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What is an earthquake?

Origine of a seism

Seism comes from the Greek word seismos, meaning shake. Thus it is a movement, more or less violent, of the ground that can be artificially decomposed into 3 directions: north-south, east-west and vertical. The vertical component of the movement is generally weaker than the horizontal ones. Its frequency range is between 0 and 35 Hz.

Diagram of the propagation of the waves from focus to ground surface
Propagation of the waves from focus to ground surface

Example of evolution of acceleration against time

Example of evolution of acceleration against time
Tri-axial movement X, Y, Z
Acceleration 0 to 1 g
Frequencies 0.1 to 30 Hz
Displacement a few centimetres to metres
Duration 5 to 35 s

Types of earthquakes

Earthquakes can be classified into 3 categories:

Effects of 2 earthquakes of equivalent magnitude but with different focus depth
  1. Tectonic earthquakes are the most common. They occur when a fracture breaks suddenly. A fracture is a break of the earth's crust separating a group of rocks into two shifted sets.
  2. Volcanic earthquakes are those that occur in conjunction with a volcanic eruption.
  3. Collapse earthquakes are small quakes from the fall of underground caverns or mines.


The focus of an earthquake is the place where the fracture in the earth's crust takes place.

The epicenter of an earthquake is the point on the surface directly above the focus.

The intensity of an earthquake is defined in a place with respect to the effects produced by this seism, it is associated to the observation place (eg, MSK intensy scale).

The magnitude measures the energy emitted under the shape of elastic waves (eg, Richter magnitude). The magnitude of an earthquake depends on the distance between the epicenter and the focus. For a same magniture, the deeper the focus is, the larger the damaged area is and the less the intensity at the epicenter is.