Fluid Controls on the Heterogeneous Seismic Characteristics of the Cascadia Margin

The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate‐depth seismicity. In warm subduction zones, shallow dehydration can also lead to the buildup of pore‐fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate‐depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate‐depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (10 km) and indicate the presence of fluid within oceanic crust and underthrust metasediments Low‐velocity zones, seismicity, and tremor can be linked to fluids sourced from a variably hydrated and/or permeable subducting slab