When the Earth shakes: 9 questions about earthquakes

The earthquake, that moment when the earth shakes under our feet. From its birth in the depths to the sometimes devastating tremors on the surface of our planet, discover this phenomenon in 9 questions.
27 May 2021

All about earthquakes

What is an earthquake? What mechanisms are involved and what risks exist? How can we protect ourselves against them? Can we predict them? BRGM answers in 9 questions.

© BRGM

1/ What is an earthquake?

An earthquake is primarily a fracture along a fault line several kilometres below our feet. Its sudden movement generates seismic waves that propagate. This is the "quake" that we feel.

2/ Why do earthquakes happen?

Earthquakes occur because the rocks beneath our feet are put under pressure, mainly due to plate tectonics. This is the phenomenon whereby large plates like pieces of a jigsaw puzzle on the surface of the Earth move in relation to each other. The pressures created by plate movements result in stresses loading along fault lines, which is suddenly released when the point of failure is reached.

Some earthquakes can have other causes, for example deglaciation: tens of thousands of years after the covering of ice has left an area, the accumulated pressure and stresses on the ground are released which can generate earthquakes.

3/ Can you predict an earthquake?

Unlike many natural hazards, earthquakes are not yet predictable. It is not possible to say where and when an earthquake will occur, which limits the possibilities for preventive evacuation for example. 

However, it is possible to map the hazards, showing the probability of an earthquake of a given intensity occurring at a given location over long time scales (several hundred years). These maps are established by looking back into the past and compiling statistics on the occurrence of past earthquakes. This is a form of long-range forecasting.

4/ Can earthquakes occur anywhere?

Earthquakes do not happen just anywhere. On a global scale, they are primarily concentrated at the boundaries between the major tectonic plates. On the scale of a country or a region, forces accumulate in a privileged way where there are breaks in the ground, often in mountain chains.

In France, earthquakes can be observed even though we are not on the boundary between two plates: in the Alps, the Pyrénées, the Massif Central and the Rhine Rift Valley, but also large areas such as Brittany, were mountain ranges millions of years ago which eroded over time.

5/ How is an earthquake measured?

To characterise the scale of an earthquake, we refer to both intensity and magnitude.

Magnitude characterises the energy released at the time it occurs. In practice, earthquakes are measured on the surface with seismometers, the magnitude is then calculated which explains why its value varies depending on the institution doing the measuring. But usually it has a single value, at its source. To characterise it, we often refer to the Richter scale, which is a scale of magnitude. The largest earthquakes that have been measured and observed since seismology was first developed (a relatively recent science, dating back less than 200 years) has been about 9 - 9.5.

The intensity depends on the location of the surface observation and characterises the severity of the tremor at a given location. It is generally stronger at the epicentre, i.e. on the surface directly above the spot where the fracture occurred, and it generally attenuates with distance, like sound. However, particular configurations can amplify this intensity locally, such as surface geological formations or relief.

6/ What is the difference between the epicentre and the focus?

Earthquakes occur at depth, usually a few kilometres down - up to more than 100 kilometres deep in some so-called subduction zones.

The place where the fracture starts is called the focus. Its projection on the surface, directly over it, is called the epicentre

7/ What is a seismic wave?

A rupture along a fault results in the emission of seismic waves. There are several types of seismic waves:

  • The first ones we feel, those that travel fastest, are known as P waves. These are compression waves. They spread very quickly.
  • P waves are followed some time later by shear waves. These are slower and are called S waves, which stands for secondary.
  • Later come other waves, which only propagate on the surface. The latter are more energetic, which means they can cause more damage.

8/ What are the risks?

The best way to protect against seismic risks is therefore to make buildings that do not collapse in the event of a tremor. This is referred to as earthquake engineering and earthquake-resistant building.

But the requirements of seismic codes for building construction in France are not at all at the same level as in Japan, because the level of expected tremors is not the same. What is expected is for it not to threaten the lives of its occupants at the time of the tremors. 

9/ Are there any risks in France?

In France, too, we have to make earthquake-proof buildings. We have regulations which require us to comply with the seismic codes, depending on where we are, the earthquake risk, the probability of an earthquake occurring and the type of building - single-family houses, schools and industrial sites.

But above all in France, the most earthquake-prone area is not in mainland France but in the overseas territories; the French West Indies have a much higher level of seismicity than mainland France due to the configuration of the area, close to a plate boundary. Another territory that has been in the news recently is the department of Mayotte, which has experienced significant earthquake activity over the past two years that had not previously been observed, this is linked to volcanic activity.