Seismic Intensity

The earliest measures were estimates of the number of casualties and the economic costs of events. Eventually, shaking intensity scales were developed to standardize the measurements and ease comparison of different earthquakes. Shaking intensity varied from barely perceptible to completely destructive. Since 1933, in the United States we have used what's called the Modified-Mercalli Intensity scale, a twelve-stage scale, numbered from I to XII. The lower numbers represent imperceptible shaking levels, XII represents total destruction.

A value of IV indicates a level of shaking that is felt by most people. Damage begins at an intensity of about V, and increases with intensity level. Not every country uses the same intensity scale - Japan, for example has a different scale than the United States.

An earthquake produces a range of shaking intensity values - sometimes the maximum intensity reported is used as a measure of the earthquake size. An example intensity map constructed for the Parkfield earthquake is shown below.

Early in the study of earthquakes, intensities were assigned by scientists who traveled to regions that had recently experienced strong earthquakes. Throughout much of the 20th century, postal surveys were conducted to measure intensity. Recently, electronic surveys have been used to map out the felt areas (click here to see examples). Here's the U.S. Geological Survey's 2004 Parkefield Earthquake shakemap - the earthquakes are roughly the same size.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.