Subduction zones host most of the Earth's great earthquakes (>Mw 8) and foster tsunami that are dangerous hazards across entire ocean basins. Following a great earthquake, the interface between tectonic plates must “heal”, or strengthen, to resume the process of storing elastic strain energy to be released in the next event. Rocks from subduction interfaces that have exhumed from the conditions where great earthquakes are generated show dense arrays of veins that record precipitation from a fluid into cracks. We use geochemical studies and a numerical model for earthquakes, healing, fluid flow, and the local dissolution-precipitation between earthquakes to show that the dominant mechanism for increasing strength between earthquakes comes from local dissolution-precipitation and not precipitation due to enhanced flow of fluid in the aftermath of earthquake ruptures.