Factors that Determine Earthquake-triggered Tsunamis

Tsunami warning signsPhoto byBernard HermantonUnsplash

Many people wonder why some earthquakes trigger tsunamis’ and others don’t. The USGS closely monitors for tsunamis whenever there is an earthquake. The occurrence of tsunamis due to earthquakes depends on several different contributing factors.

Location of the Earthquake

It’s all about the location. Tsunamis are typically caused by underwater earthquakes which occur at tectonic plate boundaries, especially in subduction zones where one plate is forced under another. Not all earthquakes are created equal. Earthquakes that happen on land or in areas without a significant body of water are less likely to cause tsunamis.

Depth of the Earthquake

Shallow underwater earthquakes (those occurring at depths less than 70 kilometers) are more likely to displace the seafloor and generate tsunamis. Deep earthquakes, even if powerful, do not usually cause significant displacement of the ocean floor and hence are less likely to trigger tsunamis.

Magnitude of the Earthquake

Larger earthquakes (magnitude 8.0 or higher) are more likely to produce tsunamis because they have the energy needed to cause significant displacement of the ocean floor. Smaller earthquakes, while they can still be destructive, typically do not have enough energy to cause a tsunami.

Type of Fault Movement

The nature of the fault movement during the earthquake also plays a crucial role. Vertical displacement (uplift or subsidence) of the seafloor, commonly associated with thrust or reverse faults, is more likely to generate tsunamis. Horizontal displacement (strike-slip faults) generally does not cause significant vertical movement of the seafloor and thus is less likely to result in a tsunami.

Seafloor Topography

The shape and features of the seafloor can influence how much water is displaced during an earthquake. Certain underwater features can amplify or dampen the movement of water, affecting the potential for tsunami generation.

Signs warning about tsunamis in and around ocean towns save lives. Most warning signs also have an arrow for a tsunami evacuation route. It's important to familiarize yourself with such signage if you're visiting a coastal town. Have you ever seen these signs?

Thus, for an earthquake to cause a tsunami, it would have to occur under or near a body of water, be shallow, have a large magnitude, involve vertical displacement of the seafloor, and occur in an area where the seafloor can effectively transmit the energy to the water above. Without these contributing factors, the earthquake isn’t likely to cause a tsunami.

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