March 28, 2023

What are geological faults and when do they cause earthquakes?

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The San Andreas Fault, which runs through the US state of California, is one of the most famous faults in the world.

Turkey suffers a lot from earthquakes and we often hear that this is because it has many fault lines. But what is a fault and why is it connected to earthquakes?

The BBC spoke to a seismologist to find out.

“As fast as fingernails grow – so fast do the plates move.”

Jessica Hawthorne talks about the 15 or so large slabs of rock known as tectonic plates that together make up the Earth’s outermost layer.

The professor in the Department of Earth Sciences at the University of Oxford studies the mechanics of earthquakes and explains their relationship to faults.

“A fault is a place where two plates go past each other. You have a solid block on one side and a solid block on the other and they go past each other,” explains the seismologist.

Although they move very slowly with each other (only a few centimeters per year), a sudden movement or landslide can release large amounts of energy, breaking the rock and causing an earthquake.

This happens relatively close to the surface because the hottest rock closest to Earth’s core is viscous and deforms like a fluid, Hawthorne explains.

“For an earthquake, you need a place where there’s frictional failure, and that means things have to be brittle enough to break quickly.”

Earthquakes occur mostly at tectonic plate boundaries (although there are exceptions within continents).

More than 80% of large earthquakes occur on the plates beneath the Pacific Ocean, in an area called the Ring of Fire, where the Pacific plate is pushed beneath the plates around it.

types of failures

Although earthquakes create new faults, in most cases the fault already exists, according to Hawthorne.

Slip occurs on a preexisting fault and can be classified into three basic types: normal fault, reverse fault, and transient fault.

one-time failures

This type of error occurs when two blocks cross each other horizontally.

The strike-slip faults are usually vertical, sometimes extending to a depth of 15 to 20 km.

An example is the 700 km long East Anatolian Fault that runs along the boundary between Anatolia and the Arabian Plate in Turkey.

The magnitude 7.5 and 7.8 earthquakes that struck Turkey and Syria last week—two of the strongest in nearly a century—occurred in this region, on a fault.

The earthquake and aftershocks occurred at a shallow depth (just a few kilometers below the Earth’s surface), which helps demonstrate its devastating consequences: more than 33,000 people have lost their lives so far.

Another example is the San Andreas fault system in California (USA), formed by several faults that accommodate movement between the Pacific plate in the west and the North American plate in the east.

Relative to the rest of the continent, western California is moving toward Alaska, Hawthorne says, as these two plates slide horizontally against each other, resulting in earthquakes.

The San Andreas Fault is a complex zone of crushed and fractured rock that stretches 1,200 kilometers long and at least 25 kilometers deep.

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An earthquake along the San Andreas Fault in 1906 leveled buildings in San Francisco

The California earthquake of April 18, 1906 was centered on the northernmost point of the San Andreas fault, according to the US Geological Survey, flattening San Francisco.

To this day it is considered “one of the most important earthquakes of all time”.

normal damage

Normal faults are breaks where the blocks separate (diverge) and one of the blocks falls vertically.

Afar depression in East Africa

In the Afar region of Ethiopia, three distinct parts of the Earth’s crust meet. The spot is known as the Afar Triangle.

The Somali plate is moving away from the rest of the continent – which has formed a trough.

Meanwhile, the African and Somali plates are also moving away from the Arabian plate to the north, forming a Y-shaped rift system.

The movement creates tension in the rock, creating cracks, fissures, volcanoes, and other ground deformations.

In 2005, a series of fissures appeared along the depression – with earthquakes and ash clouds – and opened a chasm eight meters wide and 60 kilometers long.

mid atlantic mountain range

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Volcanic activity in Iceland is caused by tectonic plates moving apart

The North American and Eurasian tectonic plates are slowly moving away from each other, and a rift known as the Mid-Atlantic Ridge has formed at the edge of the diverging plate.

The mountain range stretches from north to south for thousands of kilometers in the middle of the North and South Atlantic oceans.

As the plates move apart, the molten magma beneath the Earth’s surface is constantly erupting, forming new rock, which is then pushed out by younger material.

This has created a mostly underwater mountain range that appears in places like islands – for example Iceland, the Azores or Ascension Island.

It also creates surface faults, earthquakes and massive volcanic activity.

reverse errors

Reverse or thrust faults occur when blocks come together and one of them is pushed up.

They’re usually the largest faults because they “tend to cut the Earth at an angle, which creates a wider area where brittle deformation can occur,” Hawthorne says.

Japanese trench

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In 2011, a powerful earthquake caused a devastating tsunami in Japan.

The Japan Trench fault zone is a deep submarine trench that runs north to southeast of the Japanese Islands, separating the Eurasian plate from the Pacific plate.

The devastating Tohoku-Oki earthquake of magnitude 9.1 that struck off the coast of Japan in March 2011 was the result of displacement of 50 meters along the fault.

This rupture of a section along Japan’s trench caused massive damage and death as it triggered a tsunami that devastated the country’s coastline and led to a nuclear meltdown at the Fukushima plant. It was a “triple disaster”: earthquake, tsunami and nuclear accident.

Peru and Chile Trench

Also known as the Atacama Fault System, it is located in the eastern Pacific Ocean, about 160 kilometers off the coast of Peru and Chile, between the Nazca and South American tectonic plates.

The oceanic crust of the Nazca plate is moving under the continental crust of the South American plate, causing a lot of seismic activity.

On May 22, 1960, a massive 9.5-magnitude earthquake struck near the city of Valdivia in southern Chile – believed to be the largest earthquake ever recorded.

Scientists estimated that the energy released by the earthquake was 20,000 times greater than that of the bomb dropped on Hiroshima in World War II.

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An earthquake destroyed houses in Valdivia, Chile in 1960

However, those who live near a fault do not necessarily need to be on constant alert.

Hawthorne says not all faults have earthquakes.

“Many fault segments have no earthquakes or only small tremors,” he says.

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