By Terri Cook
When a fault ruptures during an earthquake, its motion deforms the surrounding crust. The resulting changes in stress often generate additional, smaller earthquakes known as aftershocks. Since the late 19th century, scientists have described how the rate of aftershocks decreases systematically over time. However, the equally fundamental effect of how a main shock influences the aftershock size distribution has not yet been quantified.
Now Gulia et al. present a new approach to determining this impact. They applied a stacking procedure to 31 high-fidelity records of earthquake sequences that included large (magnitude ≥ 6) tremors in California, Alaska, Japan, and Italy to analyze the effects of main shocks on subsequent earthquake statistics. Stacking is a commonly applied technique in signal processing to enhance the signal-to-noise ratio; this is the first time this approach has been applied to time series of earthquake size distribution.
The researchers’ results indicate that immediately following each main shock, the average size distribution of the aftershocks—called the b value—increases by 20%–30% and typically remains elevated for at least the next 5 years. This trend implies that the chance that larger earthquakes will subsequently occur decreases considerably, especially in the immediate vicinity of the affected fault, where the observed b value increase is the most pronounced.
On the basis of these findings, the authors propose a new empirical relationship to describe how bvalues change over time. Because most current forecasting models typically overestimate the risk associated with aftershocks, the proposed equation should provide an important basis for more realistic statistical assessments of aftershock hazard. (Geophysical Research Letters, 2018)