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

Abstract

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.

Modeling of Continental Normal Fault Earthquakes

“…Also, the dependency of the coupling on W / h* is similar to the results of Mark et al. (2018), however, coupling in our models is slightly higher. This can be related to the viscous component of the crust.…”

“…3 , which is similar to Mark et al. (2018). Also, the dependency of the coupling on W / h* is similar to the results of Mark et al.…”

Controls on the magmatic fraction of extension at mid-ocean ridges

“…The significant deficit of MOR earthquakes compared to their expected moment release however suggests that a sizable portion of tectonic extension may occur aseismically, particularly at faster-spreading ridges (Cowie et al, 1993;Frohlich and Wetzel, 2007;Olive and Escartín, 2016). This idea is backed up by recent numerical models of MOR seismic cycles (Mark et al, 2018).…”

Kinematics of Footwall Exhumation at Oceanic Detachment faults: Solid‐Block Rotation and Apparent Unbending

“…Nevertheless, it is clear the footwall earthquake cluster imaged by Demartin et al (2007) is significantly more limited in its spatial extent than compared to the region of high strain rates developed in the model (e.g., Figure 5). A few points are worth bearing in mind, however: the seismic deployment detailed in Demartin et al (2007) was relatively short (8 months), and seismicity patterns may be biased with respect to the long-term tectonic strain rates; there may be additional variability in terms of whether faulting occurs as unstable sliding versus aseismic slip (Mark et al, 2018), as well as the level of microseismicity versus larger events (i.e., the b value). Similarly, procedures on the numerical modeling side could be implicated: we omit physical processes such as melting, hydrothermal heat transport as well as any three-dimensional aspects of dynamics which may affect thermal and dynamic structure of the footwall.…”