Comparison of the maximum amplitudes recorded at different epicentral distances by the torsion seismometers of the Southern California group makes it possible to rate earthquakes in this region in terms of a magnitude scale. The magnitude assigned is characteristic of the shock as a whole; it thus differs from the intensity, which varies from point to point of the affected area.
The magnitude of a shock is defined as the logarithm of the calculated maximum trace amplitude, expressed in microns, with which the standard short-period torsion seismometer (T0) = 0.8, V = 2,800, h = 0.8) would register that shock at an epicentral distance of 100 kilometers.
Shock magnitudes can be assigned in routine to the nearest half-unit, and in cases specially studied to the nearest tenth. The smallest shocks recorded are of magnitude 0; the smallest reported felt are of magnitude 1.5. Shocks of magnitude 3 are perceptible over an area some 20 kilometers in radius; those of magnitude 4.5 are capable of causing slight damage near the epicenter; those of magnitude 6 are destructive over a restricted area; those of magnitude 7.5 are at the lower limit of major earthquakes.
Representative shocks of various magnitudes are discussed. The Santa Monica Bay earthquake of August 30, 1930, is assigned magnitude 5.2; the Long Beach earthquake of March 10, 1933, 6.2; the Utah earthquake of March 12, 1934, 7.0; the Nevada earthquake of December 20, 1932, 7.5.
The amplitude registered by the torsion seismometer at the lower limit of perceptibility to persons is found to be about 50 millimeters. With a period of 0.6 of a second, which applies at moderate distances, this corresponds to an acceleration of 250 milligals. At larger distances the period of the maximum waves increases, and perceptibility decreases more rapidly than amplitude of registration.
Beyond 200 kilometers the maximum registered amplitude for a given shock appears to vary nearly as the inverse cube of the epicentral distance.
The maximum range to which the torsion seismometer usefully records shocks of various magnitudes is discussed; shocks of magnitude 2.5 and over should be detected within 500 kilometers of Pasadena.
Statistics of the frequency of shocks of various magnitudes are given. It is found that the seismic energy liberated in a given region during a given period is almost wholly accounted for by the larger shocks; the smaller shocks are not sufficiently frequent to contribute more than a small fraction of this energy. It follows that the smaller shocks do not appreciably mitigate the strains which are released in the larger earthquakes, but must be regarded as minor incidents in and symptoms of the accumulation of such strains.