Why some underwater earthquakes cause tsunamis – and others, just little ripples

Yahoo

3 days ago

After a massive earthquake off the coast of Kamchatka, a peninsula in the far east of Russia, on July 30 2025, the world watched as the resultant tsunami spread from the epicentre and across the Pacific Ocean at the speed of a jet plane.
In some local areas, such as in Russia's northern Kuril Islands, tsunami waves reached heights of over three metres. However, across the Pacific there was widespread relief in the hours that followed as the feared scenario of large waves striking coastal communities did not materialise. Why was this?
Not all underwater earthquakes result in tsunamis. For a tsunami to be generated, the Earth's crust at the earthquake site must be pushed upwards in a movement known as vertical displacement. This typically occurs during reverse faulting, or its shallow-angled form known as thrust faulting, where one block of the Earth's crust is forced up and over another, along what is called a fault plane.
It is no coincidence that this type of faulting movement occurred at a subduction zone on 'the Pacific ring of fire', where the dense oceanic Pacific plate is being forced beneath the less dense Eurasian continental plate.
These zones are known for generating powerful earthquakes and tsunamis because they are sites of intense compression, which leads to thrust faulting and the sudden vertical movement of the seafloor. Indeed, it was the ring of fire that was also responsible for the two most significant tsunami-generating earthquakes of recent times: the 2004 Indonesian Boxing Day and March 2011 Tohoku earthquakes.
Why did the Indonesian and Japanese earthquakes generate waves over 30 metres high, but the recent magnitude 8.8 earthquake off Kamchatka (one of the strongest ever recorded) didn't? The answer lies in the geology involved in these events.
In the case of the 2004 Indonesian tsunami, the sea floor was measured to have risen by up to five metres within a rupture zone of 750,000 sq km.
For the tsunami that struck Japan in March 2011, estimates indicate the seafloor was thrust upwards by nearly three metres within a rupture zone of 90,000 sq km.
Preliminary data from the recent Kamchatka event has been processed into what geologists call a finite fault model. Rather than representing the earthquake as a single point, these models show where and how the crust ruptured, including the length of that rupture in Earth's crust, its depth and what direction it followed.
The model results show that the two sides of the fault slipped by up to ten metres along a fault plane of 18°, resulting in about three metres of vertical uplift. Think of it like walking ten metres up an 18° slope: you don't rise ten metres into the air, you only rise about three metres, because most of your movement is forward rather than upward.
However, since much of this occurred at depths greater than 20km (over an area of 70,000 sq km) the seabed displacement would probably have been reduced as the overlying rock layers absorbed and diffused the motion before it reached the surface.
For comparison, the associated slippage for the Tohoku and Indonesian events was as shallow as 5km in places.
An added complication
So, while the size of sea floor uplift is key to determining how much energy a tsunami begins with, it is the processes that follow – as the wave travels and interacts with the coastline – that can transform an insignificant tsunami into a devastating wall of water at the shore.
As a tsunami travels across the open ocean it is often barely noticeable – a long, low ripple spread over tens of kilometres. But as it nears land, the front of the wave slows down due to friction with the seabed, while the back continues at speed, causing the wave to rise in height. This effect is strongest in places where the sea floor gets shallow quickly near the coast.
The shape of the coastline is also important. Bays, inlets and estuaries can act like funnels that further amplify the wave as it reaches shore. Crescent City in California is a prime example. Fortunately however, when the wave arrived in Crescent City on July 30 2025, it reached a height of just 1.22 metres – still the highest recorded in the continental US.
So, not every powerful undersea earthquake leads to a devastating tsunami — it depends not just on the magnitude, but on how much the sea floor is lifted and whether that vertical movement reaches the ocean surface.
In the case of the recent Russian quake, although the slip was substantial, much of it occurred at depth, meaning the energy wasn't transferred effectively to the water above. All of this shows that while earthquake size is important, it's the precise characteristics of the rupture that truly decide whether a tsunami becomes destructive or remains largely insignificant.
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Matthew Blackett does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.