The earliest earthquake measurements were simple descriptions called intensity ratings. These results were unreliable depending on the distance between the quake'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 measuring the damage done to buildings. 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, this method, like the intensity ratings, was influenced by the measurement's distance from the earthquake's epicenter. 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 determined 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 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 , compared the reading to others taken at various distances from the epicenter, then calculated an average magnitude from all reports. The results are plotted on a logarithmic scale, in whole numbers and tenths, from 1 to 9. Each whole number increase means that the magnitude 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 force of one with a magnitude of 5.5; an earthquake of 7.5 has 100 times the intensity of the 5.5 earthquake. An 8.5 measurement is 1,000 times stronger, and so on.
The amount of energy an earthquake releases is calculated in a different manner. Instead of tenfold jumps with each increase in magnitude, energy released is measured in roughly thirtyfold increments. Thus, an earthquake with a value of 7 releases 30 times the amount of energy as an earthquake measured at 6, while an earthquake of 8 would have 900 times the energy as one valued at 6.
Today the modified Mercalli scale is often used in combination with the Richter scale because both methods are helpful in gauging the total impact of an earthquake.
See also Seismology
Rich·ter scale / ˈriktər/ • n. Geol. a numerical scale for expressing the magnitude of an earthquake on the basis of seismograph oscillations. The more destructive earthquakes typically have magnitudes between about 5.5 and 8.9; the scale is logarithmic and a difference of one represents an approximate thirtyfold difference in magnitude.