In 1935, seismologists Charles Francis Richter and Beno Gutenberg of the California Institute of Technology developed a scale, later coined as the “Richter magnitude scale” with the purpose of computing the magnitude of earthquakes, specifically those recorded with the Wood-Anderson torsion seismograph in a particular area of California. Initially, Richter reported mathematical values to the nearest quarter of a unit, but the values later were reported with one decimal place; the local magnitude scale compared the magnitudes of different earthquakes. Surprisingly, Richter derived his earthquake-magnitude scale from the apparent magnitude scale used to measure the brightness of stars.
Richter established a magnitude 0 event to be an earthquake that would show a maximum, combined horizontal displacement of 1.0 µm (3.9×10−5 in) on a seismogram recorded with a Wood-Anderson torsion seismograph 100 km (62 mi) from the earthquake epicenter. That fixed measure was chosen to avoid negative values for magnitude, given that the slightest earthquakes that could be recorded and located at the time were around magnitude 3.0. The Richter magnitude scale itself has no lower limit, and contemporary seismometers can register, record, and measure earthquakes with negative magnitudes.
Later, to express the size of earthquakes around the planet, Gutenberg and Richter developed a surface wave magnitude scale and a body wave magnitude scale. These are types of waves that are recorded at teleseismic distances. The two scales were adjusted such that they were consistent with the scale. Each scale saturates when the earthquake is greater than magnitude 8.0.
Because of this, researchers in the 1970s developed the moment magnitude scale. The older magnitude-scales were superseded by methods for calculating the seismic moment, from which was derived the moment magnitude scale.
The Richter scale was defined in 1935 for particular circumstances and instruments; the particular circumstances refer to it being defined for Southern California and “implicitly incorporates the attenuative properties of Southern California crust and mantle.” The particular instrument used would become saturated by strong earthquakes and unable to record high values. The scale was replaced in the 1970s by the moment magnitude scale); for earthquakes adequately measured by the Richter scale, numerical values are approximately the same.
The following describes the typical effects of earthquakes of various magnitudes near the epicenter. They should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, the depth of the earthquake’s focus beneath the epicenter, the location of the epicenter and geological conditions (certain terrains can amplify seismic signals).
The intensity and death toll depend on several factors (earthquake depth, epicenter location, population density, to name a few) and can vary widely.
Minor earthquakes occur every day and hour. On the other hand, great earthquakes occur once a year, on average. The largest recorded earthquake was the Great Chilean earthquake of May 22, 1960, which had a magnitude of 9.5 on the moment magnitude scale. The larger the magnitude, the less frequently the earthquake happens.
Beyond 9.5, while extremely strong earthquakes are theoretically possible, the energies involved rapidly make such earthquakes on Earth effectively impossible without an extremely destructive source of external energy. For example, the asteroid impact that created the Chicxulub crater and caused the mass extinction that may have killed the dinosaurs has been estimated as causing a magnitude 13 earthquake , while a magnitude 15 earthquake could destroy the Earth completely. Seismologist Susan Hough has suggested that 10 may represent a very approximate upper limit, as the effect if the largest known continuous belt of faults ruptured together (along the Pacific coast of the Americas).