Key difference: The Richter magnitude scale is a scale that assigns earthquakes a number between 1 and 10 in order of increasing intensity. The Mercalli intensity scale is another seismic scale. It labels an earthquake from I to XII depending on the effects of the earthquake.

Earthquakes are one of the most dangerous disasters that humans have to face. However, most earthquakes are actually not even noticeable. There are in fact millions of earthquakes that occur in a year. Of those, only 1.3 million are even noticeable by humans, at least according to the U.S. Geological Survey. Most of these actually occur in remote areas far from people and are often so low in intensity that most people would not notice.

There are many reasons why earthquakes occur. They range from meteor impacts and volcanic eruptions, to man-made events like mine collapses and underground nuclear tests. However, the most common reason for earthquakes are the shifting of the Earth’s tectonic plates.

As earthquakes can be quite devastating, humans needed to find a way to learn when and where earthquakes would occur. In order to do this, the seismograph was developed. A seismograph is any instrument that measures the motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. The records of the seismic waves helps seismologists to map the interior of the Earth, and to locate and measure the different sources of earthquakes.

Now, that the scientists know when and where the earthquakes would occur, they next needed a way to classify the earthquakes in order of how dangerous they were. Hence, the concept of a scale developed which would assign a number or category to the earthquake depending on its intensity. The Richter Scale and Mercalli Scale are two such scales.

The Richter magnitude scale is a scale that assigns earthquakes a number between 1 and 10 in order of increasing intensity. The scale was developed in 1935 by Charles Francis Richter in partnership with Beno Gutenberg. They both hailed from the California Institute of Technology. The scale was originally was to be used to only in a particular study area in California, and on seismograms recorded on only the Wood-Anderson torsion seismograph. However, eventually the scale was developed into a worldwide accepted standard.

The Richter scale assigns the number on the basis of how much energy is released during the earthquake. The scale is a base-10 logarithmic scale, which means that an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0, and corresponds to a 31.6 times larger release of energy. While the scale is usually considered to label from 1 to 10, and 0 being the basis to which the energy is being compared, the truth is that the scale does not actually have a lower limit. Many sensitive modern seismographs now routinely record quakes with negative magnitudes.

The Mercalli intensity scale is another seismic scale. It labels an earthquake from I to XII depending on the effects of the earthquake. It differs from the Richter scale, as the Richter scale measures the amount of energy that is released during an earthquake, whereas the Mercalli scale measures the amount of damage that the earthquake is capable of. 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).”

The Mercalli intensity scale was originally revised from the simple ten-degree Rossi-Forel scale by Italian volcanologist, Giuseppe Mercalli in 1884 and 1906. In 1902, Italian physicist Adolfo Cancani expanded the Mercalli scale from ten degree to a twelve-degree classification. The German geophysicist August Heinrich Sieberg then completely re-wrote the scale and it became known as the Mercalli-Cancani-Sieberg (MCS) scale, which was modified and published in English by Harry O. Wood and Frank Neumann in 1931 as the Mercalli-Wood-Neumann (MWN) scale. The scale was then improved by Charles Richter, the father of the Richter magnitude scale. Today, the scale is known simply, and perhaps rightly, as Modified Mercalli scale (MM) or Modified Mercalli Intensity scale (MMI).

Classification of the Modified Mercalli Intensity Scale:

I - Instrumental

II - Weak

III - Slight

IV - Moderate

V - Rather Strong

VI - Strong

VII - Very Strong

VIII - Destructive

IX - Violent

X - Intense

XI - Extreme

XII - Catastrophic

Comparison between Richter Scale and Mercalli Scale:

Classification of Earthquakes as per the Richter Scale and Mercalli Scale:

Image Courtesy: alekwalker.blogspot.com, dnr.mo.gov