Slow earthquakes occur at the plate interface in subduction zones. These low-frequency tremors and slow-slip events are often located at about 30 km depth 1-3 , near the boundary between the crust and mantle (Moho) on the overriding plate. Slow earthquakes occur on fault patches with extremely low frictional strength 4-6 . This weakness is generally assumed to result from increased pore-fluid pressures and may be linked to the release of fluids from the descending plate. Here we propose that a contrast in permeability across the Moho results in the accumulation of water and the build-up of pore-fluid pressure at the corner of the mantle wedge that overlies the subducting plate. We use laboratory measurements of permeability to show that gabbroic rock layers in the crust are two orders of magnitude less permeable than serpentinite layers in the underlying hydrated mantle rocks. Inserting our experimental data into a numerical model that simulates pore pressure evolution across the Moho, we show that the porefluid pressure at this boundary can be as high as lithostatic pressure. We suggest that water released from the descending plate is trapped at the corner of the mantle wedge owing to this permeability barrier, and then causes the localized slow earthquakes that are triggered by fault instabilities.The presence of pore fluid has long been recognized as playing an important role in the crustal deformation and earthquake mechanism, because it weakens the frictional strength and can result in fault instability 7,8 . As episodic tremor and slow-slip events found in subduction zones exhibit extremely low effective stresses, the occurrence of these slow earthquakes is attributed to the effect of high pore-fluid pressures on the plate boundaries 4-6 . Low seismicwave velocities and high Poisson's ratios of the source regions also suggest the presence of aqueous fluids that facilitate these slow earthquakes 1-3 . The occurrence of these events is mostly localized at triple junctions between the subducting plate and the island-arc Moho; consequently, a mechanism must be operating to facilitate water accumulation in such localized regions.Water in the subducting plate is released into the overlying mantle wedge by the dehydration of hydrous minerals 9 . These fluids tend to migrate upwards along the plate interface because the hydrated mantle (serpentinite) can develop a strong fabric 10 that results in a significant anisotropy in permeability 11 . At the island-arc Moho, migrating fluids can become trapped at the gabbroic layer, which acts as a cap rock, similar to the structures that trap petroleum resources 12 . Thus, the permeability contrast across the Moho may be the key to water accumulation in the source regions of slow earthquakes (Fig. 1). A similar model of a permeability barrier has also been proposed to account for the anomalous pore pressures in subducted crust [13][14][15] . In this study we Figure 1 | Schematic model of water circulating in a subduction zone.Subducting plates release most of their water in...

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