Magnitude vs. Intensity

From time to time, the words magnitude and intensity are used interchangeably in writings on earthquakes. Although both terms are used to express the severity of an earthquake, they refer to two different scales.

The Richter Scale of magnitude measures an earthquake's energy at its source. This is the most commonly-used scale, but often the most misunderstood. The magnitude of an earthquake, as measured by the Richter scale, is expressed in whole numbers and decimals. Each whole number step of magnitude on the scale represents an increase of 10 times in the measured wave amplitude. Thus, an 8.3 magnitude earthquake is not twice as large as an earthquake of 4.3 magnitude, but 10,000 times as large.

The Modified Mercalli Intensity Scale measures the intensity of an earthquake's effects in a given locale, and is perhaps much more meaningful to the layman because it is based on actual observation of earthquake effects in specific places. The scale contains 12 categories ranging from I (not felt except by a very few under especially favorable circumstances) to XII (total damage - practically all works of construction are greatly damaged or destroyed.) Waves are seen on ground surface and lines of sight and level are distorted. Objects are thrown upward into the air.

It is difficult to compare magnitude and intensity because magnitude is a measure of the energy released at the focus of the earthquake while intensity is linked with the particular ground and structural conditions of a given area, as well as the distance from the epicenter of the earthquake.

Adapted from UNDRO News