Metamorphic dehydration reactions trigger slow slip events in subduction zones

Aseismic slip, particularly in the form of Slow Slip Events (SSEs), plays an undisputed role in the release of stress along faults, occurring slowly and without generating classical seismic waves. SSEs are recognized as critical phenomena influencing various stages of the seismic cycle, including postseismic phases, earthquake triggering or arresting, and interseismic transients. However, the mechanisms governing their underlying physics remain debated. Three primary hypotheses have been proposed: (1) heterogeneities in fault constitutive properties that may drive episodic SSEs; (2) stress interactions arising from geometric complexities (e.g., damage zones) that could explain the full observed slip spectrum; and (3) the influence of fluids circulating along fault zones, which increase pore pressure and reduce normal stress, thereby promoting slip. To investigate these mechanisms, we integrate SSE databases, slab thermal models, and thermodynamic metamorphic modeling.

Our study examines nine subduction zones around the Pacific region, using thermal slab models that account for uncertainties in temperature estimations. By using an extensive SSE database (1800 events, Slow Earthquake Database, from the Japanese project “Science of Slow-to-Fast Earthquakes), we compare modeled temperature and pressure conditions with observed SSE distributions. Statistical analysis reveals two distinct temperature ranges where SSEs cluster: approximately 100°C and 350–550°C. Thermodynamic modeling of mafic rocks under subduction conditions indicates that the 100°C cluster aligns with the smectite-to-illite transition, a reaction known to release significant amounts of water. The 350–550°C cluster corresponds to metamorphic transitions from greenschists to amphibolites, which also release considerable water. SSEs are notably absent at pressure-temperature conditions where mafic rocks are fully dehydrated.

The water released during such metamorphic reactions increases pore pressure, reduces normal stress, and facilitates slip. While the mechanisms sustaining slow slip—such as nucleation length or dilatant stress—remain debated, our results suggest that water release due to metamorphic reactions is a key trigger for SSEs along subduction interfaces. In addition to the release of fluids, we hypothesize that the change in resistance induced by the change in mineralogical configuration might also play a role in the nucleation of SSEs. These findings highlight the importance of integrating geophysical observations with petrological processes to better understand the dynamics of SSE in subduction zones