Intermediate-depth earthquakes (30-300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic
antigorite-
olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals,
dehydration of deforming samples containing only 5 vol% of
antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of
antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that
antigorite dehydration triggered dynamic shear failure of the
olivine load-bearing network. These laboratory analogues of intermediate-depth earthquakes demonstrate that little
dehydration is required to trigger embrittlement. We propose an alternative model to
dehydration-embrittlement in which
dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.