Homepage / EARTHQUAKES / A few definitions

Characteristics of an earthquake

faq8
The focus or hypocenter

it is the zone of the seismic fault where the fault fracture occured - the seismic waves start spreading from this point

The epicenter

it is the hypocenter's "reflection" at the surface of the globe and is generally associated to the largest seismic movement

The intensity

as opposed to the magnitude, the intensity corresponds to the effects and the damage cause by an earthquake in a given place. The further the distance, the lower the intensity : therefore, intensity is not an intrinsic criteria of the natural phenomenon. It shows the state of the human environment in the event of an earthquake (how big constructions are, what their state is, to what extent was the population prepared to the earthquake...)

The magnitude

introduced by Richter in 1935, it represents a logarithmic quantity, calculated either using the signal amplitude recorded by a seismograph, either using the duration of this signal. The magnitude is not a scale but a continuous function which can be negative and which usually has no limits. Its minimum value is usually limited by the seismographs' sensitivity, whereas its maximum value depends on how long the fault is - the fault being likely to fracture in one go.

Different types of magnitudes can be used in order to characterize an earthquake :

Local magnitude Ml :

used for local earthquakes (short distances), it depends on a relationship between the distance recorded and the maximum amplitude observed.

Magnitude of surface waves Ms :

valid for all distances, it is calculated using the Rayleigh surface waves and characterizes des large but shallow earthquakes.

Magnitude of body waves mB :

calculated using P compression waves, it is usually used for deep earthquakes with no surface waves.

Magnitude of duration MD :

used for small earthquakes, it shows the duration of the signal between the moment the first wave occured and the moment the signal goes back to the normal level of seismic noise.

Moment magnitude Mw :

directly linked to the earthquake's seismic moment, it is used for large earthquakes (Mw>7.5) where Ms and Ml underestimate the energy released.

Length of the seismic fault Magnitude Mw Earthquake
1 km < 2 Imperceptible microearthquake
1 km 4 Earthquake felt locally
10 km 6 Earthquake causing mildly important local damage
100 km 8 Very large earthquake causing considerable damage
1000 km 10 Largest known earthquakes, causing considerable damage and with an impact on the environment

Ground acceleration

In earthquake engineering, ground acceleration is used to define the earthquake because it is the richest physical parameter directly linked to the force exercised at the basis of a construction. The amplitude associated to each frequency of a gound movement enables one to know how important an acceleration a structure with a favourable vibration frequency. In France, the Réseau Accélérométrique Permanent collects ground accelerations collected in mainland France and in the West Indies.