Dynamic simulations of coseismic slickenlines on non-planar and rough faults: Towards inferring the rupture directions of paleoearthquakes

Knowing the directions of rupture propagation of paleoearthquakes is a challenging, yet important task for our understanding of earthquake physics and seismic hazard, as the rupture direction significantly influences the distribution of strong ground motion. Recent studies proposed a relationship between the direction of rupture propagation and curvature of slickenlines formed during seismic slip (Kearse and Kaneko, 2020). The relationship was established using a global catalogue of historic surface-rupturing earthquakes and dynamic models of idealized, planar faults. At the same time, some slickenlines previously documented on geometrically complex fault segments show their convexity opposite from the simple model prediction, which we refer to as `abnormal convexity'. To explain such observations, we perform simulations of spontaneous earthquake ruptures on non-planar and rough faults. We find that a non-planar fault model can lead to abnormal convexity of slickenlines in places where the fault dip angle changes abruptly from the average dip of the fault. In the case of strike-slip faults, abnormal convexity of slickenlines is produced when the initial along-dip stresses are larger than, and are opposite in direction to, the dynamic stresses imparted by the mixed-mode rupture. Such results are also confirmed in our rupture simulations with a rough fault. Our results also show that the parameter space for which abnormal convexity of slickenlines occurs near Earth's surface is narrow, especially when the fault strength and initial shear stresses increase with depth. Nevertheless, slickenlines on geometrically complex faults need to be carefully interpreted and investigation of rupture direction using curved slickenlines should focus on structurally simple parts of faults.