On Jan. 15, 2022, the Hunga Tonga-Hunga Ha’apai volcano in Tonga erupted, sending a tsunami racing across the Pacific Ocean in all directions.

As word of the eruption spread, government agencies on surrounding islands and in places as far away as New Zealand, Japan and even the U.S. West Coast issued tsunami warnings. Only about 12 hours after the initial eruption, tsunami waves a few feet tall hit California shorelines – more than 5,000 miles away from the eruption.

I’m a physical oceanographer who studies waves and turbulent mixing in the ocean. Tsunamis are one of my favorite topics to teach my students because the physics of how they move through oceans is so simple and elegant.

Waves that are a few feet tall hitting a beach in California might not sound like the destructive waves the term calls to mind, nor what you see in footage of tragic tsunamis from the past. But tsunamis are not normal waves, no matter the size. So how are tsunamis different from other ocean waves? What generates them? How do they travel so fast? And why are they so destructive?

Deep displacement

Most waves are generated by wind as it blows over the ocean’s surface, transferring energy to and displacing the water. This process creates the waves you see at the beach every day.

Tsunamis are created by an entirely different mechanism. When an underwater earthquake, volcanic eruption or landslide displaces a large amount of water, that energy has to go somewhere – so it generates a series of waves. Unlike wind-driven waves where the energy is confined to the upper layer of the ocean, the energy in a series of tsunami waves extends throughout the entire depth of the ocean. Additionally, a lot more water is displaced than in a wind-driven wave.

Imagine the difference in the waves that are created if you were to blow on the surface of a swimming pool compared to the waves that are created when someone jumps in with a big cannonball dive. The cannonball dive displaces a lot more water than blowing on the surface, so it creates a much bigger set of waves.

Earthquakes can easily move huge amounts of water and cause dangerous tsunamis. Same with large undersea landslides. In the case of the Tonga tsunami, the massive explosion of the volcano displaced the water. Some scientists are speculating that the eruption also caused an undersea landslide that contributed to the large amount of displaced water. Future research will help confirm whether this is true or not.

Tsunami waves travel fast

No matter the cause of a tsunami, after the water is displaced, waves propagate outward in all directions – similarly to when a stone is thrown into a serene pond.

Because the energy in tsunami waves reaches all the way to the bottom of the ocean, the depth of the sea floor is the primary factor that determines how fast they move. Calculating the speed of a tsunami is actually quite simple. You just multiply the depth of the ocean – 13,000 feet (4,000 meters) on average – by gravity and take the square root. Doing this, you get an average speed of about 440 miles per hour (700 kilometers per hour). This is much faster than the speed of typical waves, which can range from about 10 to 30 mph (15 to 50 kph).

This equation is what oceanographers use to estimate when a tsunami will reach faraway shores. The tsunami on Jan. 15 hit Santa Cruz, California, 12 hours and 12 minutes after the initial eruption in Tonga. Santa Cruz is 5,280 miles (8,528 kilometers) from Tonga, which means that the tsunami traveled at 433 mph (697 kph) – nearly identical to the speed estimate calculated using the ocean’s average depth.

Destruction on land

Tsunamis are rare compared to ubiquitous wind-driven waves, but they are often much more destructive. The 2004 Indian Ocean tsunami killed 225,000 people. More than 20,000 lost their lives in the 2011 Japan tsunami.

What makes tsunamis so much more destructive than normal waves?

In the open ocean, tsunami waves can be small and may even be undetectable by a boat at the surface. But as the tsunami approaches land, the ocean gets progressively shallower and all the wave energy that extended thousands of feet to the bottom of the deep ocean gets compressed. The displaced water needs to go somewhere. The only place to go is up, so the waves get taller and taller as they approach shore.

When tsunamis get to shore, they often do not crest and break like a typical ocean wave. Instead, they are more like a large wall of water that can inundate land near the coast. It is as if sea level were to suddenly rise by a few feet or more. This can cause flooding and very strong currents that can easily sweep people, cars and buildings away.

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Luckily, tsunamis are rare and not nearly as much of a surprise as they once were. There is now an extensive array of bottom pressure sensors, called DART buoys, that can sense a tsunami wave and allow government agencies to send warnings prior to the arrival of the tsunami.

If you live near a coast – especially on the Pacific Ocean where the vast majority of tsunamis occur – be sure to know your tsunami escape route for getting to higher ground, and listen to tsunami warnings if you receive one.

The eruption of the Hunga Tonga-Hunga Ha’apai volcano severed the main communication cable that connects the people of Tonga to the rest of the world. While the science of tsunamis can be fascinating, these are serious natural disasters. Only a few deaths have been reported so far from Tonga, but many people are missing and the true extent of the damage from the tsunami is still unknown.

Sally Warner, Assistant Professor of Climate Science, Brandeis University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

On Feb. 11, 2020, Brazilian Maya Gabeira surfed a wave off the coast of Nazaré, Portugal, that was 73.5 feet tall. Not only was this the biggest wave ever surfed by a woman, but it also turned out to be the biggest wave surfed by anyone in the 2019-2020 winter surfing season – the first time a woman has ridden the biggest wave of the year.

As a female surfer myself – though of dubious abilities – this news made me really excited. I love it when female athletes accomplish things that typically garner headlines for men. But I am also a physical oceanographer and climate scientist at Brandeis University. Gabeira’s feat got me thinking about the waves themselves in addition to the surfers who ride them.

What makes some waves so big?

Waves start with a storm

Think for a few seconds about what happens when you throw a stone into a serene pond. It creates a ring of waves – depressions and elevations of the water’s surface – that spread out from the center.

Waves in the ocean act similarly. On rare occasions earthquakes and landslides can generate waves, but usually waves are created by wind. Generally, the biggest and most powerful wind-generated waves are produced by strong storms that blow for a sustained period over a large area.

The waves that surfers ride originate in distant storms far across the ocean. For instance, the wave that Gabeira surfed at Nazaré was likely generated by a storm somewhere between Greenland and Newfoundland a few days earlier. The waves within a storm are usually messy and chaotic, but they grow more organized as they propagate away from the storm and faster waves outrun slower waves.

This organization of the waves creates “swell,” or regularly spaced lines of waves. When describing a swell, oceanographers and surfers generally care about three attributes. First, the height – how tall a wave is from the bottom to the top. Then the wavelength – the distance between the top of one wave and the top of the wave behind it. And finally the period – the time it takes for two consecutive waves to reach a fixed location.

Seafloors control the waves

Waves are not just sitting on top of the ocean. Their energy extends far below the surface, sometimes as deep as 500 feet. When waves move into shallower water close to shore, they start to “feel” the ocean’s bottom. When the bottom pulls and drags on the waves, they slow down, get closer together and grow taller.

As the waves move toward shore, the water gets ever more shallow and the waves keep growing until, eventually, they become unstable and the wave “breaks” as the crest spills over toward shore.

When a swell is traveling through the ocean, the waves are all more or less the same size. But when swells run into a coastline, waves at one beach can be many times bigger than waves at another beach a mere mile away. So why don’t we find large waves breaking on all shores? Why are there some spots like Nazaré in Portugal, Mavericks in California and Jaws in Maui that are notorious for having big waves?

It comes down to what’s at the bottom of the ocean.

Most coasts do not have a smooth, evenly sloping bottom extending from the deep ocean to shore. There are reefs, sand banks and canyons that shape the underwater terrain. The shape and depth of the ocean floor is called the bathymetry.

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Just as light waves and sound waves will bend when they hit something or change speed – a process called refraction – so do ocean waves. When shallow bathymetry slows down a part of a wave, this causes the waves to refract. Similar to the way a magnifying glass can bend light to focus it into one bright spot, reefs, sand banks and canyons can focus wave energy toward a single point of the coast.

This is what happens at Nazaré to create giant waves. Extending out to sea from the shore is an underwater canyon that was etched out by an ancient river when past sea level was much lower than it is today. As waves propagate toward shore over this canyon, it acts like a magnifying glass and refracts the waves toward the center of the canyon. This focusing of waves by the Nazaré Canyon helps make the largest surfable waves on the planet.

The next time you hear about someone like Maya Gabeira surfing a record-breaking wave at Nazaré, think about the faraway storms and the unique underwater bathymetry that are essential for generating such big waves. The wave she rode had been on a long journey, and at its crashing end, it was memorialized as she took off from its crest and rode down its huge, steep face.

Sally Warner, Assistant Professor of Climate Science, Brandeis University

This article is republished from The Conversation under a Creative Commons license. Read the original article.