Deep crustal fracture zones control fluid escape and the seismic cycle in the Cascadia subduction zone

•We seismically image fracture zones in the terranes of the Cascadia forearc.•The extent of fractures controls the permeability properties of the forearc.•This extent also determines the presence of shallow non-volcanic tremors.•Variations of pore-fluid pressure explain the segmentation of frictional behavior.

Seismic activity and non-volcanic tremors are often associated with fluid circulation resulting from the dehydration of subducting plates. Tremors in the overriding continental crust of several subduction zones suggest fluid circulation at shallower depths, but potential fluid pathways are still poorly documented. Using receiver function analysis in the Cascadia subduction zone, we provide evidence for a seismic discontinuity near 15 km depth in the crust of the overriding North American plate. This interface is segmented, and its interruptions are spatially correlated with conductive regions of the forearc and shallow swarms of seismicity and non-volcanic tremors. These observations suggest that fluid circulation in the overriding plate is controlled by fault zones separating blocks of accreted terranes. These zones constitute fluid escape routes that may influence the seismic cycle by releasing fluid pressure from the megathrust.