Why do some ocean earthquakes cause tsunamis while others do not?

We live on an active planet, the surface of which is constantly moving, although it is imperceptible to us most of the time. Until the earthquake happens.

This morning, such an event occurred in the seas north of the Indonesian archipelago, that a powerful earthquake (magnitude 7.6 on the Richter scale) shook the region and was felt as far away as Darwin in Australia.

The Met Office advises that there is no tsunami warning for Australia, while some parts of Indonesia are on a watch-and-wait mode. But what determines the occurrence of a tsunami?

rock grinding

Just 70 years ago, our planet was considered rigid and affected only by slight sagging and elevation of the landscape.

However, as technology advanced in the 1950s, we began to map the sea floor using sonar. We can also measure the magnetic properties of the sea floor.

As a result, we discovered that the ocean floors are divided into undersea mountain ranges known as mid-ocean ridges.

In addition, oceanic crust (the portion of Earth’s crust underlying ocean basins) is being lost around the edges of most continents. It is pushed back deep into the Earth’s mantle – the thick layer of semi-molten rock beneath the Earth’s surface crust.

This occurs in what are known as “subduction zones”. Subduction zones are deep trenches in the oceans where one tectonic plate sinks below the other, triggering earthquakes as rocks slowly converge into one another.

Tsunami source

So why do some earthquakes generate deadly tsunamis and others do not?

Earth’s tectonic plates move across the planet’s surface at an average speed of about 10 cm per year. This speed was originally estimated based on changes in the sea floor’s magnetic properties, but has now been measured by satellites in space.

This movement is not a smooth process, because the Earth’s crust is rigid and experiences strong friction when tectonic plates come into contact with each other.

As they move, this friction increases pressure in the rock, which is occasionally released in the form of earthquakes. In some places, earthquakes only happen occasionally but are very strong, while in other places they occur more frequently and are weaker.

But earthquakes also vary greatly in terms of how deep below the surface they are generated. This is because subduction zones persist for a long time in the mantle. The rock remains cold and hard for hundreds of kilometers before it heats up enough from the planet’s internal heat to become soft.

Read more: Why are shallow earthquakes more destructive? The Java disaster is a devastating example

This is the main reason why some earthquakes generate tsunamis and others do not. Earthquakes in the shallow subduction zone shift the sea floor—either up or down—as well as the ocean above it.

This happened with devastating effect in the 2011 Tohoku earthquake in Japan, which is located at a depth of 24 kilometers and has a magnitude of 9.1 on the Richter scale. This single earthquake moved the crust 26 meters in seconds and lifted the ocean, sending tsunamis crashing across the Pacific Ocean.

Meanwhile, the 7.6-magnitude Maluku earthquake that struck Indonesian waters last night was not as strong and occurred at a depth of 105 kilometers. At this depth, the energy and motion associated with an earthquake dissipates into a million tiny fractures in the overlying rock.

The energy also has to pass through a wedge of semi-molten mantle. Thus, the surface expression of the earthquake is greatly weakened and we don’t get ocean waves, or we get only small waves.

Since the Earth’s plates are moving at a relatively constant rate and we have a record of seismic activity for any given part of the Earth’s crust in the form of the geological record, we can roughly predict how many earthquakes should occur over any large location.

Unfortunately, we do not yet have the technology to predict exactly when or where an earthquake will occur. But what we can do is identify vulnerable areas and build earthquake-resistant infrastructure in earthquake-prone areas to prevent damage and loss of life.

Read more: Always be prepared: the tsunami message from the recent New Zealand earthquake

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