A Cascadia Slab Model From Receiver Functions

Abstract

We map the characteristic signature of the subducting Juan de Fuca and Gorda plates along the entire Cascadia forearc from northern Vancouver Island, Canada, to Cape Mendocino in northern California, USA, using teleseismic receiver functions. The subducting oceanic crustal complex, possibly including subcreted material, is parameterized by three horizons capable of generating mode‐converted waves: a negative velocity contrast at the top of a low velocity zone underlain by two horizons representing positive contrasts. The amplitude of the conversions varies likely due to differences in composition and/or fluid content. We analyzed the slab signature for 298 long‐running land seismic stations, estimated the depth of the three interfaces through inverse modeling and fitted regularized spline surfaces through the station control points to construct a margin‐wide, double‐layered slab model. Crystalline terranes that act as the static backstop appear to form the major structural barrier that controls the slab morphology. Where the backstop recedes landward beneath the Olympic Peninsula and Cape Mendocino, the slab subducts sub‐horizontally, while the seaward‐protruding and thickened Siletz terrane beneath central Oregon causes steepening of the slab. A tight bend in slab morphology south of the Olympic Peninsula coincides with the location of recurring large intermediate depth earthquakes. The top‐to‐Moho thickness of the slab generally exceeds the thickness of the oceanic crust by 2–12 km, suggesting thickening of the slab or underplating of slab material to the overriding North American plate.

Subducting plate structure and megathrust morphology from deep seismic imaging linked to earthquake rupture segmentation at Cascadia

“…S11 and 12). Further onshore, receiver function studies ( 47 , 61 ) indicate plate interface depths under the Olympic peninsula that are up to 10 km shallower than the prior regional slab models, consistent with our findings. Modeling of onshore geodetic data indicates strong seismic locking at present within this portion of the margin ( 21 , 62 ) and recent observations of plate interface microseismicity suggest that the limit of the locked zone extends further landward in this region than some prior estimates ( 20 ) (fig.…”

“…The sense of fault offset in the lower plate under the shelf, up to the north, is consistent with lower vertical load from the upper plate associated with the presence of less dense Olympic accretionary complex rocks to the north. Observations of the location of oceanic Moho further onshore derived from receiver functions indicate a step up to the north coincident with the northern boundary of Siletz, indicating a lower plate tear coincident with the edge of Siletz terrane is also present deep within the CSZ ( 61 ).…”