Measuring robust b-values from magnitude differences

Nicholas J. van der Elst

Published August 12, 2020, SCEC Contribution #10404, 2020 SCEC Annual Meeting Poster #088 (PDF)

The earthquake magnitude-frequency distribution is characterized by the b-value, which describes the relative frequency of large vs. small earthquakes. There is evidence to suggest that the b-value for aftershocks is larger than for background events, and that exceptions to this rule tend to occur exclusively in foreshock sequences, with low b-values indicative of larger earthquakes to come [Gulia and Wiemer, 2019]. However, the measurement of b-value during an active aftershock sequence is strongly biased by short-term incompleteness of the earthquake catalog. Since smaller earthquakes are obscured by larger ones, the b-value in an active aftershock sequence tends to be biased low by incomplete detection across the entire range of magnitudes. Here I develop a new estimator of the b-value that is relatively insensitive to transient changes in catalog completeness and allows for the real-time, unbiased measurement of b-value during an ongoing sequence. The estimator is based on the differences in magnitude between successive earthquakes, which are described by a double-exponential distribution with the same b-value as the exponential magnitude distribution itself. The new estimator broadly confirms the findings of Gulia and Wiemer [2019], showing a decrease in b-value for events between the foreshock and mainshock for several prominent foreshock sequences over the past decade – with some caveats. The new estimator greatly improves the robustness of b-value measurement during active earthquake sequences, as well as in historical catalogs with variable completeness.

Citation
van der Elst, N. J. (2020, 08). Measuring robust b-values from magnitude differences. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)