Controls on Mid-ocean Ridge Normal Fault Seismicity Across Spreading Rates From Rate-and-State Friction Models

Recent seismic and geodetic observations have led to a growing realization that a significant amount of fault slip at plate boundaries occurs aseismically and that the amount of aseismic slip varies across tectonic settings. Seismic moment release rates measured along the fast-spreading East Pacific Rise suggest that the majority of fault slip occurs aseismically. By contrast, at the slow-spreading Mid-Atlantic Ridge seismic slip may represent up to 60% of total fault displacement. In this study, we use rate-and-state friction models to quantify the seismic coupling coefficient, defined as the fraction of total fault slip that occurs seismically, on mid-ocean ridge normal faults and investigate controls on fault behavior that might produce variations in coupling observed at oceanic spreading centers. We find that the seismic coupling coefficient scales with the ratio of the downdip width of the seismogenic area (W) to the critical earthquake nucleation size (h*). At mid-ocean ridges, W is expected to increase with decreasing spreading rate. Thus, the relationship between seismic coupling and W/h* predicted from our models explains the first-order variations in seismic coupling coefficient as a function of spreading rate.