On 23 June 2020, a large ( 7.4) interplate thrust earthquake struck near the town of La Crucecita in the state of Oaxaca in southern Mexico, following a 55‐yr interseismic period. A seismic source model is well constrained by teleseismic waveforms, static Global Positioning System offsets, and tsunami data, suggesting that the earthquake occurred on the slab interface at a dip of , with a narrow elliptical asperity concentrating around a shallow depth of . The rupture propagates bilaterally from the hypocenter, and the down‐dip rupture is restricted to by slow slip events (SSEs). The down‐dip shear stress is released by SSEs during the interseismic period, limiting the earthquake magnitude and possibly resulting in the characteristic earthquake. The 2020 La Crucecita event, thus, is a good reminder to assess the seismic and tsunami potential in this region. The stress changes caused by the coseismic slip of the 2017 8.2 Chiapas earthquake are too small to trigger the 2020 La Crucecita earthquake. However, combined with the postseismic afterslip effects that play a leading role, it greatly promotes the eventual occurrence of the La Crucecita event. The results demonstrate the importance of considering postseismic afterslip, when evaluating seismic hazard and its migratory pattern.