The $b$-value in earthquake magnitude-frequency distribution quantifies the relative frequency of large versus small earthquakes. Monitoring its evolution could provide fundamental insights into temporal variations of stress on different fault patches. However, genuine $b$-value changes are often difficult to distinguish from artificial ones induced by temporal variations of the detection threshold.
A highly innovative and effective solution to this issue has recently been proposed by van der Elst (2021) through the b-positive method, which is based on analyzing only the positive differences in magnitude between successive earthquakes.
Here, we provide support to the robustness of the method, largely unaffected by detection issues due to the properties of conditional probability. However, we show that the b-positive method becomes less efficient when earthquakes below the threshold are reported, leading to the paradoxical behavior that it is more efficient when the catalog is more incomplete. Thus, we propose the b-more-incomplete method, where the b-method is applied only after artificially filtering the instrumental catalog to be more incomplete. We also present other modifications of the b-method, such as the b-more-positive method, and demonstrate when these approaches can be efficient in managing time-independent incompleteness present when the seismic network is sparse.
We provide analytical and numerical results and apply the methods to fore-mainshock sequences investigated by van der Elst (2021) for validation. The results support the observed small changes in $b$-value as genuine foreshock features.