In natural earthquakes, it is well known that generally small earthquakes occur frequently, but large earthquakes occur rarely. Gutenberg and Richter (1944) found the following statistical formula about the relation between the magnitude of earthquakes and their frequency:

log N = c + b(8 − M ) (6.1) where N is the number of earthquakes and M is the magnitude, introduced by Richter (1935) based on the records made by seismographs.