Determining the Size of an Earthquake
Distinguish between intensity scales and magnitude scales.
Seismologists use a variety of methods to determine two fundamentally different measures that describe the size of an earthquake: intensity and magnitude. The first of these to be used was intensity—a measure of the amount of ground shaking at a particular location, based on observed property damage. Later, the development of seismographs allowed scientists to measure ground motion using instruments. This quantitative measurement, called magnitude, relies on data gleaned from seismic records to estimate the amount of energy released at an earthquake’s source.
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 great Italian earthquake of 1857. By systematically mapping effects of the earthquake, a measure of the intensity of ground shaking was established. 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 located near the center of maximum ground shaking and often (but not always) the earthquake epicenter.
In 1902, Giuseppe Mercalli developed a more reliable intensity scale, which is still used today in a modified form. The Modified Mercalli Intensity scale, shown in Table 1, was developed using California buildings as its standard. For example, on the 12-point Mercalli Intensity scale, when some well-built wood structures and most masonry buildings are destroyed by an earthquake, the affected area is assigned a Roman numeral X (10).
More recently, the U.S. Geological Survey has developed a webpage called “Did You Feel It,” where Internet users enter their zip code and answer questions such as “Did objects fall off shelves?” Within a few hours, a Community Internet Intensity Map, like the one in Figure 1 for the 2011 central Virginia earthquake (M5.8), is generated. Shaking 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. Because the crustal rocks east of the Rocky Mountains are cool and strong, earthquakes are felt over a much larger area than those of similar magnitudes in the west (see Figure 1).
To more accurately compare earthquakes around the globe, scientists searched for a way to describe the energy released by earthquakes that did not rely on factors such as building practices, which vary considerably from one part of the world to another. As a result, several magnitude scales were developed.
In 1935 Charles Richter of the California Institute of Technology developed the first magnitude scale to use seismic records. As shown in Figure 2 (top), the Richter scale is calculated by measuring the amplitude of the largest seismic wave (usually an S wave or a surface wave) recorded on a seismogram.
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.
Theoretically, as long as equivalent instruments are used, monitoring stations at different locations will obtain the same Richter magnitude for each recorded earthquake.
In practice, however, different recording stations often obtain slightly different magnitudes for the same earthquake— a result of the variations in rock types through which the waves travel.
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 (ML) of 4 (Figure 3).
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 (32 × 32) 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 (Figure 4).
The convenience of describing the size of an earthquake by a single number that can be calculated quickly from seismograms makes the Richter scale a powerful tool. Seismologists have since modified Richter’s work and developed other Richter-like magnitude scales.
Despite its usefulness, the Richter scale is not adequate for describing very large earthquakes. For example, the 1906 San Francisco earthquake and the 1964 Alaska earthquake have 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 for major earthquakes because it cannot distinguish among them. Despite this shortcoming, Richter-like scales are still used because they can be calculated quickly.
For measuring medium and large earthquakes, seismologists now favor a newer scale, called moment magnitude (MW), which measures the total energy released during an earthquake. Moment magnitude is calculated by determining the average amount of slip on the fault, the area of the fault surface that slipped, and the strength of the faulted rock.
Moment magnitude can also be calculated by modeling data obtained from seismograms. The results are converted to a magnitude number, as in other magnitude scales. As with the Richter scale, each unit increase in moment magnitude equates to roughly a 32-fold increase in the energy released.
Because moment magnitude estimates the total energy released, it is better than the Richter scale for measuring very large earthquakes. Seismologists have used the moment magnitude scale to recalculate the magnitudes of older strong earthquakes. For example, the 1964 Alaska earthquake, originally given a Richter magnitude of 8.3, has since been recalculated using the moment magnitude scale, resulting in an upgrade to MW 9.2. Conversely, the 1906 San Francisco earthquake that was given a Richter magnitude of 8.3 was downgraded to MW 7.9. The strongest earthquake on record is the 1960 Chilean subduction zone earthquake, with a moment magnitude of 9.5.