Defining the “Size” of EarthquakesSome earthquakes shake the ground violently, whereas others can barely be felt. Seismologists havedeveloped two scales to define size in a uniform way, so that they can systematically describe andcompare earthquakes. The first scale focuses on the severity of damage at a locality and is called theMercalli Intensity scale. The second focuses on the amount of ground motion at a specific distancefrom the epicentre, as measured by a seismometer, and is called the magnitude scale.

Mercalli Intensity Scale

The intensity of an earthquake refers to the effect or consequence of an earthquake’s ground shaking ata locality on the Earth’s surface. In 1902, an Italian scientist named Giuseppe Mercalli devised a scalefor defining intensity by systematically assessing the damage that the earthquake caused. A version ofthis scale, called the Modified Mercalli Intensity scale (MMI), with roman numerals, continues to beused 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, andof the perception of shaking, not on a direct measurement with an instrument. Also, the Mercalliintensity value varies with location for a given seismic event we cannot assign a single Mercallinumber to a given earthquake. Typically, the intensity is greater near the epicentre, and decreasesprogressively 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 agiven 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 thatreads something like, “An earthquake with a magnitude of 7.2 struck the city yesterday at noon.” Whatdoes this mean? The magnitude of an earthquake is a number that represents the maximum amplitudeof ground motion that would be measured by a seismometer placed at a specified, standard distancefrom 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 seismometerpen or needle as it traces out a seismogram. Since the magnitude is calculated to represent motion at astandard distance from the epicentre, there is one magnitude for an earthquake a magnitude value doesnot depend on this distance.The American seismologist Charles Richter developed a method for defining and measuring earthquakemagnitude in 1935. The scale he proposed came to be known as the Richter scale and is based on themaximum 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 toadjust for distance of the station from the epicentre (figure above a, b). Richter’s scale became sowidely used that news reports often include wording such as, “The earthquake registered a 7.2 on theRichter 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 theseismometer station. Because of the distance limitation, a number on the original Richter scale is nowalso called a local magnitude (ML). The moment magnitude scale (MW) provides the most accuraterepresentation of an earthquake’s size. To calculate the moment magnitude, seismologists measure theamplitude 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 1960Chilean quake, registered as a 9.5 on the MW scale and the catastrophic 2011 Tohoku earthquake had amagnitude of MW 9.0.

Using the Richter magnitude scale.

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

Adjectives for Describing Earthquakes.

Energy Release by Earthquakes

As we pointed out earlier, earthquakes release energy. Seismologists can calculate the energy releasefrom equations that relate moment magnitude to energy. Not all versions of this calculation yield thesame result, so energy estimates must be taken as an approximation. According to some researchers, amagnitude 6 earthquake releases about as much energy as the atomic bomb that was dropped onHiroshima in 1945. The 1964 Alaska Good Friday earthquake, during which up to 15 m of slipoccurred on a thrust fault, near Anchorage, released significantly more energy than the largest hydrogenbomb 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 energythan a magnitude 4 earthquake (figure above). In fact, a single magnitude 8.9 earthquake releases asmuch energy as the entire average global annual release of seismic energy coming from all otherearthquakes 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 8earthquake happens only about once or twice a year.

Energy released by earthquakes increases dramatically with magnitude.

Credits: Stephen Marshak (Essentials of Geology)