The Effect of a Mainshock on the Size Distribution of the Aftershocks

A systematic decay of the aftershock rate over time is one of the most fundamental empirical laws in Earth science. However, the equally fundamental effect of a mainshock on the size distribution of subsequent earthquakes has still not been quantified today and is therefore not used in earthquake hazard assessment. We apply a stacking approach to well‐recorded earthquake sequences to extract this effect. Immediately after a mainshock, the mean size distribution of events, or bvalue, increases by 20–30%, considerably decreasing the chance of subsequent larger events. This increase is strongest in the immediate vicinity of the mainshock, decreasing rapidly with distance but only gradually over time. We present a model that explains these observations as a consequence of the stress changes in the surrounding area caused by the mainshocks slip. Our results have substantial implications for how seismic risk during earthquake sequences is assessed. The effect of a mainshock on the size distribution of subsequent earthquakes has not been quantified and is therefore not used in earthquake hazard assessment. To quantify this effect, we develop a stacking approach centered on the mainshock time and apply it to for 31 well‐recorded aftershock sequences from around the world. We found that after a mainshock the earthquake size distribution shifts toward relative more smaller events, increasing the so‐called bvalue by 20–30%. One of the consequences of our finding is that the rates of large aftershocks are overestimated by the currently used models. Our result is fully consistent with both laboratory measurements and modeling, and we present a conceptual model that explains our findings. We develop a stacking approach to bvalue time series centered on the mainshock time in order to extract the generic behaviorApplying this approach to well‐recorded aftershock sequences, we demonstrate that the bvalue increases by 20–30% after a mainshockWe develop a Coulomb stress‐based model explaining the postmainshock bvalue increase and propose an empirical relationship to be used to forecast aftershock hazard