RICHTER SCALE

widely used quantitative measure of the magnitude of an earthquake, introduced in 1935 by the seismologists Beno Gutenberg and Charles Francis Richter (q.v.). The scale was originally devised to measure the magnitude of local earthquakes in southern California as recorded by a certain kind of seismograph. The weakest earthquakes that had been detected up to that time were assigned scale values close to zero, and the scale was arranged so that each increase of one unit represents a 10-fold increase in the magnitude of an earthquake (i.e., the numbers on the Richter scale are proportional to the common logarithms of the magnitudes). Later and more sensitive seismographs can detect earthquakes even fainter than the ones that were originally chosen to define magnitude zero; their magnitudes are accommodated on the Richter scale by the use of negative numbers. Although the scale has no theoretical upper limit, the largest earthquakes have not exceeded a scale value of 9. The scale was refined by Richter and Beno Gutenberg to remove the restrictions of distance and type of seismograph used.