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Understanding the dynamics of microearthquakes is a timely challengewith the potential to address current paradoxes in earthquake mechanics,and to better understand earthquake ruptures induced by fluid injection.We perform fully 3D dynamic rupture simulations caused by fluidinjection on a target fault for FEAR experiments generating Mw ≤ 1earthquakes. We investigate the dynamics of rupture propagation withspatially variable stress drop caused by pore pressure changes andassuming different constitutive parameters. We show that the spontaneousarrest of propagating ruptures is possible by assuming a high faultstrength parameter S, that is, a high ratio between strength excess anddynamic stress drop. In faults with high S values (low rupturingpotential), even minor variations in Dc (from 0.45 to 0.6 mm) have asubstantial effect on the rupture propagation and the ultimateearthquake size. Our results show that modest spatial variations ofdynamic stress drop determine the rupture mode, distinguishingself-arresting from run-away ruptures. Our results suggest that severalcharacteristics inferred for accelerating dynamic ruptures differ fromthose observed during rupture deceleration of a self-arrestingearthquake. During deceleration, a decrease of peak slip velocity isassociated with a nearly constant cohesive zone size. Moreover, theresidual slip velocity value (asymptotic value for a crack-like rupture)decreases to nearly zero. This means that an initially crack-likerupture becomes a pulse-like rupture during spontaneous arrest. Insummary, our findings highlight the complex dynamics of smallearthquakes, which are partially contrasting with established crack-likemodels of earthquake rupture.