Seismologists use a variety of methods to determine two fundamentally different measures that describe the size of an earthquake: intensity and magnitude. An intensity scale uses reported damages and effects at a certain location to estimate the amount of ground shaking at a particular location. Magnitude scales, which were developed more recently, use data from seismographs to estimate the amount of energy released at an earthquake’s source.

Intensity Scales

Until the mid-1800s, historical records provided the only accounts of the severity of earthquake shaking and destruction. Perhaps the first attempt to scientifically describe the aftermath of an earthquake came following the 1857 earthquake that hit Basilicata, Italy. By systematically mapping effects of the earthquake, Irish geophysicist Robert Mallet established a measure of the intensity of ground shaking. The map generated by this study used lines to connect places of equal damage and hence equal ground shaking. Using this technique, zones of intensity were identified, with the zone of highest intensity representing the location of maximum ground shaking, which is often (but not always) found surrounding the earthquake epicenter.

In 1902, Giuseppe Mercalli, an Italian volcanologist, developed a more reliable intensity scale, which is still used today in a modified form. The Modified Mercalli Intensity Scale, shown in Table 1, takes into account observations of common phenomena experienced by people during an earthquake and is based on a 12-point scale (represented by roman numerals, where I is the least intense and XII is the most intense).

More recently, the U.S. Geological Survey (USGS) has developed a website called “Did You Feel It?” a citizen science program where online users experiencing a quake can enter their geographic locations and answer questions, such as “Did objects rattle, topple over, or fall off shelves?” From the data collected, a Community Internet Intensity Map, like the one in Figure 16 for the 2011 central Virginia earthquake (M 5.8), is generated. As shown in Figure 5.16, shaking strong enough to be felt was reported from Maine to Florida, an area occupied by one-third of the U.S. population. Several national landmarks were damaged, including the Washington Monument and the National Cathedral, located about 130 kilometers (80 miles) away from the epicenter. Programs like “Did you Feel it?” help provide rapid information for disaster management, confirm shaking experiences for others, and contribute to the advance of earthquake science.

To more accurately compare earthquakes around the globe, scientists searched for a way to describe the energy released by earthquakes using geological factors, such as ground displacement, as opposed to other factors, such as building practices, which vary considerably worldwide. As a result, several magnitude scales were developed.

Richter Magnitude

In 1935, Charles Richter of the California Institute of Technology developed the first magnitude scale using seismic records, called the Richter scale. As shown in Figure 17, the Richter magnitude (ML, where “L” means “local”) of an earthquake is calculated by measuring the amplitude of the largest seismic wave recorded on a seismogram at a specific distance from the earthquake. Because seismic waves weaken as the distance between the hypocenter and the seismograph increases, Richter developed a method that accounts for the decrease in wave amplitude with increasing distance, which meant that monitoring stations at different locations could obtain the same Richter magnitude for each recorded earthquake.

Earthquakes vary enormously in strength, and great earthquakes produce wave amplitudes thousands of times larger than those generated by weak tremors. To accommodate this wide variation, Richter used a logarithmic scale to express magnitude, in which a 10-fold increase in wave amplitude corresponds to an increase of 1 on the magnitude scale. Thus, the intensity of ground shaking for a magnitude 5 earthquake is 10 times greater than that produced by an earthquake having a Richter magnitude of 4 (Figure 18).

In addition, each unit of increase in Richter magnitude equates to roughly a 32-fold increase in the energy released. Thus, an earthquake with a magnitude of 6.5 releases 32 times more energy than one with a magnitude of 5.5 and roughly 1000 times  more energy than a magnitude 4.5 quake. A major earthquake with a magnitude of 8.5 releases millions of times more energy than the smallest earthquakes felt by humans (refer to Figure 18).

The convenience of describing the size of an earthquake by a single number that can be calculated quickly from seismograms made the Richter scale a powerful tool. However, the scale was originally designed for earthquakes occurring in Southern California, making it less reliable for earthquakes occurring in other areas with different geological characteristics and at distances far from a recording seismograph. Further, the Richter scale is not adequate for describing very large earthquakes. For example, the 1906 San Francisco earthquake and the 1964 Alaska earthquake had roughly the same Richter magnitudes. However, based on the relative size of the affected areas and the associated tectonic changes, the Alaska earthquake released considerably more energy than the San Francisco quake. Thus, the Richter scale is considered saturated (or maxed out) for major earthquakes because it cannot distinguish among them in terms of actual energy release.

Moment Magnitude

To address these limitations, seismologists developed a scale called moment magnitude (Mw), which estimates the total energy released during an earthquake. Moment magnitude for an earthquake is calculated by determining the average amount of slip on the fault, the area of the fault surface that slipped, and the strength (rigidity) of the faulted rock.

Moment magnitude is calculated by modeling seismic data obtained from global networks of seismographs. The results are converted to a magnitude number on a logarithmic scale, and as with the Richter scale, each unit increase on the moment magnitude scale equates to a 10-fold increase in seismic wave amplitude and roughly a 32-fold increase in the energy released (Figure 19).

Because the moment magnitude scale is better than the Richter scale at estimating the relative size of very large earthquakes, seismologists have used it to recalculate the magnitudes of older strong earthquakes (refer to Figure 19). For example, the 1964 Alaska earthquake, originally given a Richter magnitude of 8.4, has since been recalculated using the moment magnitude scale, resulting in an upgrade to Mw 9.2, and it is estimated this earthquake had a fault rupture length of 700 kilometers (435 miles). Conversely, the 1906 San Francisco earthquake’s Richter magnitude of 8.3 was downgraded to Mw 7.9, and its fault rupture length is estimated to be 250 kilometers (155 miles). The strongest earthquake on record is the 1960 Chilean megathrust earthquake, at Mw 9.5 and a fault rupture of nearly 1000 kilometers (620 miles).

• Intensity and magnitude are different measures of earthquake strength. Intensity measures the amount of ground shaking at a location due to an earthquake, and magnitude is an estimate of the amount of energy released during an earthquake.

• The Modified Mercalli Intensity Scale is a tool for measuring an earthquake’s intensity at different locations. The scale is based on verifiable physical evidence that is used to quantify intensity on a 12-point scale.

• The Richter scale takes into account both the maximum amplitude of the seismic waves measured at a given seismograph and that seismograph’s distance from the earthquake. The Richter scale is logarithmic, meaning that the next higher number on the scale represents seismic amplitudes that are 10 times greater than those represented by the number below. Furthermore, each larger number on the Richter scale represents the release of about 32 times more energy than the number below it.

• Because the Richter scale does not effectively differentiate between very large earthquakes, the moment magnitude scale was devised. This scale measures the total energy released from an earthquake by considering the strength of the faulted rock, the amount of slippage, and the area of the fault that slipped. Moment magnitude is the modern standard for measuring the size of earthquakes.

• intensity scale: A categorical scale measuring the degree of shaking at a given location that experiences an earthquake, based upon the amount of damage that occurs.

• magnituce scales: Numerical scales used to objectively measure the total amount of energy released during an earthquake.

• Modified Mercalli Intensity Scale: A 12-point scale developed to evaluate earthquake intensity based on the amount of damage to various structures.

• moment magnitude scale: A more reliable measure of earthquake magnitude than the Richter scale that is derived from the amount of displacement that occurs along a fault zone during an earthquake.

• Richter scale: A logarithmic scale of earthquake magnitude based on the motion of a seismograph.