Earthquake intensity

Macroseismic map of the Dudley Earthquake of 2002.

Intensity is a measure of an earthquake determined from the observed effects, especially damage.

For a given earthquake, intensity normally decreases with distance from the epicentre.

The observations can then be compiled to make macroseismic maps showing lines of equal intensity.

European Macroseismic Intensity Scale

  1. Not felt — detected by seismic instruments only
  2. Very weak — felt by very few people
  3. Weak — felt by a few people indoors
  4. Noticed by many people, windows and doors rattle
  5. Strong — some small objects fall over
  6. Slightly damaging — objects fall off shelves
  7. Damaging — parts of chimneys fall
  8. Very damaging — large cracks in walls
  9. Destructive — some houses collapse
  10. Very destructive — many houses collapse
  11. Devastating — most ordinary buildings collapse
  12. Completely devastating — practically all structures above and below ground are heavily damaged or destroyed
  13. Felt Reports

    Since earthquake intensity can be related to what people feel or experience during the shaking from an earthquake it is possible to use people as "intensity sensors" and collect earthquake felt reports for significant earthquakes.

    Scientists can make use of these felt report surveys to compare the size of recent earthquakes to ones which occurred before modern scientific measurements (from about 1970 in the UK), but for which contemporary written reports are available. In this way earthquake records in the UK can be extrapolated back to the 14th century.

    Should intensity be the true test of seismic power?

    Scientist can now predict the likely intensity of an earthquake's effects as soon as it happens and produce 'shake maps' showing how the shaking will vary from place to place. Maybe it is time more emphasis was put on these intensity values in communicating earthquake size to the general public?

    Pawnee, Oklahoma, shakemap.

     

    Related topics

    Magnitude spheres.Earthquake magnitude

    Magnitude is a number that indicates how much energy an earthquake has released.