Dynamic triggering of earthquakes is promoted by crustal heterogeneities and bimaterial faults

Remotely triggered earthquakes and aftershocks constitute a great challenge in assessing seismic risk. A growing body of observations indicates that significant earthquakes can be triggered by moderate to great earthquakes occurring at distances of up to thousands of kilometres. Currently we lack the knowledge to predict the location of triggered events. We present numerical simulations showing that dynamic interactions between material heterogeneities (e.g. compliant fault zones, sedimentary basins) and seismic waves focus and enhance stresses sufficiently to remotely trigger earthquakes. Numerical simulations indicate that even at great distances (>100 km), the amplified transient dynamic stress near heterogeneities is equivalent to stress levels near the source rupture tip (<5 km). Such stress levels are widely considered capable of nucleating an earthquake rupture on a pre-stressed fault. Analysis of stress patterns in dynamic rupture simulations which include a heterogeneous zone with a range of material and geometrical properties reveals various mechanisms of stress enhancement. We conclude that both stiff and weak heterogeneities may focus stress waves to form zones of enhanced stress, and that bimaterial interfaces distort under static and dynamic loading in a way that induces local stress concentrations. Our work provides insights for understanding non-uniform distribution of remotely triggered seismicity and recurrence of such events along complex fault-systems and near magmatic intrusions and geothermal zones.