why doesn't every ocean earthquake generate a tsunami

Why doesn’t every ocean earthquake generate a tsunami? Earthquakes are a fascinating and awe-inspiring display of nature’s forces. And ocean earthquakes are even more intriguing.

The thought of an ocean earthquake usually makes you believe a tsunami is brewing. But, in reality, not all those ocean earthquakes lead to tsunamis.

This makes people confused and ask questions. How do tsunamis occur? Do deep earthquakes cause tsunamis?

Let’s find out the truth now!

Not every earthquake generates a tsunami because it may not have a high magnitude or is not a shallow-focus earthquake. 

More about the Science of Ocean Earthquakes

more about the science of ocean earthquakes

Quite like their terrestrial counterparts, ocean earthquakes are the result of a sudden release of piled-up stress deep underneath the Earth’s crust.

There is a large network of tectonic plate boundaries under the earth and they all crisscross the ocean floor as well. In fact, this is where the seismic activities turn quite strong.

These boundaries include convergent, divergent, and transform plate boundaries, and they each come with distinct characteristics.

And due to those characteristics, they all play a different role in generating a tsunami. 

More about the Anatomy of a Tsunami

more about the anatomy of a tsunami

By digging deeper into the details, you will quickly notice that tsunamis result from the seabed’s vertical displacement.

When the seafloor displaces, it also disturbs a large volume of water. When it happens, it leads to a powerful surge that spreads across the ocean.

Because of the power behind them, these are no ordinary waves usually resulting from wind energy.

But, there are many other factors at play when it comes to determining exactly what causes a tsunami to occur

Why Doesn’t Every Ocean Earthquake Generate a Tsunami?

not all ocean quakes trigger tsunamis

Most ocean earthquakes sound intimidating, but the truth is that not all of them are devastating.

The reason is that a large majority of ocean earthquakes just won’t turn into tsunamis.

But, why does that happen? Why don’t we get to see those colossal waves we associate with ocean earthquakes?

Well, this phenomenon has definitely confused scientists for a long time, making them look deeply into the interplay of tectonic, geological, and hydrodynamic factors.

Here are some possible explanations as to why it is highly unlikely for an ocean earthquake to turn into a tsunami:

The Magnitude is Not Always High

the magnitude is not always high

Whether or not an earthquake would lead to a tsunami depends heavily on the magnitude of an ocean earthquake.

Magnitude RangeTsunami Potential
7.9 and greaterPotential for a destructive tsunami
7.8 to 7.6Destructive tsunami close to the epicenter
7.5 to 6.5Little potential to produce destructive tsunamis
Below 6.5Unlikely to generate a tsunami

The higher the magnitude, the stronger the energy released during the process.

In fact, a 7.5 magnitude earthquake is capable of releasing energy at least 32 times more than an earthquake of 6.5 magnitude.

With a slight increase in the magnitude, there is a huge change in the energy released, which can directly impact the displacement of the seafloor.

And a substantial vertical seafloor displacement can displace enormous volumes of water, leading to a tsunami.

But not all ocean earthquakes have high magnitudes, which is why they do not turn into tsunamis. 

An Example to Consider

The 2004 Indian Ocean earthquake had around 9.3 magnitude, which displaced the seafloor by several meters.

And as a result, it displaced trillions of gallons of water and quickly changed into a catastrophic tsunami. 

The Ocean Earthquakes are Not Deep Enough

ocean earthquakes are not deep enough

The extent of a seaquake’s depth plays a crucial role in determining its ability to generate a tsunami.

Shallow earthquakes are considered more dangerous. It is mainly because they take place in the upper 60km of the earth’s crust.

Earthquakes that occur at greater depths, like those beneath 100 kilometers, might not cause sufficient water displacement to create a significant tsunami.   

 An Example to Consider

The Maule earthquake occurred in Chile in 2010 and it was only 35km deep.

The occurrence of this seismic event caused the seafloor to rise, resulting in a massive displacement of water, ultimately causing a tsunami that affected the entire Pacific region.

Fact: The earthquake that occurred in Japan in 2011, known as the Tohoku earthquake, caused the seafloor to rise by several meters due to its depth of approximately 32 kilometers. 

The Underwater Location Plays a Role

the underwater location plays a role

The precise positioning of an earthquake plays a vital role in the creation of tsunamis, as it is primarily triggered by seismic activities occurring beneath the ocean’s surface.

Subduction zones, known for the collision of tectonic plates, are remarkable locations for these earthquakes and the subsequent tsunamis they generate.

For instance, the Indian Ocean earthquake of 2004 transpired on the interface of the Indian and Burma tectonic plates.

During this event, the Indian plate was compelled to subduct beneath the Burma plate, resulting in the uplifting of the ocean floor and the displacement of an immense amount of water.

Fact: The Indian Ocean tsunami ranks among the most fatal disasters ever recorded.   

The Fault Movement May Not Be Suitable for Tsunamis

faults may not cause tsunamis

A tsunami may not occur if the fault movement is not suitable.

The likelihood of tsunami generation is heavily influenced by the specific type of fault movement that occurs during an earthquake.

Thrust faults are identified by the vertical movement of tectonic plates.

They possess the capacity to induce substantial changes in the seafloor, resulting in the occurrence of massive tsunamis.

Conversely, strike-slip faults, such as the San Andreas Fault located in California, entail horizontal plate movements and do not displace substantial amounts of water, thus reducing the likelihood of generating tsunamis.

Fact: The 2011 Tohoku earthquake in Japan was the result of the Pacific plate sliding beneath the North American plate, resulting in a catastrophic tsunami. 

The Displacement of Seafloor is Not Substantial

the displacement of seafloor is not substantial

In order for an earthquake to produce a tsunami, it must cause a quick and significant vertical movement of the ocean floor.

When the ocean floor experiences a sudden rise or fall, it causes the water above to move, setting off a chain of waves that travel across the sea and transform into a tsunami.

The Chilean Valdivia earthquake of 1960 is notable, showcasing a vertical displacement of around 10 meters.

The immense movement set off a tsunami that swept across the Pacific, wreaking havoc on faraway shores such as:

  • Hawaii
  • Japan
  • The Philippines 

The Inappropriate Coastal Configuration

the inappropriate coastal configuration

The arrangement of the coastline, encompassing the form and shallowness of the nearby region, has a notable influence on molding the effect of a tsunami as it approaches the land.

As a tsunami approaches a narrow and shallow coastline, the wave energy becomes compressed, resulting in a substantial increase in wave height.

The term used to describe this occurrence is “wave shoaling.” 

An Example to Consider

The effect of the 2011 Tohoku earthquake in Japan was evident when the tsunami waves reached the confined and shallow Sendai Plain along the coast.

The magnitude of the wave height increased significantly, surpassing 40 meters in certain regions, leading to catastrophic devastation along the shoreline. 

Presence of Many Tsunami Barriers

presence of many tsunami barriers

The presence of many tools and natural barriers helps lower the chances of tsunamis becoming too dangerous.

For instance:

FactorsInfluence of Tsunami GenerationExamples
Bathymetry Analyzes tsunami wave-seafloor interactionA narrow continental shelf increases the wave effect.
Coastal MorphologyAmplifies wave energyTsunamis increase in narrow estuaries.
Natural barriersReduce tsunami force as wave attenuators.Coral reefs and mangroves reduce wave impact.

The Role of Underwater Landslides

the role of underwater landslides

Apart from seismic activities, underwater landslides and volcanic eruptions also play a crucial role in the formation of tsunamis.

If they do not occur, you are less likely to see a tsunami even after an earthquake.

Tsunamis can cause significant damage when landslides take place in volcanic island environments, displacing large quantities of water. 

An Example to Consider

In California’s Long Valley Caldera, the eruption of Mount Ritter in 1888 resulted in a landslide that led to a significant debris flow into the neighboring Lake Crowley.

The force caused the water in the lake to shift, resulting in a tsunami that produced waves reaching heights of 9 meters.

Fact: The 1883 outburst of Krakatoa in Indonesia triggered an abrupt caldera collapse, resulting in the displacement of ocean water and the formation of devastating tsunamis. 


not all ocean earthquakes generate tsunamis

Why doesn’t every ocean earthquake generate a tsunami? Many conditions and factors are at play when an earthquake leads to a tsunami.

For instance, it has to be an earthquake with a higher magnitude to cause vertical displacement of the sea floor.

Similarly, shallow focus earthquakes are more likely to produce tsunamis.

Since these conditions are not always present, you do not get to see tsunamis that often, and we should thank God for that!