Earthquakes

The Earth's crust is divided into several tectonic plates. These plates move a few centimetres a year as a result of the convection of magma within the mantle.

Since the lithosphere is mainly solid, when subjected to internal forces from the Earth, this part stores energy and gradually deforms. When the tension is too great, it ruptures. In short, when there is an earthquake, the ground will suddenly move from a few centimetres to several metres. The vibrations of the ground can then be felt. More precisely, an earthquake is the result of a sudden release of energy accumulated by the forces exerted on the rocks.

Earthquakes can be of natural origin (tectonic or volcanic) or man-made.

• Tectonic earthquakes are caused by the movement of tectonic plates. These are the most frequent and most devastating earthquakes. When two tectonic plates rub against each other, pressure is exerted on the rocks of the lithosphere. When the lithosphere has reached the limit of its elasticity, all the energy accumulated in the lithosphere is released, causing an earthquake. Earthquakes can also occur when two plates collide or move apart.
• Volcanic earthquakes are caused by the accumulation of magma in a volcano. They are generally the first signs of a volcanic eruption. These earthquakes cause less damage than those of tectonic origin.
• Man-made earthquakes are caused by certain human activities that disrupt the integrity of the lithosphere. The impounding of a large dam, gas extraction, seismic exploration, mining and quarrying, underground nuclear testing and the collapse of old mines are all examples of earthquakes caused by human activity.

Regardless of the origin of the earthquake, very high pressure is exerted in the lithosphere. The vibrations caused by this pressure propagate in all directions. These vibrations are known as seismic waves.

The location in the Earth's crust where the seismic waves originate is the focus of an earthquake. It is sometimes called the hypocentre. This is the point of rupture where the energy is released in full force. The most intense tremor will be felt at the surface opposite the focus. This location is called the epicentre of an earthquake.

You might think that earthquakes occur just about anywhere on Earth, but that's not the case at all. The distribution of earthquakes supports the tectonic plates theory. All the plates move in relation to each other. The vast majority of earthquakes occur at the boundaries of tectonic plates.

Around 100,000 earthquakes are recorded around the world in any given year. The vast majority of these are not even felt by humans. There are several measurement scales used to determine the magnitude of an earthquake. Of these, the Richter and Mercalli scales are the most widely used in North America.

The Richter scale is the best-known way of measuring earthquakes. It is named after its creator, Charles Richter, who created it in 1935. The scale is generally graduated from 1 to 9, but an earthquake could exceed this value. The magnitude is calculated from the energy released from the focus. On the Richter scale, a one-unit increase in magnitude means a 10-fold increase in power. An earthquake of magnitude 6 is 10 times more powerful than one of magnitude 5, so an earthquake of magnitude 6 will be 100 times more powerful than one of magnitude 4. Since it is based on the energy released and not on the damage caused, the Richter scale can be described as objective.

In North America, the modified Mercalli (MM) intensity scale is also used to quantify the degree of surface effect. The intensity of an earthquake, in this case, is determined by the extent of the damage caused by the earthquake and the population's perception of it. It is therefore a subjective scale.