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.
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).
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.
A day after the quake, the editors of The New York Times sought to allay readers’ fear. The quake, they said, was an unexpected fluke never to be repeated and not worth anyone’s attention: “History and the researches of scientific men indicate that great seismic disturbances occur only within geographical limits that are now well defined,” they wrote in an editorial. “The northeastern portion of the United States . . . is not within those limits.” The editors then went on to scoff at the histrionics displayed by New York residents when confronted by the quake: “They do not stop to reason or to recall the fact that earthquakes here are harmless phenomena. They only know that the solid earth, to whose immovability they have always turned with confidence when everything else seemed transitory, uncertain, and deceptive, is trembling and in motion, and the tremor ceases long before their disturbed minds become tranquil.”
Richard Allen's Berkeley group has assembled a set of images and movies illustrating various MMI levels.