SCIENCE

Earthquakes: a brief and devastating phenomenon

The streets of Port-au-Prince after the earthquake that hit the country on January 7, 2010. © Jorge Silva / Reuters

Text by: Nenad Tomic

6 mins

An earthquake usually only lasts a few seconds but often causes considerable material and human damage without warning.

Memories are still vivid of the disasters that hit Thailand and neighboring countries in December 2004, Haiti in 2010 or the Japanese islands in March 2011. Earthquakes are the result of the perpetual movement of the Earth's mantle and lithosphere and occur in specific areas.  

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What is an earthquake? 

The earthquake or earthquake is the result of the movement of blocks of rock in the depths of the earth's crust.

Indeed, the earthquake comes from the fracturing of the rocks in depth due to a large accumulation of energy which is released when the mechanical rupture threshold of the rocks is exceeded. 

It should be noted that the earth's crust is made up of several large plates that interact with each other.

These plates are called “tectonic plates” and depending on the location, they move apart, converge or slide.

Seismic faults are located most of the time on the edges of tectonic plates and represent a kind of witness to the deformations caused by the movement between two blocks.

The vast majority of earthquakes occur in the boundary areas of these plates when the accumulated energy causes the fault to yield. 

Earthquakes occur in the earth's crust at a depth of about 15 to 20 kilometers.

The exact place where the earthquake occurs is called the focus while the place vertically on the earth's surface where the seismic waves are felt is the epicenter.  

How to measure the strength of earthquakes?  

The strength of the tremors caused by an earthquake is measured by magnitude and intensity.  

The

magnitude 

corresponds to the seismic energy released during the rupture of the fault.

It is calculated from the amplitude of ground motion recorded by seismological instruments.

When an earthquake occurs, to illustrate the power of the event the media very often mention the magnitude of Richter.

This is the scale established by the American seismologist Charles Francis Richter in 1935. He compared the earthquakes that occurred in California, measured with a certain type of instrument and at a relative proximity to the epicenter.

But the Richter scale has since evolved.

In the second half of the 20th century, researchers discovered that one degree higher on the magnitude scale corresponded to a quantity of energy released thirty times greater than the previous degree! 

To characterize the severity of the earthquake ground shaking, seismologists have established intensity scales.

They are based on the observation of effects and consequences in one place on people, buildings and the environment in general.

There are several intensity scales: the Rossi-Forel scale (RF), the Medvedev-Sponheuer-Karnik scale (MSK), the Mercalli scale (MM) or the European macroseismic scale (EMS98). 

The effects of the earthquake © RFI

Earthquake zones around the world 

By observing the history of earthquakes and their foci, scientists have established the precise areas where earthquakes occur most often.

Areas with high seismic activity correspond, as for volcanic activity, to the edges of tectonic plates.

It is the result of the permanent activity of the lithosphere. 

The rim of the Pacific Ocean is particularly exposed to earthquakes.

The oceanic plate sinks under the continents that surround it.

It is the phenomenon of subduction which results in numerous slips along faults causing powerful earthquakes such as in Japan, Chile, Indonesia, all along New Zealand or on the American and Mexican west coast. 

When two tectonic plates collide, they rub against each other.

This phenomenon of collision or extension mainly concerns regions such as North-West Africa, the Mediterranean, the Alps, the Balkans, the Caucasus, the Himalayas and northern Australia. 

The earthquake that struck Haiti on January 12, 2010 is the consequence of the collision of two plates: the North American plate to the north and the Caribbean plate to the south.

The earthquake was caused by the rupture of the Enriquillo fault which passes through the capital Port-au-Prince from west to east and over a length of about one hundred kilometers.

The consequences are disastrous: more than 230,000 dead, around 300,000 injured and cities 90% destroyed.  

In addition, seismic zones with shallow focus are also located under the oceans and at the edge of volcanic chains.

It is not uncommon for seismographs to record tremors during volcanic eruptions that are the result of moving magma and breaking rocks. 

© FMM Graphic Studio

Tsunamis 

One of the consequences of earthquakes that occur between two oceanic plates is the tidal wave phenomenon, most often called tsunami (in Japanese “harbour wave”).

When the tremors cause the displacement of large quantities of water, these will submerge the coastline, with waves sometimes measuring tens of meters high.

The speed of propagation of such waves is linked to the power of the earthquake and can range from 500 to 800 km/h.

Clearly, a tsunami caused by an earthquake whose epicenter is 1000 km from the coast can reach them about 2 hours later.

On December 26, 2004, the island of Sumatra in Indonesia experienced one of the strongest earthquakes ever recorded so far (magnitude 9.3).

The tremors generated a gigantic tsunami which

has spread throughout the Bay of Bengal and the Indian Ocean.

The consequences were disastrous throughout the region: more than 250,000 dead and missing and apocalyptic destruction in countries bordering the Indian Ocean: Indonesia, Thailand, Sri Lanka, India, Maldives, Malaysia , Somalia, Tanzania and Kenya.

On March 11, 2011, the powerful earthquake off the coast of Japan caused the tsunami that caused one of the biggest nuclear disasters at the Fukushima power plant.

Our selection on the subject:

  • To read : 

→ Why does the earth keep shaking in Italy?


→ The new seismic map of France extends the area of ​​municipalities at risk


→ A magnitude 7.2 earthquake shakes Haiti


→ Japan: two major earthquakes linked by the same fault

  • To listen : 

→ Why is the earth shaking?


→ How to predict earthquakes?


→ Is Istanbul ready for its “Big one”?


→ A network of seismograph smartphones


→ What do we know about earthquakes?

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