Imagine a wave so powerful it can level entire coastal cities and travel across oceans at the speed of a jet plane. This isn't science fiction — it's the awe-inspiring and terrifying reality of tsunamis, one of nature’s most destructive forces.

What are tsunamis?

Tsunamis, often mistakenly called “tidal waves,” are series of ocean waves triggered by the sudden displacement of large volumes of water. Unlike wind-generated waves that only affect the ocean’s surface, tsunamis involve water movement from the surface all the way to the ocean floor.

The name “tsunami” comes from Japanese and translates literally to “harbor wave.” Although its original meaning isn’t entirely descriptive, it’s the term we’ve adopted for these phenomena.

When a tsunami is generated, it can travel at astonishing speeds in the open ocean — up to 500 mph (800 km/h), comparable to a jet airplane. However, in deep water, tsunami waves may be barely noticeable, often only a foot or two high (30–60 cm). It’s only as they approach shallower coastal waters that tsunamis slow down and grow in height, potentially reaching over 100 feet (30 meters) tall.

The destructive potential of tsunamis is staggering. Between 1998 and 2017, tsunamis caused more than 250,000 deaths globally, including over 227,000 deaths in the 2004 Indian Ocean tsunami alone. The 2011 Tohoku tsunami in Japan reached heights of up to 133 feet (40.5 meters) and penetrated up to 6 miles (10 km) inland, causing widespread destruction and triggering a major nuclear disaster.

What causes tsunamis?

The most common cause of tsunamis is underwater earthquakes. When tectonic plates suddenly shift along a fault line beneath the ocean, they can displace enormous amounts of water, setting a tsunami in motion. However, tsunamis can also be triggered by:

• Underwater landslides
• Onshore landslides in which large volumes of debris fall into the water
• Volcanic eruptions
• Meteorite impacts (rare but possible)

How does a tsunami wave work?

The mechanics of a tsunami wave are fascinating, involving complex physical processes that transform a barely noticeable disturbance in the open ocean into a potentially devastating force at the coastline. Let’s explore the journey of a tsunami from its origin to its coastal impact:

1. Activation: When an underwater disturbance occurs, such as an earthquake, landslide, or volcanic eruption, it displaces a large volume of water. This displacement can be caused by vertical movement of the ocean floor, often less than three feet (one meter), but covering a vast area. The enormous volume of water displaced creates a series of waves that spread out in all directions from the source.
2. Build: In the open ocean, tsunami waves are deceptively small. However, what makes tsunamis unique is their incredibly long wavelength, which can stretch up to 125 miles (200 km) between wave crests. This long wavelength means that the entire water column, from surface to seafloor, is in motion, even in the deepest parts of the ocean. The long wavelength also contributes to the tsunami’s remarkable speed.
3. Formation: Each wave has a peak and a trough, and sometimes the trough of a tsunami reaches land before the peak. This causes a drawback where the tide seems to recede hundreds of meters further than usual. This drawback lasts for about six minutes before the peak reaches shore.
4. Approach: As the tsunami approaches shallower coastal waters, its behavior changes dramatically. The wave speed decreases due to friction with the seafloor, but the energy of the wave remains largely constant. This causes the wave height to increase significantly, a process known as shoaling.
5. Impact: When a tsunami reaches the shore, it can manifest as a massive wall of water or, deceptively, as a rapid rise in water level — like a river flowing incredibly fast. Tsunamis don’t just crash and recede like regular waves do. They can continue to surge inland for several minutes, carrying debris and causing widespread destruction.

Measuring tsunami strength

Several scales and methods are used to quantify the size and impact of tsunamis:

Tsunami intensity scales

• Sieberg-Ambraseys scale: This is a 6-point scale that describes the physical effects of a tsunami. However, it has been criticized for not being detailed enough for modern use.
• Papadopoulos-Imamura scale: A more recent 12-point scale that aims to provide a more detailed and sensitive measure of tsunami intensity. It’s based on the tsunami’s effects on humans, objects, and buildings, similar to earthquake intensity scales.

Tsunami magnitude scales

• Soloviev-Imamura scale: This scale uses the maximum tsunami wave height to calculate a magnitude value.
• Abe magnitude scale: Developed by Katsuyuki Abe, this scale relates tsunami energy to earthquake magnitude and distance from the source.

Other measurement and description methods

• Inundation and run-up: These measure how far inland and how high above sea level the tsunami waters reach, respectively.
• Flow depth: This refers to the depth of tsunami waters at various onshore locations.
• DART (deep-ocean assessment and reporting of tsunamis) systems: These deep-ocean buoys can detect and measure tsunamis as they pass, providing real-time data for warning systems.

Mitigating tsunami damage

Given the immense destructive potential of tsunamis, early warning systems are crucial. NASA and other agencies are working on improving tsunami detection and warning capabilities. For example, a network of over 1,600 Global Navigation Satellite System (GNSS) ground stations can detect tectonic activity and feed data into models that predict potential tsunami activity.

Other mitigation strategies include building seawalls and floodgates, implementing evacuation plans, and conducting public education programs. Some countries, like Japan, even maintain or plant coastal forests to act as natural barriers against tsunamis.

What makes tsunamis particularly dangerous is their unpredictability. Unlike hurricanes or tornadoes, which can be forecast days in advance, tsunamis can strike with little warning, making rapid and accurate early warning systems vital for saving lives.

Understanding the science behind tsunamis not only satisfies our curiosity about these powerful natural phenomena but also helps us develop better prediction and protection strategies. As we continue to study and respect the awesome power of our oceans, we improve our ability to coexist with these forces of nature.

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