Relationships between earthquake magnitude, source geometry and slip mechanism

Abstract

The empirical relationships between source area, source length, source width and magnitude ( M S) are investigated for globally distributed earthquakes of different slip mechanism types. A total of nearly 400 events having magnitudes approximately between 4.5 and 8.5 were considered. The analyses of specially selected events outlined that the regional setting has no influence on the relationships between M S and source dimensions, as well as between rupture length and width; instead they depend on the slip mechanism type (strike-slip, reverse and normal, oblique-slip). As mechanisms change from strike-slip to oblique-slip, and further from oblique-slip to reverse and normal slip, sources of similar length size exhibit increasing magnitudes, in steps of 0.8–1.6 in the overall mentioned spectrum of the mechanism types, whereas sources of similar areas correspond to the different slip-type events with magnitudes increasing in steps of 0.5–0.8. Source widths increase by about three times for earthquakes of equal rupture length, when the mechanism changes from strike-slip to reverse and normal slip. Therefore, it can be concluded that sources of equal length have different areas and length/width ratios for various types of mechanism. Accordingly, stress drop may be larger for strike-slip sources than for sources with prevailing vertical displacement. The regressions of rupture dimensions on surface-wave magnitude for different single slip mechanism types, restricted in most cases to the magnitude range between 5.5 and 8.0, are strongly correlated, having standard deviations of 0.15–0.25 magnitude units. Corresponding regression coefficients are different, at 95% level of significance, for different slip mechanism types of each pair of correlated parameters. The single-slip-type regressions have smaller standard deviations and higher correlation coefficients than the all-slip-type regressions. This observation validates the conclusion about strong dependencies of source dimensions and surface-wave magnitudes on slip mechanism type. The results can be used to evaluate the event magnitude, source area, length, width and mechanism type when any two of these parameters are known.