Magnitude uncertainties affect different components of an estimate of seismic hazard in a variety of ways, but methods are available for countering such effects. Uncertainties in measured magnitudes cause bias in estimates of the Gutenberg-Richter activity-rate parameter and, if magnitude uncertainties are correlated with magnitude, in the b-value also. The combined effect can easily amount to a factor of two in estimating the frequency of occurrence of large earthquakes. The biases can be corrected if standard errors of magnitude estimates are known. In attenuation modelling, ignoring magnitude uncertainties can inflate the residual variance and lead to spurious terms being included in the model. Explicit treatment of magnitude uncertainties formally involves an extension of the usual random effects regression model. The maximum magnitude that is assumed to be possible in a given source region is highly influential on seismic hazard estimates, but also subject to much uncertainty. A substantial part of this uncertainty can be quantified from relations between earthquake magnitude and source dimensions, and used to adjust estimates of the frequency-magnitude relation. The effects of magnitude uncertainties on seismic hazard estimates are potentially large. Allowance for such uncertainties should be a standard part of seismic hazard assessment.
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