• Question: What is the difference between the "magnitude" and the "intensity" of an earthquake?

    Asked by bhavinamistry to Davie, Gemma, James P, James V, Nuala on 28 Jun 2012.
    • Photo: James Verdon

      James Verdon answered on 28 Jun 2012:

      Hi bhavinamistry,

      The magnitude of an earthquake measures the amount of energy released by the earthquake. It’s an absolute measure of an earthquakes size.

      The intensity of an earthquake describes the amount of shaking felt by people on the surface. So, for example, you could have two earthquakes of the same magnitude, but if one happened much closer to the surface, it would cause more shaking, and so would have a higher intensity.

      An earthquake can only have one magnitude. However, the intensity will vary depending on how far away the quake happens. So if you are nearer the epicentre, you will experience a higher intensity than if you were further away.

      So, for

    • Photo: James Pope

      James Pope answered on 28 Jun 2012:

      Hi Bhavinamistry,

      Magnitude is a measurement from instruments, where as the intensity is the way people feel the Earthquake. A great quesiton though!

    • Photo: Davie Galloway

      Davie Galloway answered on 2 Jul 2012:

      Hi bhavinamistry,

      Magnitude is a measure of earthquake size and remains unchanged with distance from the earthquake.

      Intensity, however, describes the degree of shaking caused by an earthquake at a given place and decreases with distance from the earthquake epicentre.

      We can, therefore talk about a magnitude 5.4 event with an intensity of 6 EMS in the epicentral area, on the Lleyn Peninsula, but intensity 3 EMS at Carlisle.

      Magnitude measurement requires instrumental monitoring for its calculation, however, assigning an intensity requires a sample of the felt responses of the population. This is then graded according to the EMS intensity scale. For example, Intensity 1, Not felt, 2, Scarcely perceptible, 3, weak, felt by a few, up to 12 assigned for total devastation.

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