On the variation of b-values with earthquake size

Emile A. Okal*, Barbara A. Romanowicz

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

109 Scopus citations


We investigate the effect on Gutenberg and Richter's parameter b of the saturation of moment-magnitude relationships caused by source finiteness. Any conventional magnitude scale measured at a constant period features a saturation which results in a stepwise increase in the slope of the log10 moment-magnitude relationship. We predict that this leads to an increase in b value with earthquake size. This is in addition to the effect of the physical saturation of the transverse dimension of the fault, previously described in the literature. A number of scaling models are used to predict the behavior of b with increasing magnitude, in the case of both the 20 s surface-wave magnitude Ms, and the 1 s body-wave magnitude mb. We show in particular that a b value of unity can be expected only in a range of earthquake size where the relevant magnitude has already started to saturate: it should be the exception, not the rule, and cannot be extended to a wide range of magnitudes, except at the cost of significant curvature in the frequency-magnitude curve. The widely reported b = 1 stems from the common practice of using a heterogeneous magnitude scale, e.g. Ms for large events and m1 for smaller ones.

Original languageEnglish (US)
Pages (from-to)55-76
Number of pages22
JournalPhysics of the Earth and Planetary Interiors
Issue number1-2
StatePublished - Dec 1994

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Geophysics
  • Physics and Astronomy (miscellaneous)
  • Space and Planetary Science


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