Water accumulation and pore pressure build-up in mantle wedge due to permeability barrier

Slow earthquakes that occur at the subducting plate interface, including low-frequency tremors and slow-slip events, are extremely weak in terms of frictional strength. The weakening mechanism is generally assumed to be a result of increased pore fluid pressure in the source regions, which are mostly located at the interface between the subducting plate and island arc Moho. Laboratory measurements of permeability show that gabbro layers are two orders of magnitude less permeable than are serpentinite (hydrated mantle) layer, which results in the accumulation of water and pore pressure build-up at the mantle wedge corner. Numerical modelling of our experimental data shows that the pore fluid pressure at the boundary can reach as high as lithospheric pressure. This indicates that water released from the descending plate is trapped at the mantle wedge corner due to the permeability barrier; water accumulation then causes localized slow earthquakes triggered by fault instability.