Seismic Intensity
Seismic intensity scales are used to measure the intensity of an earthquake. A seismic intensity scale measures the effects of an earthquake, and is distinct from earthquake magnitude scales, usually reported for an earthquake in the United States. Earthquake Magnitude scales can be thought of as a measure of the total energy released by the earthquake. The intensity of an earthquake varies by sites, and is not totally determined by its magnitude.
The seismic scale most commonly used in the United States is the Mercalli Intensity scale (MMI). MMI quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures. The USGS version of the MMI scale ranges from I (not felt) to X+ (very violent shaking). Values depend upon the distance to the earthquake, with the highest intensities often being around the epicentral area. Data gathered from people who have experienced the quake are used to determine an intensity value for their location.
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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.
