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In the Nazca‐South American subduction zone, the subducted slab is flattened beneath northern Andes, called the Peruvian flat slab. The 2019 Mw 8.0 northern Peru intermediate‐depth normal‐faulting earthquake occurred at the leading edge of the Peruvian flat slab, where the slab rebends and sinks into greater depths. Here we investigate this earthquake by back projection analysis and finite fault inversion using seismic waveforms at teleseismic distances. The rupture process indicates that this earthquake ruptured mainly along strike (353°) ~150 km north‐northwestward within ~55 s (average rupture velocity ~2.7 km/s), resulting in two major slip areas with three high slip rate areas, which are consistent with three high‐frequency energy radiation subevents. Our study suggests that such a heterogeneous rupture may be caused by slab bending forces and dehydration embrittlement associated with morphology of the slab. Plain Language Summary: In the subduction zone, the oceanic plate and the continental plate converge, and then the oceanic plate subducts beneath the continental plate and sinks into great depths. In South America, the subducted oceanic plate is flattened beneath northern Peru instead of steeply sinking, called the Peruvian flat slab. After flattened, the oceanic plate starts to sink again. In the region where the oceanic plate starts to sink again, an earthquake with magnitude of 8.0 occurred at a depth of ~120 km on 26 May 2019. How and why this earthquake occurred at such a depth? Does the occurrence of this earthquake is related to the subducted oceanic plate? We try to answer these questions in this paper. By using two techniques, we obtained a scenario about this earthquake: In the region where the oceanic plate starts to sink again, the water in the oceanic plate releases, along with the effect of sinking to cause this earthquake. After this earthquake initiated, it ruptured mainly unilaterally north‐northwestward ~150 km with a speed of ~2.7 km/s, resulting in two large slip areas with three high slip rate areas. Such an uneven rupture may be produced by the heterogeneous structure of the slab. Key Points: Rupture process of the 26 May 2019 northern Peru earthquake is obtained from back projection analysis and finite fault inversionThe effects of slab bending forces and dehydration embrittlement together may cause this earthquake at an intermediate depthThis intermediate‐depth earthquake is probably associated with morphology of the Peruvian flat slab [ABSTRACT FROM AUTHOR]