Simplified estimation of seismic moment from seismograms

This study proposes a method to estimate the seismic moment of regional and local earthquakes based on simple measurements made directly on Wood-Anderson seismograms. The relation used is log M (sub o) = a + b log(C X D X (super p) ) where C is the maximum peak-to-peak amplitude read on a Wood-Anderson seismogram, D is the duration between the S arrival and the onset with amplitude C/d, is epicentral distance, and a, b, p, and d are constants. The form of the logarithmic term is suggested by the analytical expression for moment (Keilis-Borok, 1960). Least-squares fits were made to data from 73 Wood-Anderson records of 16 central California earthquakes with seismic moments evaluated independently from spectral analysis or broadband displacement rocords. The values p = 1, d, = 3 proved appropriate and subsequent regression yielded log M (sub o) = (16.74 + or - 0.20) + (1.22 + or - 0.14) log(C X D X ) where M (sub o) is in dyne-cm, C in millimeters, D in seconds, and in kilometers. The corresponding moment-magnitude relation is log M (sub o) = (17.92 + or - 1.02) + (1.11 + or - 0.15)M (sub L) , for 3 < or = M (sub L) < or = 6.2. The latter fit is close to an earlier empirical result (Johnson and McEvilly, 1974) for central California based on fewer cases and a different range of magnitude (2.4 < or = M (sub L) < or = 5.1).--Modified journal abstract.