Metamorphic dehydration reactions trigger slow slip events in subduction zones

Slow Slip Events (SSEs), widespread along most subduction zones, play a critical role in the seismic cycle as they are known to gradually release stress and trigger earthquakes. Yet, the conditions driving SSEs remain poorly constrained. Here, based on thermal state modeling of subduction zones worldwide, we reveal that SSEs predominantly occur within two clusters at ~100°C and ~450 ± 100°C. Using phase equilibrium modeling, we show that these clusters coincide with conditions where significant dehydration reactions are expected in subducting metasedimentary and oceanic crustal rocks. These findings are consistent with geological observations from exhumed subduction interfaces, laboratory experiments, and numerical modeling, indicating that mineral transformations and associated fluid release can promote SSE nucleation by locally and transiently weakening the megathrust.