A technique is presented for determination of local magnitude, ML, from strong-motion accelerograms. The accelerograph records are used as an acceleration input to the equation of motion of the Wood-Anderson torsion seismograph to produce a synthetic seismogram which is then read in the standard manner. When applied to 14 records from the San Fernando earthquake, the resulting ML is 6.35, with a standard deviation of 0.26. This is in good agreement with the previously reported value of 6.3. The technique is also applied to other earthquakes in the western United States for which strong-motion records are available. An average value of ML = 7.2 is obtained for the 1952 Kern County earthquake; this number is significantly smaller than the commonly used value of 7.7, which is more nearly a surface-wave magnitude.
The method presented broadens the base from which ML can be found and allows ML to be determined in large earthquakes for which no standard assessment of local magnitude is possible. In addition, in instances where a large number of accelerograms are available, reliable values of ML can be determined by averaging.