The Effect of Declustering on the Size Distribution of Mainshocks

TL;DR

Declustering earthquake catalogs significantly affects the estimated b-value of the magnitude distribution, introducing potential bias in seismic hazard assessments, with the extent depending on the method and parameters used.

Contribution

This study systematically evaluates how different declustering methods influence the b-value, revealing that declustering can cause substantial bias in seismicity analysis.

Findings

Declustering reduces b-value by up to 30% in California earthquake data.

The reduction in b-value varies with declustering method and parameters.

Synthetic catalogs confirm that declustering alone can cause the observed b-value decrease.

Abstract

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.