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Summary. Large earthquakes are nearly always followed by aftershocks. A persistent question in earthquake triggering is, what proportion of aftershocks is caused by the passage of seismic waves (called dynamic triggering) as compared with those triggered by the rearrangement of the Earth’s crust during the mainshock (called static triggering). This proportion is important to earthquake forecasting. The two sources of triggering stress have different features; dynamic stresses are larger, but temporary, lasting only as long as the crust is shaken by seismic waves. Static stress changes are small, but permanent. They can be increases or decreases, and are thus expected to both trigger and suppress earthquakes. Dynamic waves are expected to only raise stress, triggering earthquakes, but not suppressing them. Both sources of triggering stress correlate with seismicity rate increases, but only static stress changes correlate with seismicity rate decreases.

A key diagnostic for static vs. dynamic earthquake triggering then is observation of earthquake suppression. It is thought that examples of post-mainshock declines in earthquake rate are confirmation of static stress changes dominating earthquake triggering. However, clear cases of earthquake rate declines have proven difficult to demonstrate. In this paper these assumptions are questioned. It is shown numerically that if seismic waves can cause damage to fault zones - like they damage many other things - all the features we observe in aftershock sequences can occur.