Effect of lithological heterogeneities on shallow slow slip events: An example from the northern Hikurangi margin, New Zealand

Shallow slow slip events (SSEs) have been documented at subduction zones worldwide as part of a spectrum of fault slip modes. While a universal mechanism that may explain their slip rate-limiting behavior has been elusive, rheological or frictional heterogeneities, potentially arising from mixed lithologies along faults, have been invoked as a possible ingredient for slow slip genesis. Motivated by laboratory friction experiments conducted on sediments entering the trench at the northern Hikurangi subduction zone (where shallow SSEs are well-documented), we simulate a mildly velocity-weakening fault with explicit velocity-dependence of rate-state frictional (RSF) parameters a, b and the characteristic slip distance, Dc. Here, we explore the role of lithological mixing at different scales, and the relative importance of these heterogeneities over the velocity dependence of the frictional parameters. We take a cross-section of the subduction interface along the drilling transect of IODP expeditions 372/375 as a template for downdip geometry for the northern Hikurangi margin. We then model macro-scale heterogeneities by simulating the incorporation of strong calcite-rich patches at randomly seeded locations along the fault, with sizes ranging from ~200 m to ~10h* where h* is a critical nucleation patch size (~1-10 km). Broadly, our results indicate that homogeneous phyllosilicate-bearing faults and heterogeneous faults are both capable of hosting SSEs, with the rate-dependent RSF parameters stabilizing fault slip. Moreover, phyllosilicate-dominant faults host fault-spanning SSEs likely due to their low strength whereas faults with larger lithological heterogeneities are marked by complex slip behavior including bimodal ruptures and downdip rupture asymmetry. Our results have important implications for understanding the mechanics of SSEs and trench-rupturing subduction zone earthquakes, and potential for co-location of these processes.