Earthquakes...

volcanoes...

tsunamis...

each of these offers dramatic evidence of Earth's power and can be overwhelmingly destructive.

Are these natural hazards related? If so, what do they have in common? In this lesson, you'll find out, as we explore where tsunamis occur.

Tsunamis are not scattered randomly around Earth. They occur more frequently in certain locations. The most common sites are associated with earthquakes that occur at tectonic plate boundaries called "subduction zones." A subduction zone is a place where one plate is moving under another.

Here is a map that shows Earth's subduction zones (red lines). On top of that are plotted the locations of earthquakes that produced major tsunamis in the past 110 years. As you may have noticed, they overlap a lot.

At subduction zones, one of the tectonic plates that make up Earth's crust is traveling down into the mantle underneath an overriding plate. The plates catch on one another because the surface between them is irregular and jagged. As the lower plate continues to travel down, stress builds between the two plates.

Eventually, the boundary, or fault, ruptures and the upper plate suddenly rises, while further back from the fault, the plate drops. These zones are mostly under oceans or seas near coastlines, so as the plate moves up and down, so does the water on top of it. The rising rock pushes water into a momentary ridge, which gravity immediately forces into series of spreading tsunami waves.

Another far less common reason subduction zones make tsunamis is that these zones also tend to create volcanoes, although volcanoes form elsewhere too.

Erupting volcanoes at the coast can also sometimes create tsunamis.

Earthquakes can also trigger landslides both underwater and at the surface. Tsunamis caused by landslides can also be created around subduction zones.

Earthquakes can happen at other types of plate boundaries too (including "strike-slip" or "transform" faults) where plates move past each other...

...and also at divergent faults (or spreading centers), where plates move away from each other. But earthquakes at these places are typically not as large and do not produce much uplift, so waves are less likely to be created.

Earthquakes cause tsunamis much less commonly in these places.

Submarine landslides can occur anywhere slopes become overly steep underwater.

Note the large brown area of delta at the mouth of the Amazon in the center of the image.

This often happens in the sediment fans of river deltas, where muddy rivers drop their silt upon reaching the ocean.

Note the steep continental shelf off the east coast of the U.S.

But anywhere a steep slope exists underwater—whether around the base of an island, atoll, underwater seamount, continental slope or canyon—landslide-generated tsunamis can occur. These are frequently (but not always) triggered by earthquakes.

Finally, you may have noticed that some tsunamis have occurred far from any tectonic boundary, for example, in Hawaii and on other islands in the Pacific.

A Computer Model Animation of the 2010 Chile Tsunam

Because tsunamis can travel through the sea to distant locations, they can affect some islands and coasts that are far from any tectonic boundary or source of landslides. One of the most famous of these was the 1960 tsunami that struck in Hawaii after traveling all the way from Chile, much larger than the recent 2010 tsunami shown here, also from an earthquake near Chile.

The March 2011 earthquake off the coast of Japan resulted in a tsunami that not only devastated parts of Japan, but caused damage in Hawaii and other islands, including the Galapagos. Waves were as high as 2 ½ meters (over 8 feet) in California and 1 ½ meters (nearly 5 feet) in Chile on the extreme far side of the basin from the earthquake source.

In reality, while coasts closest to subduction zones have the most threat for a tsunami, nearly every coast can be vulnerable to a large tsunami that travels from far away.