Background: The Modified Mercalli Intensity Scale

Several hundred years ago, before instruments for measuring earthquakes were developed, scientists observed that the amount of shaking felt by people and amount of damage to structures depended on their location. For big earthquakes in which the ground ruptured, the strongest shaking and the most damage were observed close to the rupture. In many smaller earthquakes, the ground did not rupture, but the strongest shaking and damage were still concentrated. Scientists developed a scale to quantify an earthquake's shaking by what was felt and by its effects on structures and the landscape. The most widely used scale is the Modified Mercalli Intensity Scale (MMI scale). It was originally introduced by the Italian seismologist Giuseppe Mercalli in 1902, and was modified and updated for "modern" building practices in 1931. It has twelve levels describing the strength of shaking.

There are two basic differences between the intensity of an earthquake and its magnitude. The magnitude of an earthquake is based on measurements from instruments, so it is objective. Intensity is a subjective measure. It is based on the observations and descriptions of people, those living in the area where the earthquake occurred and also the engineers or scientists estimating the damage to structures. The second difference is that an earthquake has only one magnitude, while its intensity will be different at the different locations. A single earthquake will usually generate a whole range of intensities. The values generally decrease as the distance from the epicenter increases.

How do scientists use intensity? People report their observations and experience. For example, after the Loma Prieta earthquake in 1989, USGS scientists sent out hundreds of postcards to people in the Bay Area and asked where they were at the time of the earthquake and what they experienced. Nowadays, if you experience an earthquake, you can report it online on the USGS "Did you feel it?" webpage. From the reports and observations they collect, scientists estimate the intensity for each location, and plot it on a map. For example, here are intensity maps for the most recent earthquakes in California.

Even before seismometers were invented, scientists collected reports from earthquakes and determined the epicentral region from where the shaking and damage were greatest. In some cases, only one intensity is listed for an earthquake. Then it is the maximum value that was observed. For example, the recent San Simeon earthquake before Christmas 2003 had a maximum intensity of VIII.

Intensity is usually given in Roman numerals in order to avoid confusion with magnitudes.

Click to open the Modified Mercalli Intensity Scale descriptions in a new window. Keep the table open and refer to it as you complete this exercise. You may wish to print a copy.

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