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Various earthquake models predict that aseismic slip modulates the seismic rupture process but actual observations of such seismic‐aseismic interaction are scarce. We analyze seismic and aseismic processes during the 2014 Iquique earthquake sequence. High‐rate Global Positioning System displacements demonstrate that most of the early afterslip is located downdip of the M 8.1 mainshock and is accompanied by decaying aftershock activity. An intriguing secondary afterslip peak is located ∼120 km south of the mainshock epicenter. The area of this secondary afterslip peak likely acted as a barrier to the propagating mainshock rupture and delayed the M 7.6 largest aftershock, which occurred 27 hr later. Interevent seismicity in this secondary afterslip area ended with a M 6.1 near the largest aftershock epicenter, kicking the largest aftershock rupture in the same area. Hence, the interevent afterslip likely promoted the largest aftershock nucleation by destabilizing its source area, favoring a rate‐dependent cascade‐up model. Plain Language Summary: Subduction zone faults host both fast (regular earthquakes, seismic) and slow (aseismic) slip. Simulation models predict that slow slip can affect fast slip processes. We explored such an interaction taking place during the 2014 Iquique earthquake offshore northern Chile using observation data of crustal deformation by Global Positioning System and earthquakes. We discovered that the fast mainshock slip was terminated by a slowly slipping fault zone, which prevented the simultaneous occurrence of the largest aftershock. Furthermore, afterslip, one type of slow slip following the mainshock, helped the occurrence of the largest aftershock 27 hr after the mainshock. Therefore, the sequential occurrence of large earthquakes can be controlled by slowly slipping faults. Key Points: Global Positioning System captured crustal deformation during 27 hr between the 2014 Iquique mainshock and its largest aftershockThe mainshock and the largest aftershock areas are separated by an aseismic area, likely preventing both from rupturing as a single eventThe largest aftershock nucleation is a mixture of seismicity and decelerating afterslip, favoring a rate‐dependent cascade‐up model [ABSTRACT FROM AUTHOR]