A Seismic Moment Magnitude Scale

The aim of obtaining a single scale for earthquake magnitudes has led many studies in the past to either develop relationships among various existing scales or develop an altogether new scale to represent a wide range of magnitudes on a single scale. Although a reliable and standardized estimation of earthquake size is a basic requirement for all tectonophysical and engineering applications, different magnitude scales estimate different values for the same earthquake, thereby making such studies inadequate. The moment magnitude (M-w) scale has been referred to by various researchers as the best scale, one that matches well with the observed surface-wave magnitudes with M-s >= 7.5 at a global level. The formulation and validation of the M-w scale were carried out considering the southern California region for lower and intermediate earthquakes. In this study, an endeavor has been made to extend the moment magnitude scale to include lower and intermediate magnitudes in a global context emphasizing the use of body waves, particularly P waves, in which data are abundant. We first investigate the degree of closeness of M-w values with other observed magnitudes (e.g., M-s and m(b)) for smaller and intermediate magnitude ranges considering global International Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) databases. To improve upon the consistency of the M-w scale for a wider range, a uniform generalized seismic moment magnitude scale M-wg = log M-0/1.36 - 12.68, for magnitudes >= 4.5, has been developed, considering 25,708 global earthquake events having m(b) and M-0 values from ISC and Global CMT databases, respectively, during the period 1976-2006. The M-wg scale is also valid for 3.5 <= m(b) <= 7.0 because the relations between seismic moment and the magnitudes m(b) and M-wg are same. The greater accuracy of the M-wg scale over the M-w scale at different magnitudes (i.e., m(b) or M-s) is found to be statistically significant in the range including smaller and intermediate events. The similarity of the M-wg scale is also tested on 394 global seismic radiated energy values collected from Choy and Boatwright (1995). It is observed that 76% of estimated radiated energy values obtained through the M-wg scale show closer agreement (than with M-w) to the observed radiated energy values. M-wg is computed from low- and high-frequency spectra, and because it is consistent for small, intermediate, and large earthquake events, it will play a useful role as an earthquake magnitude estimator for all earthquake related studies.