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.
|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|
Earthquakes can be classified into 3 categories:
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.