  Declustering aims to divide earthquake catalogs into independent events
(mainshocks), and dependent (clustered) events, and is an integral component of
many seismicity studies, including seismic hazard assessment. We assess the
effect of declustering on the frequency-magnitude distribution of mainshocks.
In particular, we examine the dependence of the b-value of declustered catalogs
on the choice of declustering approach and algorithm-specific parameters. Using
the catalog of earthquakes in California since 1980, we show that the b-value
decreases by up to 30% due to declustering with respect to the undeclustered
catalog. The extent of the reduction is highly dependent on the declustering
method and parameters applied. We then reproduce a similar effect by
declustering synthetic earthquake catalogs with known b-value, which have been
generated using an Epidemic-Type Aftershock Sequence (ETAS) model. Our analysis
suggests that the observed decrease in b-value must, at least partially, arise
from the application of the declustering algorithm on the catalog, rather than
from differences in the nature of mainshocks versus fore- or aftershocks. We
conclude that declustering should be considered as a potential source of bias
in seismicity and hazard studies.
