The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude
Contents
- Modified Mercalli Intensity scale
- Correlation with magnitude
- Correlation with physical quantities
- Comparison to the moment magnitude scale
- References
The Mercalli scale quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction). Values depend upon the distance from the earthquake, with the highest intensities being around the epicentral area. Data gathered from people who have experienced the quake are used to determine an intensity value for their location. The Italian volcanologist Giuseppe Mercalli revised the widely used simple ten-degree Rossi–Forel scale between 1884 and 1906, creating the Mercalli Intensity scale which is still used nowadays.
In 1902, the ten-degree Mercalli scale was expanded to twelve degrees by Italian physicist Adolfo Cancani. It was later completely re-written by the German geophysicist August Heinrich Sieberg and became known as the Mercalli–Cancani–Sieberg (MCS) scale.
The Mercalli–Cancani–Sieberg scale was later modified and published in English by Harry O. Wood and Frank Neumann in 1931 as the Mercalli–Wood–Neumann (MWN) scale. It was later improved by Charles Richter, the father of the Richter magnitude scale.
The scale is known today as the Modified Mercalli scale (MM) or Modified Mercalli Intensity scale (MMI).
Modified Mercalli Intensity scale
The lower degrees of the Modified Mercalli Intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage.
This table gives Modified Mercalli scale intensities that are typically observed at locations near the epicenter of the earthquake.
Correlation with magnitude
The correlation between magnitude and intensity is far from total, depending upon several factors including the depth of the hypocenter, terrain, distance from the epicenter. For example, on May 19, 2011, an earthquake of magnitude 0.7 in Central California, United States, 4 km deep was classified as of intensity III by the United States Geological Survey (USGS) over 100 miles (160 km) away from the epicenter (and II intensity almost 300 miles (480 km) from the epicenter), while a 4.5 magnitude quake in Salta, Argentina, 164 km deep was of intensity I.
The small table is a rough guide to the degrees of the Modified Mercalli Intensity scale. The colors and descriptive names shown here differ from those used on certain shake maps in other articles.
Correlation with physical quantities
The Mercalli scale is not defined in terms of more rigorous, objectively quantifiable measurements such as shake amplitude, shake frequency, peak velocity, or peak acceleration. Human-perceived shaking and building damages are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events.
Comparison to the moment magnitude scale
The effects of any one earthquake can vary greatly from place to place, so there may be many Mercalli intensity values measured for the same earthquake. These values can be best displayed using a contoured map of equal intensity, known as an isoseismal map. However, each earthquake has only one magnitude.