Earthquake Measurement Scale

The earliest earthquake measurements were simple descriptions of seismic events and their consequences. These results were unreliable depending on the distance between the earthquake's source (epicenter), and the people evaluating the event.

A more systematic approach was developed by an Italian seismologist, Guiseppe Mercalli in 1902. He gauged earthquake intensity by qualitatively rating observer's experiences and the damage done to structures. The United States Coast and Geodetic Survey adapted his method, which they called the modified Mercalli Scale, dividing the measurements into 12 categories: level II was "felt by persons at rest," but at level VII it was "difficult to stand." Level X caused most buildings to collapse, and level XII, the most intense, combined ground fissures with tsunamis (tidal waves) and almost total destruction. Despite the specific detail of descriptions, Mercalli intensity, like the earlier descriptions, was influenced by the measurement's distance from the earthquake's epicenter, as well as building design, and the character of the bedrock and soils.

Seismologists needed a way to determine the size, or magnitude, of an earthquake. They needed a quantitative, numerical measurement that would compare the strength of earthquakes in a meaningful way, not merely catalog damage or record perceptions as Mercalli's qualitative method did. This critical factor was finally developed in 1935 by American seismologist Charles F. Richter, a professor of seismology at the California Institute of Technology. His system of measurement, called the Richter magnitude scale, was based on his studies of earthquakes in southern California. It has become the most widely used assessment of earthquake severity in the world.

Richter measured ground movement with a seismograph and calculated the magnitude based on the height of the waves on the seismogram (the paper record). He then compared the reading to others taken at various distances from the epicenter and calculated an average magnitude after correcting for distance. The results are plotted on a logarithmic scale, in whole numbers and tenths. The scale is open-ended, meaning there is no upper limit to the magnitude of an earthquake. However, earthquake magnitude, as measured by the Richter magnitude scale, probably cannot exceed 9. Although rarely reported, very small earthquakes may have negative magnitude values. There are several different methods for determining earthquake magnitude and seismologists in different areas use different methods. This is why, in part, you may hear different magnitudes for the same quake, depending on the source of the information.

Each whole number increase in Richter magnitude means that the ground motion of the quake is ten times greater than the previous whole number. Thus, an earthquake with a magnitude of 6.5 has ten times the ground motion of one with a magnitude of 5.5; an earthquake of 7.5 has 100 times the ground motion of the 5.5 earthquake, and so on.

The relative difference in the amount of energy from different earthquakes follows a different pattern. Instead of tenfold jumps with each increase in magnitude, energy released differs in roughly thirtyfold increments. Thus, an earthquake with a value of 7 releases about 30 times the amount of energy as an earthquake measured at 6, while an earthquake of 8 would release about 900 times the energy of a Richter magnitude 6 earthquake.

Today the modified Mercalli intensity scale is still used in combination with the Richter magnitude scale because both methods are helpful in gauging the total impact of an earthquake. Building destruction and people's perceptions are considered important means of measurement for several reasons. Mercalli intensity allows geologists to better understand the role that bedrock and soil type play in the degree of earthquake damage. Deep earthquakes are much less damaging than shallow earthquakes. The type of fault movement (horizontal, vertical, oblique) can be related to earthquake damage as well. Finally, engineers can study how various construction methods and building materials respond to shaking, in hopes of developing more earthquake-resistant structures in the future.

For very damaging earthquakes, those above magnitude 7, Richter magnitude does not accurately express the amount of energy released during an earthquake. Therefore, a new scale, called moment magnitude, has been implemented by seismologists. It is generally the scale used when seismologists report seismic events to news agencies and, therefore, is the scale that is most often reported to the public. The moment magnitude scale takes into consideration the amount of slip on the fault, the area of rupture, and the strength of the rocks that failed, and thereby more correctly expresses the size of the largest earthquakes. Unlike Richter magnitude, moment magnitudes may exceed 9. For the 1960 Chilean earthquake, the strongest earthquake recorded by a seismograph, the Richter magnitude was about 8.5 and the moment magnitude about 9.5. Magnitudes for less powerful earthquakes are very similar to those based on Richter magnitude.