We study a phenomenon occurring by hydraulic fracturing stimulations, where events are triggered during injection and also after the end of injection, sometimes over hours or even days. We assume that pore-pressure diffusion is the main triggering mechanism. Based on the theory of linear poroelasticity, an equation is derived, that describes the distance from injection point at which seismicity is terminated at a given time after the end of fluid injection. We call this distance the back front of induced seismicity. Using numerical modeling and data from three case studies we show that the back front is observed in reality. The existence of the back front is an important phenomenon supporting the idea of the pore-pressure diffusion nature of fluid injection triggered seismicity. Moreover, it provides an additional means of rock characterization allowing the estimation of the scalar hydraulic diffusivity of the seismically active area.