Scaling of Repeating Earthquakes at the Transition From Aseismic to Seismic Slip

Some observations of repeating earthquakes show an unusual, non‐self‐similar scaling between seismic moment and corner frequency, a source property related to rupture size. These observations have been mostly reported in regions at the transition from stable to unstable slip, in geothermal reservoirs and subduction zones. What controls the non self‐similarity of these ruptures and how this is linked to the frictional stability of the interface are still open questions. Here we develop seismic cycle simulations of a single unstable slipping patch to investigate the mechanisms underlying this behavior. We show that temporal changes of normal stress on a fault can produce ruptures that exhibit the observed anomalous scaling. Our results highlight the role of fault zone fluid pressure in modulating the effective normal stress and contributing to the sliding stability of the fault. Plain Language Summary The observation that some earthquakes have nearly similar source lengths but the varying magnitude is at odds with empirical earthquake scaling relations observed worldwide. Here we test how the influence of fluid pressure (or, equivalently, effective normal stress) on the fault could explain atypical size‐duration scaling. We run numerical simulations of a fault containing an asperity that can produce repeating earthquakes. We observe that this asperity can slip seismically or aseismically depending on the value of the effective normal stress imposed on the fault. For a given asperity size, there exists a range of effective normal stress that leads to earthquakes with quasi identical lengths but strongly varying magnitudes. The relation between these two quantities is close to the one observed for these atypical earthquakes on natural faults. We thus propose that an explanation for anomalous size‐duration scaling can be related to the fluctuations of fluid pressure within a fault. Key Points Effective normal stress influences the properties of repeating earthquakes in a rate‐and‐state friction model Fluctuations of effective normal stress reproduce the unusual scaling between moment and rupture size of repeating earthquakes Non self‐similar scaling is an indicator of proximity to the transition between the seismic and aseismic slip