Figure S14 (above): 3D representation of the CASC16-P model. The box shows a volume from 36°N to 51°N, 112°W to 133°W, and 0 km to 800 km depth. A map showing political boundaries and tectonic plate boundaries is on the surface. A vertical slice at about 49°N shows the slab descending to depth, outlined by the dashed lines. The blue isosurface shows the high-velocity Juan de Fuca slab descending beneath the Pacific Northwest of the US. The orange isosurface to the west of the slab outlines the imaged low-velocity feature beneath the slab that we interpret as an accumulation of low-viscosity buoyant material.
The boundary between Earth’s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth, perhaps indicating a thin, weak layer that may strongly influence plate motion dynamics. The behavior of such a layer at subduction zones remains unexplored. We present a tomographic model, derived from on and offshore seismic experiments, that reveals a strong low-velocity feature beneath the subducting Juan de Fuca slab along the entire Cascadia Subduction Zone. Through simple geodynamic arguments, we propose that this low-velocity feature is the accumulation of material from a thin, weak, buoyant layer present beneath oceanic lithosphere everywhere. The presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics.
Cascadia Initiative seismic deployemnt (above): Overview map of the Cascadia Subduction Zone and the Cascadia Initiative seismic deployment. The ocean floor Cascadia Initiative stations are shown in orange and red; the onshore USArray station are purple. Moderate earthquakes around the Juan de Fuca oceanic plate are shown grey; the purple swath illustrates the Cascadia megathrust fault that has and will generate magnitude 9 earthquakes.
3D rendering showing the structure of the Cascadia subduction zone. Prepared by William Hawley.