Stress triggering of large earthquakes complicated by transient aseismic slip episodes

We investigate how a static stress change, caused by a nearby earthquake, perturbs sliding processes on a fault and the time to the next earthquake. For this purpose, we model numerically a reverse fault in a two‐dimensional, semi‐infinite and elastic medium, partly stable and partly unstable under a rate‐ and state‐dependent friction law. When no stress change intervenes, steady sliding spreads out of the deep zone of frictional stability and keeps expanding updip during the interseismic period. Aseismic slip episodes (ASEs) develop spontaneously within the region of steady sliding and repeat quasi‐periodically. When steady sliding has spread beyond a certain critical length (the nucleation length) above the zone of frictional stability, one of the ASEs undergoes rapid buildup and avalanches into high‐speed seismic sliding. When a step‐like stress change intervenes, ASEs of larger amplitudes arise and evolve similarly to their spontaneous counterparts until one of them avalanches into an earthquake. If the stress change arrives not too early nor too late in the seismic cycle, the basic picture is as follows: delaying the stress step, be it positive or negative, delays the timing of the ASE sequence and thereby postpones the next earthquake. When the stress step is delayed to more than a certain extent, however, an earlier ASE gets to evolve into an earthquake, advancing its time discontinuously. Only in the final stage of a seismic cycle does a positive stress step systematically advance the next earthquake and a negative stress step systematically delay it.