Seafloor Displacements Across the Mid‐Ocean Ridge Eruption Cycle Modulated by Mush Zone Viscosity

Abstract Mid‐ocean ridges (MORs) are active volcanic systems where axial magma lenses (AMLs) are replenished on decadal time scales, enabling frequent dike intrusions. The associated seafloor deformation, however, remains scarcely understood. Here we assess this deformation by simulating cycles of steady AML inflation followed by instantaneous dike intrusion and AML drainage. We do so using finite element models whereby an AML lies at the base of an elastic lithosphere, and atop a Maxwell visco‐elastic mush zone, with viscosity , that is laterally confined to the ridge axis. AML replenishment manifests as distributed seafloor uplift, and a weak viscous mush ( Pa s) exerts a damping effect on seafloor uplift rates. AML drainage and dike intrusions further trigger lasting viscous relaxation manifesting as seafloor uplift and subsidence, respectively. We use our numerical simulations to revisit estimates of AML inflation at the East Pacific Rise (EPR) 9°50′N, where seafloor uplift rates up to cm/yr were measured 4–6 years following the 2005–2006 eruption. If we assume Pa s, the observed uplift requires a replenishment rate of /yr per meter along axis, whereas a very weak mush ( Pa s) requires rates /yr/m. Interestingly, the observed cross‐axis profile of seafloor displacements appears incompatible with our post‐eruption relaxation models, implying either that such relaxation did not take place, or that it was effectively over within 4 years. The latter implies that the effective viscosity of the mush zone should be Pa s or less.