Defining the “Size” of Earthquakes

Defining the “Size” of Earthquakes 

Some earthquakes shake the ground violently, whereas others can barely be felt. Seismologists have developed two scales to define size in a uniform way, so that they can systematically describe and compare earthquakes. The first scale focuses on the severity of damage at a locality and is called the Mercalli Intensity scale. The second focuses on the amount of ground motion at a specific distance from the epicentre, as measured by a seismometer, and is called the magnitude scale.

Mercalli Intensity Scale 

Modified Mercalli Intensity Scale.
The intensity of an earthquake refers to the effect or consequence of an earthquake’s ground shaking at a locality on the Earth’s surface. In 1902, an Italian scientist named Giuseppe Mercalli devised a scale for defining intensity by systematically assessing the damage that the earthquake caused. A version of this scale, called the Modified Mercalli Intensity scale (MMI), with roman numerals, continues to be used today (Table above). 

This map shows Modified Mercalli Intensity contours for the 1886 Charleston, South Carolina, earthquake. Note that near the epicentre, ground shaking reached MMI of X, and in New York City, ground shaking reached MMI of II to III.
Note that the specification of earthquake intensity depends on a subjective assessment of damage, and of the perception of shaking, not on a direct measurement with an instrument. Also, the Mercalli intensity value varies with location for a given seismic event we cannot assign a single Mercalli number to a given earthquake. Typically, the intensity is greater near the epicentre, and decreases progressively away from the epicentre. To illustrate how intensity varies over a region for a given earthquake, seismologists draw contour lines on a map, to delimit zones in which the earthquake had a given intensity (figure above).

Earthquake Magnitude Scales 

When you read a report of an earthquake disaster in the news, you will likely come across a phrase that reads something like, “An earthquake with a magnitude of 7.2 struck the city yesterday at noon.” What does this mean? The magnitude of an earthquake is a number that represents the maximum amplitude of ground motion that would be measured by a seismometer placed at a specified, standard distance from the epicentre. By amplitude of ground motion, we mean the amount of up-and-down or backand-forth motion of the ground. The larger the ground motion, the greater the deflection of a seismometer pen or needle as it traces out a seismogram. Since the magnitude is calculated to represent motion at a standard distance from the epicentre, there is one magnitude for an earthquake a magnitude value does not depend on this distance. 

Using the Richter magnitude scale.
The American seismologist Charles Richter developed a method for defining and measuring earthquake magnitude in 1935. The scale he proposed came to be known as the Richter scale and is based on the maximum amplitude of motion that would be recorded at a station about 100 km from the epicentre. Since there’s not necessarily a seismometer exactly at this distance, Richter developed a simple chart to adjust for distance of the station from the epicentre (figure above a, b). Richter’s scale became so widely used that news reports often include wording such as, “The earthquake registered a 7.2 on the Richter scale.” 
These days, seismologists actually use several different magnitude scales, not just the Richter scale, because the original Richter scale works well only for shallow earthquakes that are close to the seismometer station. Because of the distance limitation, a number on the original Richter scale is now also called a local magnitude (ML). The moment magnitude scale (MW) provides the most accurate representation of an earthquake’s size. To calculate the moment magnitude, seismologists measure the amplitude of several different seismic waves, determine the dimensions of the slipped area on the fault, and estimate the displacement that occurred. The largest recorded earthquake in history, the great 1960 Chilean quake, registered as a 9.5 on the MW scale and the catastrophic 2011 Tohoku earthquake had a magnitude of MW 9.0.

Adjectives for Describing Earthquakes.
All magnitude scales are logarithmic, meaning that an increase of one unit of magnitude represents a tenfold increase in the maximum amplitude of ground motion. Thus, a magnitude 8 earthquake results in ground motion that is 10 times greater than that of a magnitude 7 earthquake, and 1,000 times greater than that of a magnitude 5 earthquake. To make discussion easier, seismologists use familiar adjectives to describe the size of an earthquake, as listed in Table above. 

Energy Release by Earthquakes 

Energy released by earthquakes increases dramatically with magnitude.
As we pointed out earlier, earthquakes release energy. Seismologists can calculate the energy release from equations that relate moment magnitude to energy. Not all versions of this calculation yield the same result, so energy estimates must be taken as an approximation. According to some researchers, a magnitude 6 earthquake releases about as much energy as the atomic bomb that was dropped on Hiroshima in 1945. The 1964 Alaska Good Friday earthquake, during which up to 15 m of slip occurred on a thrust fault, near Anchorage, released significantly more energy than the largest hydrogen bomb ever detonated. Notably, an increase in magnitude by one integer represents approximately a 32-fold increase in energy. Thus, a magnitude 8 earthquake releases about 1 million times more energy than a magnitude 4 earthquake (figure above). In fact, a single magnitude 8.9 earthquake releases as much energy as the entire average global annual release of seismic energy coming from all other earthquakes combined! Fortunately, such large earthquakes occur much less frequently than small earthquakes. There are about 100,000 magnitude 3 earthquakes every year, but a magnitude 8 earthquake happens only about once or twice a year.
Credits: Stephen Marshak (Essentials of Geology)