Magnitude and Intensity of Earthquake

What is the difference between magnitude and intensity?

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 ML (Moment Magnitude) event with intensity of 6 EMS European macroseismic scale) 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.

Study of intensity and the production of isoseismal maps, contouring areas of equal intensity, is particularly important for the study of earthquakes which occurred prior to instrumental monitoring.

The article above was taken from British Geological Survey FAQ webpage.