Plot showing the distribution of magnitudes (blue) and positive magnitude differences (red) for aftershocks of the 2019 M7.1 Ridgecrest California earthquake
Distribution of magnitudes (blue) and positive magnitude differences (red) for aftershocks of the 2019 M7.1 Ridgecrest California earthquake. The slope of these curves yields the b-value. Because of missing undetected small earthquakes after the mainshock, the GR estimator under-estimates b-value while the b+ estimator is much more robust. Credit: van der Elst [2021], Figure 5, top left panel
Source: Journal of Geophysical Research: Solid Earth

The b-value is widely used to characterize seismicity. It has been suggested to depend on stress and to be a potential precursor of large earthquakes. The classical estimator (GR) is based on the slope of the frequency-magnitude distribution above a completeness magnitude.

van der Elst [2021] presents a new b+ estimator that is based on the magnitude difference between successive events considering only positive values. When the completeness magnitude increases, for instance after a large earthquake, the GR estimator is under-estimated due to undetected small earthquakes, while the b+ estimator is much more robust.

This new tool confirms recent observations of low b-value in some sequences that turn out to be foreshocks. It also highlights a more complex picture than previous studies. This shows that care should be taken when interpreting the b-value as a stress-meter.

Citation: van der Elst, N. J. [2021]. B‐positive: A robust estimator of aftershock magnitude distribution in transiently incomplete catalogs. Journal of Geophysical Research: Solid Earth, 126, e2020JB021027. https://doi.org/10.1029/2020JB021027

—Agnes Helmstetter, Associate Editor, JGR: Solid Earth

Text © 2021. The authors. CC BY-NC-ND 3.0
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