the leading edge of the tide to form a turbulent, wall-like wave that travels against the river’s current.
How Tsunamis Form and Behave
Tsunamis, which translates from Japanese as “harbor wave,” are a series of ocean waves caused by the sudden, large-scale displacement of water. These events are geological in origin, most commonly triggered by major underwater earthquakes in subduction zones where one tectonic plate slides beneath another. The sudden vertical motion of the seafloor lifts or drops the entire water column above it, generating a wave that propagates outward.
Unlike the surface-level waves generated by wind, a tsunami involves the movement of water from the surface to the ocean floor, acting as a shallow-water wave even in the deep ocean due to its enormous wavelength. In the deep ocean, where water depths can exceed 4,000 meters, a tsunami can travel at speeds comparable to a jet airliner, often exceeding 500 miles per hour. Despite this high speed, the wave’s height is typically less than one meter in the open ocean, making it virtually undetectable by ships.
As the tsunami approaches the coast and moves into shallower water, a process known as shoaling occurs. Friction with the seafloor causes the wave’s speed to decrease dramatically, slowing down to approximately 20 to 30 miles per hour near the shore. To conserve energy, this reduction in speed is accompanied by a decrease in wavelength and a corresponding increase in wave height. This transformation causes the tsunami to pile up into a devastating wall of water, reaching heights of many meters upon inundating the coastline.
Defining the Key Distinctions
The fundamental differences between tsunamis and astronomical tides lie in their origin, period, and speed. A tsunami originates from a sudden, high-energy geological event, such as an underwater earthquake or landslide, that violently displaces water. Conversely, astronomical tides result from the constant, predictable gravitational pull exerted on Earth by the Moon and Sun.
The wave period, the time interval between successive wave crests, is a second distinction. Tsunamis have a period ranging from minutes to a couple of hours, meaning the destructive force arrives as a series of waves over an extended time. Astronomical tides have a period of approximately 12 to 24 hours, reflecting the Earth’s rotation relative to the Moon and Sun.
Speed and resulting impact also differ significantly, especially near the coast. The tidal wave’s movement is experienced as a slow, gradual rise and fall of water over many hours. In contrast, a tsunami approaches the shore at a speed faster than a person can run, leading to catastrophic, rapid inundation.
Hazard Levels and Coastal Preparedness
The predictability of astronomical tides makes their associated hazard manageable and routine. Tide tables, which are calculated years in advance based on known astronomical cycles, allow coastal communities to plan for high water levels and tidal currents. Extreme high tides, sometimes coupled with storm surge, result in predictable, though sometimes damaging, coastal flooding.
Tsunami hazards are catastrophic and require specialized, real-time warning systems due to their unpredictable nature. The Deep-ocean Assessment and Reporting of Tsunamis (DART) network, operated by the National Oceanic and Atmospheric Administration (NOAA), is the primary detection system. DART buoys use seafloor pressure recorders to detect the minuscule pressure changes caused by a tsunami passing overhead.
When a potential tsunami is detected by the DART network or seismic monitoring, Tsunami Warning Centers issue immediate alerts. This system provides coastal populations with a crucial time window for evacuation before the fast-moving wave reaches the shore. Coastal preparedness relies on these real-time observations to evaluate the threat, contrasting sharply with the long-term planning afforded by astronomical tide cycles.