Earthquake nucleation on dip‐slip faults

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Abstract
The nucleation of unstable slip on a fault is of key importance in our understanding of the seismic cycle. We investigate how the asymmetric geometry of dip‐slip faults affects the nucleation of unstable slip on such faults. Previous researchers have devoted much effort to understanding this nucleation process on geometrically simple faults, using a variety of frictional parameterizations. However, there are many reasons to believe that earthquake nucleation may be affected by fault complexity. The breakdown of symmetry on a nonvertical dip‐slip fault means that normal stress is not constant during the slip process and that the hanging wall and footwall may not necessarily move equally. Using a slip‐strengthening and ‐weakening friction law in a two‐dimensional quasi‐static model based on a variational boundary integral method, we show that nucleation on dip‐slip faults is affected by both the dip angle and the direction of slip (normal versus thrust/reverse). Under otherwise identical conditions in a homogeneous half‐space (i.e., neglecting depth‐dependent frictional or material properties), thrust faults nucleate closer to the Earth's surface than normal faults and take less time to do so. These differences decrease as the dip angle of the fault increases to 90°. The amount of preseismic surface slip is a much more complicated function of dip angle. The results show that fault geometry may have an important role in the nucleation process as well as the process of dynamic rupture. Further research is needed to combine rigorous nucleation models with full rupture dynamics.