Deep intra-slab rupture and mechanism transition of the 2024 M 7.4 Calama earthquake.

While subduction zone hazard is dominated by the megathrust, intermediate-depth (70-300 km) earthquakes within the slab can likewise have catastrophic impacts. Their physics remains enigmatic, with suggested mechanisms including dehydration embrittlement and thermal runaway. Here, we investigate the 2024 Chile, M 7.4 intermediate-depth earthquake and compare the rupture extent with temperature conditions from thermo-mechanical models. We record regional geodetic co-seismic deformation and high-resolution seismicity associated with this type of event. Our analyses reveal a complex rupture spanning an exceptional depth range, with distinct asperities propagating deep into the subducting lithosphere. Comparison with thermal models shows that while the rupture initiated within the cold slab core, it extended well beyond the ~650 °C isotherm that typically delineates the boundary for efficient serpentine dehydration. We suggest that the rupture likely initiated with dehydration embrittlement within the cold core but then propagated into the warmer regions through shear thermal runaway. This implies a transition of mechanisms that facilitates large-scale rupture and activates typically aseismic, high-temperature slab regions. Our findings highlight the importance of considering interactions between rupture mechanisms as well as slab thermal and compositional settings to better understand the processes governing intermediate-depth earthquakes.