What is the difference between a tsunami and a tidal wave?

Tidal waves and tsunamis – the two most powerful types of waves on Earth – are often confused in popular discourse. Although these terms are sometimes used synonymously, tidal waves and tsunamis actually have different causes.

“The English term tidal wave prevailed until the 2004 Indian Ocean tsunami, in part because most tsunami observations to date described water phenomena resembling rapidly rising or rapidly retreating tides,” he added. Costas Synolakisdirector of the Tsunami Research Center at the University of Southern California, told Live Science. “In 2004, we accessed several videos of tsunamis in Indonesia and Thailand and realized that giant tsunamis were nothing like tidal waves.”

Tidal waves are created by the gravitational interaction between the Earth and the Moon – and, to a lesser extent, the Sun. These waves are created by tidal patterns that cause the tides to ebb and flow daily in coastal areas, which means they are generally predictable and linked to the phases of the moon.

Tides are highest during new moons – when the moon is between the Earth and the sun; and full moon, when the Earth is between the moon and the sun.

The moon’s gravity exerts a greater force on the areas of Earth closest to the moon, which pulls water there, causing the ocean to rise in a bulge. Meanwhile, the oceans on the side of Earth opposite the Moon are also experiencing a bulge due to inertia – the natural tendency of a moving object to keep moving or a stationary object to remain still. As water moves away from the Moon, it resists gravitational forces that try to pull it in the opposite direction.

These two bulges move around the Earth as our planet rotates and the Moon orbits us, which means that in most regions the tide occurs twice every 24 hours and 50 minutes. Meanwhile, outflows occur in areas that are neither closest nor farthest from the Moon.

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A tidal wave can extend for thousands of miles. In many cases, tidal waves are small. However, certain geographic features, such as narrow bays and estuaries, can concentrate tidal energy, creating huge waves in some areas.

However, tidal waves are no match for the destructive power of a tsunami – which means “harbour wave” in Japanese. Unlike tidal waves, tsunamis are largely unpredictable. They are the result of underwater earthquakes, landslides, volcanoes and even meteorites.

(Image credit: ttsz/Getty Images)

Underwater earthquakes in subduction zones, where one continental plate slides under another, often produce large tsunamis. Earthquakes of magnitude 6.5 or greater that occur at relatively shallow depths and lift the Earth’s crust are likely to cause tsunamis. In addition, volcanoes and landslides – both underwater and on land adjacent to the ocean – move large amounts of magma and rock that can trigger tsunamis. Synolakis said these events could be missed by early detection systems.

The force created by such events displaces the water, and the energy resulting from this displacement propagates in the form of a wave. A tsunami may be local, regional or distant, depending on its size. Tsunamis can result from events that occur near where the wave hits the coastline, but they can also occur thousands of miles away.

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A tsunami may be barely visible, raising the ocean surface by just a few centimeters. But they can travel at a speed of approx 500 miles per hour (800 kilometers per hour). Periods range from a few minutes to two hours. While shallower depths near coastlines slow waves, they increase height as waves following the initial wave front catch up, increasing the strength behind it. This phenomenon is responsible for the massive walls of water that can form when a tsunami makes landfall.

Because tsunamis are largely unpredictable, residents in sensitive coastal areas may have only a few minutes’ warning to get to higher ground. Some of the largest tsunamis created waves that flooded areas several miles inland. Following the devastating 2004 Indian Ocean tsunami that caused: almost 230 thousand deaths, sensors were installed in at-risk regions to create an early warning system.

“Tsunamis are monitored through the Deep-ocean Assessment and Reporting of Tsunamis (DART) system. It is a network of coastal buoys that transmit signals from pressure recorders on the ocean floor to the ocean surface and then to satellites, which in turn connect this signal to warning centers,” Synolakis said. But the system is far from perfect.

“The problem is that there are currently about 50 DARTs covering the Pacific and Indian Oceans. About half the work at any given time. We need at least 150 DARTs deployed across the world’s oceans to create an effective system with targeted warnings,” he said.