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Mode of slip and crust–mantle interaction at oceanic transform faults. The relationship between seismicity and fault structure on the Discovery transform fault, East Pacific Rise. Seismic velocity constraints on the material properties that control earthquake behavior at the Quebrada–Discovery–Gofar transform faults, East Pacific Rise. Variations in earthquake rupture properties along the Gofar transform fault, East Pacific Rise. Olivine friction at the base of oceanic seismogenic zones. Earthquake slip on oceanic transform faults. Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere. Age dependence of oceanic intraplate seismicity and implications for lithospheric evolution. Our results suggest that seawater infiltration into ocean transform faults controls the extent of seismicity and spatiotemporal variations in the mode of slip. The ability of seawater to penetrate faults determines whether slip is accommodated at depth by seismic asperities or by aseismic creep in weak, hydrous shear zones. Combined with observations of grain size reduction and hydrous mineralogy from high-strain mylonites, our model shows that brittle and ductile deformation can occur over a broad temperature range, 300–1,000 ☌. Deep fluid circulation is accounted for in a modified friction-effective pressure law and in ductile flow laws for olivine and serpentine. Here we present a model for the mechanical structure of oceanic transform faults based on fault thermal structure and the impacts of hydration and metamorphic reactions on mantle rheology. Rocks from both fast- and slow-slipping transforms show evidence of interactions with seawater up to temperatures of at least 900 ☌. The mode of slip varies with depth and along strike, with some fault patches that rupture in large, quasi-periodic earthquakes at temperatures <600 ☌, and others that slip through creep and microearthquakes at temperatures up to 1,000 ☌. Oceanic transform faults that offset mid-ocean ridges slip through earthquakes and aseismic creep.