Geometry of curved slickenlines as a function of rupture direction, asperity durability and coseismic roughening of fault surfaces

Abstract

Global data indicate slickenlines inscribed during surface rupturing earthquakes are typically curved. Dynamic rupture modelling relates slip curvature to time-varying stresses in rupture process zones. Such models generate striation curvature depending on rupture propagation direction and Andersonian slip type. Using 2D kinematic models in a new MATLAB program called Slicks, we explore expected patterns of curved slickenline on fault surfaces, comparing them to observations of natural slip striae on scarps of the Kekerengu Fault after the 2016 Kaikōura Earthquake, New Zealand, and the Alpine Fault, New Zealand which last ruptured in 1717 CE. As predicted by the dynamic rupture models, some slickenlines on both faults are curved at their upstream (older) ends and transition downstream to a longer, straighter trajectory. Slicks predicts that curved tracks should intersect, a relationship that we observed in the field, and that, for a given slip history, slickenline patterns and track-length distributions should vary depending on initial density of plowing elements, their mean durability or longevity, and the rates at which new asperities are introduced and smoothed. Striation patterns on the Kekerengu Fault suggest that inscribing asperities were established at the beginning of the earthquake rupture, with this roughness being quickly smoothed during the earthquake.