Oral Presentation

The earthquake magnitude-frequency distribution is characterized by the b-value. Some observations suggest that the b-value for aftershocks is larger than for spontaneous events (mainshocks), and that exceptions to this rule occur exclusively in foreshock sequences. Unfortunately, measurements of b-value are biased during aftershock sequences by short-term incompleteness of the earthquake catalog. I present an alternative estimator of the b-value, ‘b-positive’ that is insensitive to transient changes in catalog completeness and that 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 magnitude distribution itself. The new estimator mostly confirms the findings of Gulia and others [Nature, 2019; SRL 2020], showing a decrease in b-value between the foreshock and mainshock in several prominent foreshock sequences over the past decade. B-positive gives stable estimates of the b-value with essentially zero blind time, improving the timeliness of sequence-specific aftershock forecasts. The ‘positive’ statistical approach (targeting successive earthquake pairs with positive magnitude increment) can be extended to measure earthquake rates in historical catalogs and fit the Epidemic Type Aftershock Sequence model without reference to a completeness magnitude.